Biomembrane‐inspired design of medical micro/nanorobots: From cytomembrane stealth cloaks to cellularized Trojan horses

Micro/nanorobots are promising for a wide range of biomedical applications (such as targeted tumor, thrombus, and infection therapies in hard‐to‐reach body sites) because of their tiny size and high maneuverability through the actuation of external fields (e.g., magnetic field, light, ultrasound, electric field, and/or heat). However, fully synthetic micro/nanorobots as foreign objects are susceptible to phagocytosis and clearance by diverse phagocytes. To address this issue, researchers have attempted to develop various cytomembrane‐camouflaged micro/nanorobots by two means: (1) direct coating of micro/nanorobots with cytomembranes derived from living cells and (2) the swallowing of micro/nanorobots by living immunocytes via phagocytosis. The camouflaging with cytomembranes or living immunocytes not only protects micro/nanorobots from phagocytosis, but also endows them with new characteristics or functionalities, such as prolonging propulsion in biofluids, targeting diseased areas, or neutralizing bacterial toxins. In this review, we comprehensively summarize the recent advances and developments of cytomembrane‐camouflaged medical micro/nanorobots. We first discuss how cytomembrane coating nanotechnology has been employed to engineer synthetic nanomaterials, and then we review in detail how cytomembrane camouflage tactic can be exploited to functionalize micro/nanorobots. We aim to bridge the gap between cytomembrane‐cloaked micro/nanorobots and nanomaterials and to provide design guidance for developing cytomembrane‐camouflaged micro/nanorobots.

[36] Hence, it is difficult for nanomaterials to penetrate the poorly vascularized, hypoxic areas of tumor tissues and to cross the blood-brain barrier (BBB).Transcytosis is a metabolic process that transports various cargos within membrane-bounded carriers (e.g., vesicles) from one side of a cell to the other, and it is extensively employed by diverse cells in the body to exchange materials between two distinct environments. [37,38][41][42][43] Once a nanomaterial enters the dynamic bloodstream, it tends to be nonspecifically adsorbed by various blood serum proteins, such as albumin and globulin. [44]Furthermore, these nanomaterials are easily captured and phagocytosed by the mononuclear phagocyte system (MPS; e.g., monocytes, macrophages, and dendritic cells (DCs)), thus impeding them from targeting and reaching disease sites.Nanomaterials possess poor physiological stability in the body's circulatory system.After intravenous administration, they tend to aggregate and may severely block the capillaries, eventually accumulating in the reticuloendothelial system (RES), such as in the liver, spleen, and other reticular connective tissues, and cause chronic inflammatory reactions. [45]Moreover, the presence of different physiological barriers in the human body leads to inefficient delivery of nanomaterials to disease sites. [46]o overcome these obstacles, researchers have conducted numerous studies.[49][50][51] Nevertheless, PEGylated nanomaterials may trigger an unfavorable anti-PEG immune reaction, and great efforts have therefore been made to develop alternative approaches to tackling these issues.One example is the use of cytomembranes or living cells to camouflage nanomaterials.To evade clearance by the immune system and increase circulation in the body, various cytomembranes have been exploited to cloak NPs and create functional nanomaterials.For instance, to improve the delivery efficiency of therapeutic agents, diverse nanocarriers have been camouflaged with different cytomembranes, such as erythrocyte membrane, [52][53][54] macrophage membrane, [55] platelet membrane, [56,57] and hybrid cytomembranes derived from erythrocytes and platelets. [58][61] Moreover, biohybrid microcarriers have been created by camouflaging synthetic nanomaterials with living cells, such as macrophages, [62] neutrophils, [63,64] and backpack-carrying cells, [65] to efficiently deliver therapeutic agents, thereby exploiting the intrinsic advantages of living cells.
Although great advances have been made in cytomembrane coating nanotechnology, most of the current approaches to disease targeting and therapy are relatively passive.Thus, there is an urgent need to develop new strategies for actively targeting and treating various diseases.[85][86] However, like conventional artificial nanomaterials, micro/nanorobots have the drawbacks of potential high toxicity against cells, tissues, or organs; fast clearance by the immune system; poor disease recognition capabilities; and low accumulation at disease sites, which hinder their clinical applications.Various biological barriers must be overcome before they can reach specific target locations. [70,87,88]owadays, with the rapid convergence of medical micro/nanorobots and cytomembrane coating nanotechnology, the development of cytomembrane-camouflaged micro/nanorobots has become an emerging research field.91] Table 1 summarizes a systematic comparison of cytomembrane-cloaked micro/nanorobots and traditional NP delivery systems.In recent years, researchers have made great contributions by creating an increasing number of cytomembrane-camouflaged micro/nanorobots.Among the very few reviews on the topic of cytomembrane-camouflaged micro/nanorobots, [92] we noted that a comprehensive review remains absent in this field, especially one that aims to bridge the gap between cytomembrane coating nanotechnology and cytomembrane-camouflaged micro/nanorobots.
In this review, we first summarize the use of cytomembrane coating nanotechnology for engineering nanomaterials, including cloaking nanomaterials with various cytomembranes and directly employing living cells to engulf nanomaterials.Thereafter, we highlight state-of-theart design strategies for active cytomembrane-camouflaged micro/nanorobots, encompassing micro/nanorobot camouflaging with diverse cytomembranes and micro/nanorobot cellularization with living cells.Finally, we present suggestions for future research in this field.In this review, we discuss two types of cytomembrane camouflage

What makes active micro/nanorobots different?
Passive drug delivery systems (DDS) refer to transporting drugs through blood plasma and arriving at disease sites; they directly depend on blood circulation parameters, such as flow gradient, phagocyte activity, and circulation time.Thus, all oral and systemic DDS based on conventional NPs are intrinsically passive nanocarriers.By contrast, motile micro/nanorobots are essentially active DDS and can achieve site-specific targeting and drug delivery by means of self-propulsion or external actuation. [106,108,110]ctive micro/nanorobot-based DDS have significant advantages given the following two factors: (1) The blood flow rate is larger than 5 L/min and reaches approximately 1.5-33 cm/s in the capillaries and venules; [111] (2) the total length of human blood vessels is approximately 100,000 km. [112]Consequently, when circulating in the bloodstream, a passive drug nanocarrier undergoes a long and arduous journey at extremely high movement velocities.In only subseconds, a passive nanocarrier can bypass a tumor sized in a few centimeters, which leaves an extremely narrow "window of opportunity" for ligand-receptor interaction in diseased tissues rather than accumulation in normal tissues.Therefore, a novel, active DDS is urgently required to overcome these challenges.It should possess the capabilities of directional locomotion to a target site, low dependence on local blood flow (even the capacity to go against the bloodstream), and ability to release drugs at specific sites.
In this context, significant advances have been made in the development of micro/nanorobots for active drug delivery.With their characteristic locomotion properties, micro/nanorobots can be autonomously propelled or externally driven for localized, on-demand drug delivery.Consequently, their directed movement can exhibit independence (or low dependence) on local flow gradients and can deliver drugs to specific sites that are very difficult for passive DDS to reach. [75,113,114]

Actuation modes for active micro/nanorobots
Micro/nanorobots are capable of directional locomotion and can actively carry out specific medical tasks through autonomous or externally powered propulsion.Since motility is their fundamental feature, actuation methods are extremely important for enabling micro/nanorobots to conduct complex biomedical tasks.Nowadays, methods for propelling micro/nanorobots include self-propulsion, external propulsion (such as magnetic, ultrasonic, electric, and light propulsion), biohybrid propulsion, and multimode propulsion.Self-propulsion is chemical propulsion that employs catalytic engines to convert chemical fuel energy into propulsion for autonomous motion. [115]Magnetic actuation can be generated by a rotating, oscillating, or on/off magnetic field to drive micro/nanorobotic movement. [116]Ultrasound can propel micro/nanorobots and trigger their rapid localized aggregation. [117]Light propulsion is also an effective actuation propulsion method due to tunable size of light beams and high spatiotemporal resolution. [118]121] Delivery strategies based on micro/nanorobots can take full advantage of their autonomously propelled or externally actuated locomotion performance and capacities, such as stimuli-responsiveness, [122] specific recognition, [123] and taxes (e.g., chemotaxis, rheotaxis, and magnetotaxis), [124] to actively transport therapeutic drugs to target sites and release drugs locally on demand.According to the specific tasks to be executed, the delivery sites or targets may vary, including tumor tissues, inflamed or infected tissues, lymphoid organs, or combinations of these.

Active micro/nanorobots for tumor treatment
Medical micro/nanorobots, as an emerging technology, have demonstrated huge potential for targeted drug delivery and precision tumor therapy. [125]However, most of the studies have focused on in vitro antitumoral effects and proof-ofconcept research.For instance, sperm-driven microrobots have been developed as targeted DDS to potentially treat ovarian cancer or cervical cancer in the female reproductive tract, making full use of the native chemotaxis of sperms toward oocytes. [126]Guided by an external magnetic field, sperm microrobots carrying doxorubicin (DOX) can move into a tumor spheroid cultured in vitro and deliver the anticancer drug locally.Pt NP-loaded stomatocyte nanomotors comprising PEG-b-PCL and PEG-b-PS can sense the local acidic environment of tumors and achieve controlled delivery and release of DOX to cancer cells in vitro. [127]ew researchers have investigated the in vivo antitumor effects of micro/nanorobots.They have mainly employed magnetically actuated micro/nanorobots as delivery systems to move toward/within and transport anticancer drugs into solid tumors.For example, neutrophil-based microrobots were used to actively deliver paclitaxel-loaded magnetic nanogels to malignant gliomas in vivo. [128]The neutrophil microrobots were able to perform controlled intravascular locomotion in a rotating magnetic field, autonomously aggregate in mouse brains, and then cross the BBB via their positive chemotactic movement along the inflammatory factor gradient.Based on DNA origami, autonomous DNA nanorobots have been constructed and programmed to transport thrombin specifically into tumors with the assistance of blood flow and targeting moiety on the nanorobot surface. [75]ested on a tumor-bearing mouse model, DNA nanorobots delivered thrombin specifically to tumor-associated blood vessels after intravenous injection and caused intravascular thrombosis, which finally led to tumor necrosis and growth inhibition.With rational design, DNA nanorobots can achieve selective, targeted lysosomal degradation of tumor-specific proteins on cancer cells, exhibiting high efficiency in a mouse model and holding promise for precision breast cancer therapy. [129]s an emerging drug carrier system, a recent study evaluated the systemic distribution of micro/nanorobots. [130]lgae-based microrobots demonstrated few differences across blood parameters based on a comprehensive analysis of blood chemistry and main blood cell populations after in vivo administration into mouse lungs.Furthermore, the hematoxylin and eosin staining results of mouse hearts, livers, spleens, lungs, and kidneys showed that the structural integrity of these major organs was almost identical to those of control mice, thereby confirming no significant acute toxicity caused by algae microrobots and supporting their in vivo safety.Nevertheless, it should be noted that there are few references related to the systemic distribution or intratumoral distribution of micro/nanorobots, and there is a lack of relevant research on their pharmacokinetics.Therefore, more efforts should be made to fully ascertain the in vivo biosafety and pharmacokinetics of micro/nanorobots.During the course of these studies, the principles found in micro/nanorobots need to be compared with those followed by other DDS.

Passive nanoparticles cloaked with living cell-derived cytomembranes
Tactics that utilize biogenic cell membrane components to cloak synthetic nanomaterials can provide unique cell-like functions and enrich the concept of surface modification and functionalization of NPs.32] A wide range of cells, including DCs, [133] lymphocytes, [134] erythrocytes, [135,136] platelets, [137] natural killer (NK) cells, [138] myeloid-derived suppressor cells, [139] cancer cells, [140] cancer stem cells, [141] mesenchymal stem cells, [142,143] neurons, [144] and bacteria, [145] have been employed to camouflage nanomaterials with cytomembranederived constituents.Table 2 summarizes the biomarkers, properties, and functions of different types of cells to elucidate the differences between cytomembrane choices and their particular effects on the functions of biomimetic materials. [146]In keeping with the purpose of this review, this section centers on the use of erythrocytes, leukocytes (including monocytes, macrophages, neutrophils, and DCs), platelets, and cancer cells to engineer functional nanomaterials for versatile biomedical applications.Although the word "cloaked" is used throughout this review, it should be noted that some cytomembranes may fuse with NPs, such as liposome-based NPs.

Erythrocyte membrane-cloaked passive nanoparticles
Native cellular materials are well compatible with the human body and have attracted considerable attention in the biomedical field.Erythrocytes, also referred to as red blood cells (RBCs), possess the advantages of accessibility, biocompatibility, biodegradability, and long-term blood circulation; they are a good example of natural materials being used for multiple medical applications, such as drug delivery, [136,156] imaging, [157] detoxification, [158] vaccination, [159] and transfusion medicine. [160]In addition to the direct use of pristine erythrocytes, erythrocyte membranes have been widely used to camouflage nanomaterials.In an early study, Zhang and coworkers developed a biomimetic delivery platform by extruding poly(lactic-co-glycolic acid) (PLGA) NPs with erythrocyte membrane vesicles (Figure 1A). [161]These erythrocyte membrane-cloaked PLGA NPs exhibited excellent circulation half-life and particle retention in the blood after injection into mice.In brief, the fabrication of erythrocyte membrane-camouflaged nanomaterials involves two steps: [162] (1) preparation of erythrocyte membrane vesicles of diverse sizes via mechanical extrusion through polycarbonate porous membrane filters (0.1-10 μm) after hypotonic treatment and (2) repeated coextrusion of erythrocyte membranes and nanomaterials to fuse them.Surface charges and cytomembrane-nanomaterial ratios play important roles in erythrocyte membrane camouflage. [163]For example, a high sialic acid content endows erythrocyte membranes with negative charges, thereby leading to negatively charged nanomaterials after cloaking.
Atherosclerosis (a common cardiovascular disease) is characterized by the accumulation of lipids, immunocytes, and fibrous elements in the artery walls. [164]Nanomaterials can act as potent drug delivery platforms for atherosclerosis therapy.Nevertheless, therapeutic efficacy is extremely limited in vivo because of the nonspecific clearance of nanomaterials by the MPS.To tackle this issue, researchers have used erythrocyte membranes to cloak rapamycin (RAP)-loaded PLGA NPs, thus creating biomimetic core-shell structured nanocomplexes for managing atherosclerosis, as shown in Figure 1B. [165]The presence of erythrocyte membranes may lead to lower phagocytosis by macrophages in the blood and increased accumulation of NPs in established atherosclerotic plaques, facilitating targeted on-site drug delivery.Collectively, such biomimetic nanocomplexes are capable of significantly attenuating the progression of atherosclerosis.Tumor hypoxia may limit the efficacy of cancer radiotherapy, requiring oxygen to boost radiation-induced damage to tumor cells.To solve this problem, Liu's group designed erythrocyte membrane-cloaked, perfluorocarbon-PLGA core-shell NPs. [166]The perfluorocarbon cores exhibited efficient oxygen loading, and erythrocyte membrane coating largely prolonged the blood circulation duration of NPs.Taken together, such core-shell NPs can achieve efficient delivery of oxygen into tumor tissues after intravenous injection, largely relieve tumor hypoxia, and thereby significantly improve the therapeutic efficacy of cancer radiotherapy.Erythrocyte membranes can also be used to camouflage protein NPs or other polymeric NPs, such as melanin NPs [167] and nanogels. [168]More importantly, the erythrocyte membrane camouflaging strategy applies to a broad range of inorganic NPs, such as Au NPs, [169] Au nanocages, [170] and magnetic NPs, [171,172] for various biomedical applications.

Leukocyte membrane-cloaked passive nanoparticles
Leukocytes, also known as white blood cells (WBCs), encompass monocytes, macrophages, eosinophils, neutrophils, basophiles, lymphocytes, and DCs; they play many crucial roles in the host immune response.Leukocytes are characterized by chemotaxis [173] and exhibit targeting F I G U R E 1 (A) Diagram of the fabrication process for RBC-membrane-cloaked PLGA NPs.Reproduced with permission. [161]Copyright 2011 National Academy of Science.(B) Schematic diagram of atherosclerosis treatment using RBC/RAP@PLGA.Reproduced with permission. [165]Copyright 2019, the Authors.Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.capacity via cytomembrane interactions.The therapeutic efficacy of drug delivery devices in vivo mainly relies on their capacity to evade immune clearance, cross biological barriers, and reach target disease sites.In this context, Tasciotti's group functionalized nanoporous silicon (NPS) particles by camouflaging them with Jurkat leukocyte membranes (Figure 2A). [134]Such hybrid NPs were called leukolike vectors (LLVs) and exhibited versatile properties, such as a decrease of opsonization by abundant serum proteins, escape from phagocytosis, clearance by the host immune system, communication with endothelial cells via receptor-ligand interactions, and delivery and release of a DOX payload through an inflamed endothelium.Furthermore, these LLVs demonstrated prolonged circulation duration and enhanced accumulation at tumor sites after injection.Researchers also developed (sodium alginate-chitosan) 8 capsules via layerby-layer assembly and then cloaked them with leukocyte membrane vesicles for favorable immune evasion, prolonged circulation time, and improved accumulation at tumor sites. [174][178] The exact cause of rheumatoid arthritis remains obscure, and current therapies mainly focus on its inflammatory. [179]Although the use of anticytokine biologics has had some efficacy for rheumatoid arthritis therapy, there are still many limitations regarding response rate, treatment efficacy, and so on. [178,180]unctional NPs have been widely engineered to target and treat rheumatoid arthritis. [181]Importantly, neutrophils modulate autoimmunity and tissue damage during rheumatoid arthritis. [182]Zhang's group developed an NP-based broadspectrum anti-inflammatory strategy for the management of rheumatoid arthritis. [175]When neutrophil membranes were fused onto PLGA nanocores (Figure 2B), the hybrid NPs inherited the membrane components, functions, and antigenic characteristics of their parent neutrophils, making them ideal decoys for the neutrophil-targeted biomolecules.These NPs were capable of neutralizing proinflammatory cytokines, inhibiting synovial inflammation, penetrating deep F I G U R E 2 (A) Schematic illustration of an LLV: The surface of the LLV, fully covered by the leukocyte-derived cell membranes, and relevant membrane proteins (e.g., LFA-1, CD3z, CD45, and Lck) interspersed in the porous structure of the NPS are highlighted.TEM images of (b) bare NPS and (c) NPS coated with leukocyte-derived membranes (scale bar: 100 nm); SEM images of (d) a bare NPS and (e) a LLV completely camouflaged with leukocyte-derived membranes (scale bar: 1 μm).Reproduced with permission. [134]Copyright 2012, Nature Publishing Group.(B) Construction of neutrophil-cloaked NPs by completely coating PLGA polymeric cores with natural human neutrophil membranes to inhibit synovial inflammation and alleviate joint destruction in inflammatory arthritis (scale bar: 100 nm).Reproduced with permission. [175]Copyright 2018, the Authors, under exclusive license to Springer Nature Limited.into the cartilage matrix, and providing a strong chondroprotection effect against joint damage.Furthermore, neutrophil membrane-cloaked PLGA NPs ameliorated joint damage and restrained overall arthritis severity in two mouse arthritis models (collagen-induced and human transgenic), indicating their remarkable therapeutic efficacy for managing rheumatoid arthritis.Based on a neutrophil membrane camouflaging strategy, artificial super neutrophils were also fabricated via two steps: (1) embedding glucose oxidase and chloroperoxidase into zeolitic imidazolate framework-8 (ZIF-8) NPs to generate hypochlorous acid (HClO) through enzymatic cascades and (2) encapsulating the NPs with neutrophil membranes to achieve inflammation-targeting ability.With around a seven times higher production rate of reactive HClO than natural neutrophils, these artificial super neutrophils were capable of eradicating malignant tumors and pathogen infections.
Macrophages, unlike circulating monocytes in the peripheral blood, [183] serve as key mediators during development, disease (such as inflammation, infection, and cancer), or tissue regeneration and remodeling. [184]Macrophages can circulate in the blood for immune surveillance.Once a danger is sensed or detected, they migrate into the target tissue.Moreover, they can also reside within multiple tissues in a steady state (termed tissue-resident macrophages). [185,186]onsidering the important roles of macrophages in the host immune system, various immunomodulatory biomaterials have been designed to regulate their cell fate and immune response. [7,187,188]Herein, we mainly focus on the macrophage membrane camouflaging strategy for nanomaterial surface functionalization, which has shown great potential for disease targeting and therapy.For example, cloaking PLGA NPs with macrophage membrane microvesicles facilitated a biomimetic approach to targeting and treating rheumatoid arthritis and loading and delivering therapeutic drugs. [189]Researchers have also developed macrophage membrane-camouflaged PLGA NPs that exhibit macrophage-like abilities to simultaneously absorb bacterial endotoxins and proinflammatory cytokines. [190]This biomimetic detoxification strategy based on macrophage TA B L E 3 Summary of representative examples of macrophage membrane-camouflaged nanomaterials.

Lipid NPs Membrane proteins from J774 macrophages
Proteolipid vesicles (leukosomes) can selectively target inflamed vasculature and efficiently deliver dexamethasone to inflamed tissues to decrease phlogosis. [12]

PLGA NPs Membranes from J774 macrophages
Macrophage-like NPs can simultaneously sequester endotoxins and inflammatory cytokines to inhibit immune activation, bacteria dissemination, and sepsis cascade for detoxification.[190]   Polymeric cskc-PPiP NPs

Membranes from mouse peritoneal macrophages
NPs can effectively escape RES clearance and home in on tumor tissue to locally release paclitaxel for amplifying anticancer efficacy.[192]   PLGA NPs Membrane microvesicles from RAW 264.7 macrophages NPs efficaciously targeted inflamed sites via Mac-1 and CD44 expression in a mouse arthritis model and delivered tacrolimus to noticeably constrain RA progression. [189] Emtansine liposome NPs Membranes from RAW 264.7 macrophages NPs can efficiently target and deliver emtansine into metastatic 4T1 cells to remarkably constrain mammary cancer metastasis.[193]   Porous silicon NPs Membrane from KG-1 macrophages Bioengineered NPs can contribute to immune activation absence and hold potential for RA treatment.
[ Biomimetic NPs can selectively target tumors with immune evasion capacity and efficiently amplify photothermal therapy against breast cancer.
[ 198]   membrane camouflaging demonstrated its superiority for sepsis management.Furthermore, Cai's group recently designed macrophage membrane-cloaked DSPE-PEG-loaded IR-792 (near-infrared Ib fluorescent dye) hybrid NPs [191] that were able to penetrate the BBB and selectively accumulate at glioblastoma sites.Concurrently acting as NIR-Ib fluorescent probes, the hybrid NPs achieved targeted tumor imaging.Moreover, such macrophage membrane-camouflaged NPs were able to effectively kill tumor cells due to their photothermal effect.Collectively, this work demonstrated that NIR-Ib imaging-guided photothermal therapy significantly suppressed glioblastoma growth and extended the life of mice.Other representative examples of macrophage membrane-camouflaged nanomaterials are summarized in Table 3. DCs are antigen-presenting cells that act as a bridge between innate and adaptive immunity. [199,200]Targeted immunoregulation of DCs in vivo can manipulate T-cell priming and amplify anticancer immune responses. [201]urthermore, DCs can be harnessed to develop novel nanotechnology-based therapies.For instance, an activated mature DC membrane-coated nanoplatform (using RAPloaded PLGA NPs) has been established to orchestrate immune responses for glioma treatment. [133]These RAPloaded PLGA NPs demonstrated the ability to cross the BBB and precisely modulate the immune microenvironment.They activated tumor-infiltrating T lymphocytes and NK cells, triggered antitumor immune responses, and thereby significantly inhibited glioma growth.A biomimetic nanovaccine consisting of DC-membrane coated, IL-2-loaded PLGA NPs was created to potentiate T-cell-based immunotherapy of ovarian cancer. [202]The nanovaccine elicited enhanced Tcell activation in vitro and in vivo and exhibited excellent therapeutic and prophylactic efficacy against ovarian cancer in a mouse model, such as by delaying tumor growth and decreasing tumor metastasis when compared with a DC vaccine.
Leukocytes (such as monocytes, macrophages, neutrophils, and lymphocytes) can sense, migrate toward, and target the sites of inflammation, infection, and tumors.The specific homing, recognition, and targeting capabilities of leukocytes to diseased cells, microenvironments, or pathogens have been exploited as transporters to selectively deliver drug molecules or nanotherapeutics to disease sites.Derived from natural leukocytes, leukocyte membranes can inherit some biological entities and functions from their parent cells, including specific recognition capacity regarding inflamed, infected, or tumoral sites.Nanomaterials cloaked with leukocyte membranes can maintain critical leukocyte transmembrane proteins that have the same orientation as their source cells.[206] Consequently, nanomaterials cloaked with leukocyte membranes possess a targeting capacity originating from source leukocytes.

F I G U R E 3 (A)
A representative platelet-mimicking NP with pathogen binding, collagen binding, and immunocompatibility: SEM images of MRSA252 bacteria after incubating with (a) bare PLGA NPs and (b) platelet membrane-cloaked); (c) A pseudocolored SEM image of PNPs (in orange) incubated with the extracellular matrix of a decellularized human umbilical cord artery, demonstrating the excellent collagen binding ability of PNPs; and (d) immunocompatibility examination results using human THP-1 macrophage-like cells for particle uptake.PNPs exhibited a good.Reproduced with permission. [137]Copyright 2015, Nature Publishing Group-a division of Macmillan Publishers Limited.(B) A representative platelet membrane-camouflaged NP for tumor vessel impairment: (a) the fabrication process for MSN@PM-C-A with drug encapsulation and platelet membrane coating, and (b) proposed anticancer mechanism of MSN@PM-C-A in tumor vessels.Reproduced with permission. [222]Copyright 2021, American Chemical Society.

Platelet membrane-cloaked passive nanoparticles
Platelets, also known as thrombocytes, are small, diskshaped, non-nucleated blood cells with fragile membranes.Platelets circulate in blood and tend to bind together when they recognize damaged blood vessels, thus playing a crucial role in blood clotting and coagulation.Platelets possess diverse functions that originate from their unique surface moieties and account for atherothrombosis, [207] immune evasion, [208,209] pathogen interaction, [210,211] subendothelium adhesion, [212,213] and hemostasis and thrombosis. [214,215][218] The design and development of NPs to deliver therapeutic drugs to target disease sites in the human body promises safer and more efficient drug delivery to tackle numerous medical challenges.[221] In an early study, researchers cloaked PLGA NPs (100 nm) with human platelet membranes (Figure 3A). [137]After fusion, the hybrid NPs inherited several platelet properties due to the presence of platelet membranes that had immunoregulatory and adhesion antigens correlated with parent platelets.These cloaked NPs exhibited decreased phagocytosis by macrophages and a lack of NP-induced complement activation in autologous human plasma.They also exhibited other platelet-like properties, such as selective adhesion to injured human and murine vasculatures and boosted binding to platelet-adhering bacteria.More importantly, such platelet membrane-camouflaged NPs can serve as an effective disease-targeted delivery platform for therapeutic drugs, such as docetaxel and vancomycin, and have demonstrated their potential to enhance therapeutic efficacy in a rat Urokinase-loaded MSNs Rat platelet membranes Specific arteriovenous thrombolysis, size-dependent penetration, and retention of NPs in thrombi [235]   coronary restenosis model and a mouse systemic infection model, respectively.Tumor blood vessels can provide nutrients and oxygen to tumor tissues and remove metabolic waste, which are crucial functions for the cell proliferation, growth, and metastasis of tumors. [223,224]Therefore, treatment strategies that target tumor blood vessels and deprive tumors of metabolic requirements have great potential for application to cancer therapy. [10,75,225][228] For this purpose, Nie's group concurrently loaded combretastatin A4 (CA4, a vascular disrupting agent) and apatinib (Apa, an antiangiogenic agent) into platelet membrane-cloaked mesoporous silica NPs (MSNs) for tumor combination therapy (Figure 3B). [222]The platelet membrane camouflage endowed MSNs with capabilities of tumor vascular targeting, intratumoral accumulation, and local delivery and release of CA4 and Apa.Taken together, these biomimetic NPs accumulated at the sites of tumor tissues owing to the ability of platelet membranes to adhere to specific impaired vessel sites, thereby leading to remarkable vascular disruption and effective antiangiogenesis in BALB/c nude mouse model with MHCC-97H liver tumors.Researchers have also prepared platelet membrane-cloaked MSNs for coloading tirapazamine (a hypoxia-activated prodrug) and 5,6-dimethylxanthenone-4-acetic acid (a vascular disrupting agent) inside for tumor targeting, intratumoral vascular disruption, and hypoxia-sensitive chemotherapy. [226]Several representative examples of platelet membrane-camouflaged nanomaterials are summarized in Table 4.

Cancer cell membrane-cloaked passive nanoparticles
[248] The first study to employ cancer cell membrane-cloaked NPs to develop nanovaccines for cancer immunotherapy was conducted by Zhang's group. [249]The fabrication involved two procedures: (i) The immunological adjuvant, CpG oligodeoxynucleotide 1826, was loaded into PLGA NPs.(ii) The CpG-loaded PLGA NPs were camouflaged with B16-F10 mouse melanoma cell membranes that serve as tumor antigen material.Collectively, these nanovaccines demonstrated a potent ability to elicit antitumor immunity in a vaccinated C57BL/6NHsd mouse model challenged with B16-F10 melanoma cells subcutaneously.In a recent study  [153] Copyright 2018, American Chemical Society.(B) Cancer cell membrane-coated NPs for tumor therapy: (a) fabrication of CCM@LM by direct fusion of cancer cell membranes and corresponding SEM images (scale bar: 100 nm); (b) ex vivo evaluation of homologous targeting ability by measuring the accumulation of IR783-labeled NPs within the tumor and other organs; (c) anticancer activity in vivo demonstrated by weighing tumors at the end of the experiment.Reproduced with permission. [259]Copyright 2019, American Chemical Society.by Liu's group, the toll-like receptor 7 agonist, imiquimod (R837), was first loaded into PLGA NPs as a nanoadjuvant (denoted as NP-R), and B16-OVA cancer cell membranes, offering tumor-specific antigens, were then exploited to camouflage these adjuvant NPs (denoted as NP-R@M). [153]After further modification with mannose, the nanovaccine (denoted as NP-R@M-M) was produced and was demonstrated that NP-R@M-M could enhance the uptake by DCs and strongly stimulate the maturation of DCs, thereby triggering antitumor immunity.As an efficient prophylactic nanovaccine to delay tumor progression, the NP-R@M-M vaccine could be combined with checkpoint blockade treatment to achieve excellent cancer therapy efficacy, as illustrated in Figure 4A.
Lymph nodes are the immune organs in which innate immune responses result in acquired immunity; thus, they play major roles in both innate and adaptive immunity. [250]With rational material design, NP delivery vehicles can achieve localized accumulation of drugs within lymph nodes and target specific lymph-node-resident cells, promising effective immune activation and cancer immunotherapy. [251,252]For example, targeting lymph nodes has been generally used for vaccination to produce adaptive immunity and induce immune tolerance. [253,254]The targeted delivery of nanovaccines to lymphoid organs is also a promising strategy for boosting the efficacy of cancer immunotherapy. [255]Furthermore, lymph node drug delivery is of great significance for eliminating lymph node-residing cancers and metastases, such as lymphomas that reside in lymph nodes. [256]258] TA B L E 5 Summary of representative examples of cancer cell membrane-camouflaged nanomaterials.Cancer cell membrane-camouflaged NPs have great potential for tumor treatment because of their favorable features, such as immune escape and homotype binding.Nevertheless, their therapeutic efficacy remains limited by their low tumor penetration efficiency and intracellular transport.To overcome this bottleneck, researchers have developed PEGylated liposome yolk supported by MSNs and cloaked with cancer cell membranes (denoted as CCM@LM) for targeted tumor chemotherapy, as shown in Figure 4B. [259]CCM@LM can penetrate multicellular tumor spheroids in vitro and be directly internalized through membrane fusion.Exhibiting boosted perinuclear aggregation, CCM@LM coloaded with DOX and mefuparib hydrochloride (a polymerase inhibitor) has demonstrated significantly stronger anticancer efficacy than the first-line chemotherapy drug Doxil.Moreover, cancer combination therapy can be developed based on cancer cell membrane-camouflaged NPs.For example, porphyrinic Zr-MOF (PCN-224) NPs can simultaneously serve as photosensitizers and a delivery carrier for apatinib. [18]After MnO 2 coating and subsequent cancer cell membrane camouflaging, such biomimetic NPs have combinatorial antiangiogenesis and PDT capabilities.Cancer cell membranecamouflaged NPs have also been designed for specific tumor imaging, [140] tumor surgery navigation under NIR-II imaging, [260] fluorescent/photoacoustic imaging (PAI), and NIR-triggered tumor photothermal therapy. [261]Table 5 lists several representative examples of cancer cell membrane-camouflaged nanomaterials.In addition to the use of a single type of cytomembrane, the combination of different types of cytomembranes may enable the use of hybrid cytomembrane components with multiple functionalities for the more versatile surface camouflaging of synthetic NPs.Such cases include erythrocyte-cancer cell hybrid membrane-cloaked hollow CuS NPs, [262] cancer stem cellplatelet hybrid membrane-cloaked magnetic Fe 3 O 4 NPs, [141] platelet-leukocyte hybrid membrane-cloaked immunomagnetic beads, [263] and erythrocyte-platelet hybrid membranecloaked PLGA NPs. [58]

Fused cytomembrane-cloaked passive nanoparticles
In recent years, researchers have increasingly made efforts to camouflage synthetic NPs with hybrid or fused cytomembranes to enhance their functionalities. [146,268]Similar to single cytomembrane-cloaked NPs, the resulting biomimetic NPs not only maintain the mechanical, physical, and chemical properties of synthetic NPs but also inherit the biological functions of their parent cells.Furthermore, compared with single cytomembranes, fused cytomembranes can endow synthetic NPs with multiple biological functions derived from two or more types of source cells.This section provides a summary of recent progress in developing fused Prolonged blood circulation, targeted tumor therapy [277]   cytomembrane-camouflaged NPs as functional nanoplatforms (e.g., DDSs) for a variety of biomedical applications, as listed in Table 6.

Passive nanoparticles cloaked with living cells
This section mainly focuses on living cell-cloaked nanomaterial delivery systems for disease diagnosis and therapy.Nanomaterial-based therapy strategies hold great promise for treating diverse diseases, such as cancer, infection, and inflammation.However, due to the existence of biological barriers against nanomaterial transport, such as the BBB, MPS, and renal system, precise delivery of diverse nanotherapeutics to specific disease sites is highly challenging.For example, according to an analysis by Chan's group, only 0.7% of the administered dose of NPs was delivered to a solid tumor. [46]As discussed previously, camouflaging nanomaterials with diverse cytomembranes can greatly improve their delivery efficiency to a target disease sites.Nevertheless, this type of CCT is a relatively passive method of nanomaterial functionalization and transportation.To further improve the delivery efficiency and targeting ability of nanomaterials, cell-based nanomaterial delivery systems (i.e., cellularized nanomaterials) have been increasingly designed and developed.Compared with cytomembrane camouflaging, living cell-based nanomaterial delivery systems have unique advantages, including (1) intrinsic taxis of living cells to target a disease, (2) high loading capacity of living cells for nanomaterials, and (3) the regulatable secretion behavior of living cells.Taken together, these living cell-based delivery systems can overcome multiple biological barriers to track and target tumoral, infected, or inflamed sites within the body, thereby constituting a relatively active strategy for targeted nanomaterial delivery.In addition, living cell-based nanomaterials can be combined with advanced biofabrication techniques, such as three-dimensional (3D) bioprinting and microfluidics, which have great potential for tissue engineering applications.To this end, various living cells, such as mesenchymal stem cells, [278,279] neural stem cells, [280][281][282][283] dental-pulp stem cells, [284] adipose-derived stem cells, [285] erythrocytes, [136,286,287] leukocytes, [288] NK cells, [289] monocytes, [290,291] macrophages, [62,[292][293][294][295][296][297] neutrophils, [63,[298][299][300] lymphocytes, [301] DCs, [302] and platelets, [303,304] have been exploited to engineer nanomaterials, which is the second type of CCT.Such cellularized nanomaterials have demonstrated great potential as "Trojan horses" for active targeting and therapy of various diseases.Herein, we mainly focus on engineering nanomaterials with erythrocytes, leukocytes, and platelets.Table 7 gives an overview of the natural characteristics of erythrocytes, leukocytes (encompassing monocytes, macrophages, eosinophils, neutrophils, basophils, lymphocytes, and DCs), and platelets in the human body. [136,219,288,305]308] Although associated mechanisms remain poorly understood, both the complement system and reactive macrophages play crucial roles.Therefore, strategies are required to prevent the interactions of particles and macrophages in the first few minutes after injection, during which period adverse cardiopulmonary reactions typically occur.To this end, Moghimi's group physically attached carboxylated polystyrene spheres (750 nm) to the surface of human and pig erythrocytes, [309] aiming to delay the recognition of polystyrene particles by macrophages in the first few minutes after injection.The particles' "hitch-hiking" on erythrocytes can protect them from robust uptake by macrophages. [310,311]This erythrocyte "hitch-hiking" strategy was capable of mitigating the adverse cardiopulmonary responses associated with nanomedicine administration.Circulating cells, such as erythrocytes, leukocytes, and stem cells, possess native disease sensing and homing capabilities, which have made them promising candidates for developing advanced living delivery vehicles for therapeutic drugs and nanomaterials. [312,313]Yang's group developed melanoma-targeting DDSs using THP-1 macrophages as living carriers and navigators (Figure 5A). [295]PLX4032 (a specific drug for treating BRAF V600E mutant melanoma) was loaded into muramyl tripeptide-conjugated biodegradable photoluminescent polylactic acid (BPLP-PLA) NPs.These PLX4032-loaded BPLP-PLA NPs were then inter-nalized within THP-1 macrophages to eventually provide living DDSs that could effectively target and kill melanoma cells.Recently, researchers designed Au-hemoglobin NPloaded platelets (Au-Hb@PLT) for deep, targeted delivery into tumor tissues (Figure 5B). [303]In this living DDS, hemoglobin served as an oxygen vector to alleviate hypoxia in tumors.Au NPs act as radiosensitizers to enhance the sensitivity of cancer cells to X-rays, and platelets have intrinsic tumor homing properties.Even under low-dose radiotherapy, the Au-Hb@PLT demonstrated its ability to potentiate the treatment effect in a BALB/c nude mouse model with HeLa tumors.
NPs have been widely used to manipulate and activate DCs and achieve boosted immunotherapy against tumors and infections.For example, RNA-loaded magnetic liposomes have been designed with dual functions that can (1) induce potent anticancer immunity and (2) serve as an early predictor of therapy response. [314]These NPs activate DCs more efficiently than electroporation and achieve superior tumor growth inhibition in tumor-bearing mouse models.Moreover, the presence of iron oxide NPs can enhance DC transfection and track DC migration through magnetic resonance imaging (MRI).The T 2 *-weighted MRI intensity within lymph nodes is strongly correlated with DC trafficking, thereby acting as an early biomarker for the antitumor response.A nanoactivator capable of upregulating autophagy was developed for in situ manipulation of DCs to facilitate high-efficiency tumor antigen presentation and the generation of antigen-specific T cells. [315]Consequently, these nanoactivators can suppress tumor growth in vivo and prolong murine survival.The size of NPs plays an important role in regulating DC fate.Spherical Au NP-based vehicles with optimized sizes can be combined for DC activator and antigen delivery. [316]Such a combination (termed NanoAu-Cocktail) was related to dual targeting of CpG oligonucleotides and OVA peptides to the subcellular compartments of DCs; it can potentiate the antigen cross-presentation, upregulate the expression of costimulatory molecules, and elevate the F I G U R E 5 Representative cell-based NPs for disease therapy.(A) Schematic illustration of a macrophage-mediated DDS for targeting and killing cancer cells, in which macrophages function as both NP carriers and cancer-oriented navigators.Reproduced with permission. [295]Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.(B) Platelet-cloaked Au-hemoglobin for alleviating tumor hypoxia and boosting the radiotherapy effect of low-dose X-rays: (a) scheme demonstrating that Au-Hb@PLT has the capacity to target tumor tissue after being activated to penetrate the tumor and finally enhance the radiotherapy; (b) in vivo evaluation of the targeting behavior of Au-Hb@PLT using the biodistribution of samples following intravenous injection; and (c) in vitro cytotoxicity of HeLa cells incubated with Au-Hb@PLT upon X-ray irradiation.Reproduced with permission. [303]Copyright 2020, American Chemical Society.secretion of T helper1 cytokines.Moreover, more than 60% of lymphoid tissue-homing DCs accumulated in the liverdraining lymph nodes, demonstrating the promoted homing capability of NanoAu-Cocktail pulsed DCs.Furthermore, the NanoAu-Cocktail pulsed DCs elicited potent antigen-specific CD8+ T-cell responses that enhanced protection from viral invasion.
Cell membrane-coating nanotechnology has developed into a thriving research topic over the past ten years, with researchers making great efforts to explore its potential for biomedical applications. [132]One biotechnology company has paid attention to the clinical adaptation of cytomembrane-cloaked NPs (i.e., platelet membrane-coated, resiquimod-loaded polylactic acid NPs) for intratumoral immunotherapy. [234]Cytomembrane-cloaked NPs belong to a new class of biosynthetic nanohybrids that are still at the preclinical stage.Thus, few clinical trials have been conducted on them for clinical adaptation, many factors need to be considered.Significant input will be required from the Food and Drug Administration or an equivalent agency to identify the most suitable pathway to regulatory approval.Because of their integration with cell membranes, it is likely that new drug candidates based on cytomembrane-cloaked NPs will be treated as biologics.Lessons can be learned from the ongoing efforts to clinically adapt extracellular vesicle-based DDSs, especially when tackling problems associated with the heterogeneity and batch-to-batch variability of cytomembrane-cloaked NPs. [317]

Cytomembrane camouflage tactic of micro/nanorobots
As discussed previously, many conventional artificial nanomaterials have been widely exploited for biomedical purposes.However, these traditional nanomaterials are intrinsically passive and insufficient in targetability.Therefore, researchers have endeavored to develop active micro/nanorobots for precision medicine and targeted therapy.Micro/nanorobots have demonstrated great potential for a variety of biomedical applications, such as targeted drug delivery, precision cancer therapy, cell microsurgery, biopsy, detoxification, and single-cell manipulation, because of two outstanding advantages: (1) They can be propelled by external fields or chemical fuels to actively move to a target site that is difficult to reach with conventional methods or materials. [116,117](2) They can carry out complicated tasks according to an instruction or a stimulus in the physiological environment, such as a magnetic field, ultrasound, light, heat, pH, and/or ionic strength. [318,319]Nevertheless, traditional artificial nanomaterials used for micro/nanorobots have the drawbacks of potentially high toxicity against cells, tissues, or organs; fast clearance by the immune system; poor disease recognition capabilities; and low accumulation at disease sites, all of which hinder their clinical development.
To address these limitations, increasing efforts have been made to create biomimetic micro/nanorobots by incorporating natural cell entities into artificial miniaturized devices.][322][323][324][325] Very recently, applying cell membranes derived from living cells for the surface functionalization of micro/nanorobots (Type 1 CCT) or directly employing living cells for phagocytose micro/nanorobots (Type 2 CCT) has allowed micro/nanorobots to not only inherit the surface membrane structures, biological substances (e.g., antigens, functional proteins, and receptors), and functions (e.g., chemotaxis, secretion, and immunoregulation) of source cells, but also to preserve the intrinsic properties of synthetic micro/nanorobots, such as propulsion by an external field and light-induced drug release. [326]For instance, neutrophil-camouflaged microrobots, inheriting the chemotaxis capability of native neutrophils, have been used for chemotaxis-guided targeted drug transport. [327]Waterpropelled Janus microrobots cloaked with RBC membranes can act as decoys to attract, capture, and neutralize biological protein toxins and nerve agent simulants in biological fluids. [328]Most importantly, CCTs can effectively protect artificial micro/nanorobots from phagocyte uptake and systemic clearance.Table 8 summarizes the classifications of cytomembrane-cloaked active micro/nanorobots.
To date, the preparation of micro/nanorobot CCTs has taken two typical routes, as shown in Figure 6.Via the first route, biomimetic micro/nanorobots are prepared by fusing cell membranes derived from living cells (such as blood cells and cancer cells) with synthetic micro/nanorobots.This route corresponds to a Type 1 CCT (cytomembrane camouflaging).The geometric shapes of the fabricated cytomembrane-cloaked micro/nanorobots are mainly determined by the shapes (e.g., tubular, Janus, and irregular architecture) of the applied synthetic micro/nanorobots.Via the second route, biohybrid micro/nanorobots are fabricated though the phagocytic function of phagocytes, such as monocytes, macrophages, and neutrophils, to phagocytose synthetic micro/nanorobots.This route corresponds to a Type 2 CCT (living cell camouflaging).Hence, the geometric shapes of the created phagocyte-based micro/nanorobots (i.e., cellularized micro/nanorobots) are governed by host phagocytes.Regardless of which fabrication technique is adopted, both the cytomembrane-cloaked micro/nanorobots and the cellularized micro/nanorobots are likely to inherit the bioactive substances and biological functions of the outermost cell membranes or living cells, facilitating the prolonged circulation lifetimes of biomimetic micro/nanorobots in physiological environments by minimizing undesired immunogenic responses.Most importantly, the presence of cell membranes or living cells can endow micro/nanorobots with disease targeting ability through intrinsic chemotaxis and targeted drug delivery and release capacity with the assistance of external fields (such as a magnetic field and an ultrasonic field).The following sections introduce representative examples of cytomembrane-camouflaged micro/nanorobots that use different kinds of cell membranes as stealth cloaks and cellularized micro/nanorobots that use living phagocytes as Trojan horses, together with their typical applications.

Active micro/nanorobots cloaked with living cell-derived cytomembranes
In this section, we discuss cytomembrane-camouflaged micro/nanorobots fabricated via the first route (Type 1 CCT).To date, blood cells (e.g., erythrocytes, leukocytes, and platelets) and cancer cells have been used as the main source cells for the creation of bioinspired micro/nanorobots.

Erythrocyte membrane-cloaked active micro/nanorobots
Blood is a vital body fluid in humans and many other creatures, and it contains blood cells, plasma, proteins, glucose, metal ions, and other biomolecules.Erythrocytes (or RBCs), leukocytes (or WBCs), and platelets (or thrombocytes) are all blood cells.Erythrocytes, the most abundant cell component in the blood, have attracted great attention for the construction of various critical biomimetic systems. [135,136,349,161]Thanks to the absence of nuclei and organelles in mature erythrocytes, the erythrocyte membrane is readily extracted, making it a perfect coating material for micro/nanorobots.To the best of our knowledge, Joseph Wang's group first attempted to use RBC membrane coating technology to construct biomimetic biohybrid micro/nanorobots. [330]An ultrasounddriven microrobot was developed by fusing erythrocyte membrane-derived vesicles (diameter 50-100 nm) onto the surface of citrate-modified gold nanowire (Figure 7A).The high surface tension of vesicles and electrostatic repulsion Citric acid-coated superparamagnetic NPs and DOX-loaded thermosensitive nanoliposomes Anticancer treatment, targeted drug delivery [338]   Poly(ethylene glycol) diacrylate (PEGDA) microhelix coated with nickel and gold nanofilms Targeted immunotherapy [339]   Neutrophil Paclitaxel-loaded magnetic nanogels Targeted drug delivery and antitumor therapy [128]   MSNPs Targeted drug transport [327]   Platelet Asymmetrically immobilized urease on surface Targeted active drug delivery [340]   Sperm Conical Ti-Fe microtube Micromanipulation, targeted drug delivery [341]   CdSe/ZnS quantum dots, DOX-coated iron oxide NPs, fluorescein isothiocyanate-modified Pt NPs Responsive payload release [342]   Magnetic tubular microstructure (i.e., "tetrapod") Targeted drug delivery, tumor targeting, and treatment [126]   Streamlined-horned microcaps Targeted heparin delivery, anticoagulation therapy [113]   Microalgae PLGA NPs Targeted drug delivery in a harsh acidic environment [343]   Ciprofloxacin-loaded, neutrophil membrane-coated PLGA NPs Active delivery of antibiotics, infection treatment [130]   Chitosan-coated iron oxide NPs Active, on-demand drug delivery, targeted tumor therapy [344]   Escherichia coli bacterium DOX-loaded polyelectrolyte multilayer microparticles embedded with magnetic NPs Targeted active drug delivery, anticancer therapy [345]   Microemulsions Targeted drug delivery [346]   Magnetotactic bacterium

Ciprofloxacin-loaded mesoporous silica microtubes
Antibiotic delivery, biofilm disruption, infection therapy [347]   Nanoliposomes Targeted drug delivery and cancer therapy [348]  F I G U R E 6 Schematic diagram of two representative fabrication approaches for micro/nanorobot CCTs.
between the vesicles and citrate-modified gold nanowire contributed to the high coverage and effective fusion of RBC vesicles onto the nanowire.Biomimetic microrobots propelled by ultrasound fields can function as decoys to efficiently absorb "on-the-fly" membrane-damaging toxins, since the vigorous ultrasound-driven movement of erythrocyte membrane-camouflaged microrobots can boost toxin neutralization dynamics due to the enhanced interaction between the toxin and microrobots.The same researchers also constructed RBC membrane-cloaked Janus microrobots by coating gold NPs, alginate, and erythrocyte membranes onto the exposed surface of Mg microparticles (diameter: 10-20 μm) in sequence (Figure 7B). [328]In contrast to that of bare Janus microrobots (165 μm s −1 ), the speed of the RBC membrane-clocked Janus microrobots (172 μm s −1 ) was almost unaffected by the introduction of cytomembranes.The hydrogen bubbles generated by the chemical reaction between Mg and water provided a robust driving force for the efficient stochastic locomotion of erythrocyte membranecamouflaged Janus microrobots.The movement directions and paths could easily be controlled when negatively charged Fe 3 O 4 NPs were assembled into membrane-cloaked microrobots via electrostatic attraction.This biohybrid Janus microrobot exhibited efficient detoxification ability to remove membrane-damaging toxins (i.e., α-toxin) and chemical warfare agents (i.e., methyl-paraoxon).Photodynamic therapy has been widely used to kill cancer cells by producing ROS from photosensitizers.Nevertheless, its therapeutic efficacy remains limited by tumor hypoxia and poorly targeted photosensitizer accumulation.To tackle this challenge, Qiang He's group developed an ultrasonically propelled RBC-mimicking (RBCM) micromotor that could actively transport oxygen and a photosensitizer to enhance PDT efficacy, as shown in Figure 7C. [331]This RBCM micromotor comprised biconcave RBC-shaped native RBC membrane shells and magnetic hemoglobin cores (encapsulating indocyanine green).Propelled by an ultrasound field, it could move through biological media, such as PBS, serum, and blood, at a speed of 56.5 μm s −1 (28.2 body lengths/s −1 ), and its locomotion direction could be controlled by an external magnetic field.Such RBCM micromotors not only avert serum contamination during their motion toward targeted HeLa cells, but also exhibit significant oxygenand photosensitizer-carrying capability.Taken together, such fuel-free RBCM micromotors are expected to provide an active, motion-based approach for efficiently and rapidly delivering oxygen and photosensitizers for future PDT.
In addition to their application for effective biodetoxification, erythrocyte membrane-camouflaged micro/nanorobots have been used for targeted cancer therapy, thrombus treatment, and image-guided therapy. [329,331,350]Gao et al. [331] prepared a microrobot by coating an RBC membrane onto the surface of magnetic RBC-shaped hemoglobin cores loaded with photosensitizers.The movement of the microrobot was powered by an acoustic field, and its direction toward the target cancer cells was governed by a magnetic field.This  [330] Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.(B) SEM image (top) and fluorescence image (bottom, dyed with Rhodamine B) of a Janus RBC-Mg micromotor.Reproduced with permission. [328]Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.(C) RBCM micromotors for cancer therapy: (a) mechanism of photodynamic therapy with RBCM micromotors, (b) ultrasonically driven "on/off" motion of RBCM micromotors in serum, (c) combined motion navigation of RBCM micromotors in serum using external magnetic and ultrasound fields; (d) motion trajectories of RBCM micromotors in different fluids (PBS, serum, and blood), and (e) their corresponding velocities in different fluids (scale bar: 50 μm).Reproduced with permission. [331]Copyright 2019, American Chemical Society.erythrocyte membrane-coated microrobot exhibited an excellent ability to carry oxygen and photosensitizers for efficient photodynamic tumor therapy.Shao et al. [329] constructed a Janus microrobot using an RBC membrane to cloak the surface of a chitosan/heparin microcapsule partially covered by a gold layer.Such an asymmetrical element distribution of gold can cause a local temperature gradient when exposed to near-infrared (NIR) light.As a result, the movement of the erythrocyte membrane-camouflaged Janus microrobot in the biofluids (e.g., serum and blood) was controlled by NIR laser irradiation through the self-thermophoresis effect.By exploiting the photothermal effect of gold based on its strong surface plasmon resonance adsorption performance, RBC membrane-camouflaged Janus microrobots have successfully ablated thrombi upon NIR irradiation.Very recently, multifunctional magnetic RBC membrane-camouflaged microrobots were fabricated by Wu et al. [330] The encapsulation of Fe 3 O 4 NPs endowed the biomimetic microrobots with the feature of magnetic response for both magnetic actuation and MRI.The artificial erythrocyte membrane-camouflaged microrobots exhibited excellent experimental biocompatibility, biodegradability, magnetically driven motion behavior, and image-guided tumor therapy ability.

4.2.2
Leukocyte membrane-cloaked active micro/nanorobots Leukocytes, possessing nuclei and mainly derived from hematopoietic stem cells, are the immune system cells that guard the body against infectious diseases, cancerous cells, and foreign invaders.According to their shape and granularity, leukocytes are mainly classified into five groups: neutrophils, lymphocytes, eosinophils, basophils, and monocytes/macrophages.The advantages of using leukocyte membranes as a coating material lie in their ability to prolong the circulation time of micro/nanorobots in biological fluids, recognize cancer cells, and then accumulate at tumor sites, which provides a smart platform for cancer diagnosis and therapy. [92,351]For instance, Wang et al. [352] fabricated a leukocyte membrane-camouflaged liquid metal nanorobot using a pressure-filter-template method and cytomembranecloaking technique (Figure 8).The autonomous locomotion behaviors of rod-like cell membrane-coated nanorobots are driven by an ultrasonic field, and their velocity and direction can be governed by the frequency and voltage of the applied ultrasound.Compared with bare gallium nanorobots, leukocyte membrane-coated nanorobots exhibit better F I G U R E 8 A representative leukocyte membrane-camouflaged gallium nanorobot actively recognizes, targets, penetrates, and kills cancer cells when propelled by an ultrasonic field.Reproduced with permission. [352]Copyright 2020, Daolin Wang et al.Exclusive licensee Science and Technology Review Publishing House.Distributed under a Creative Commons Attribution License (CC BY 4.0).locomotion velocity and time in a biological medium (e.g., blood and serum), indicating a fascinating antibiofouling capacity.Most importantly, biomimetic nanorobots can recognize cancer cells.Combined with their strong absorption in the NIR region, drug loading ability, and pH-responsive drug release characteristics, leukocyte membrane-coated nanorobots can actively target, penetrate, and be internalized by malignant cells when propelled by an external ultrasonic field and finally kill tumor cells through synergistic photothermal and chemical therapy (Figure 8).

4.2.3
Platelet membrane-cloaked active micro/nanorobots Platelets are a type of akaryote generated from megakaryocyte cytosol in bone marrow hematopoietic tissue.Based on a CCT, cloaked micro/nanorobots possess platelet-mimicking characteristics, such as immune evasion capability (i.e., immunocompatibility), selective adhesion to damaged vasculatures and tumors, and binding to platelet-adhering pathogens. [134,137]Such an active targeting ability can be used to position the inflammation and tumor sites. [340]Joseph Wang's group [353] developed a dual-cytomembrane-cloaked nanorobot (RBC-PL-robot) by fusing a hybrid erythrocyte and platelet membrane onto a gold nanowire (Figure 9A).Such hybrid nanorobots propelled by ultrasound not only selectively adhere and bind to Staphylococcus aureus bacteria due to the presence of platelet membranes, but also neutralize erythrocyte-targeted pore-forming toxins (e.g., α-toxin).Wan et al. [333] fabricated a platelet-derived porous nanomotor, which could identify thrombus sites, transport thrombolytic and anticoagulant drugs to the targeted sites under nearinfrared illumination via the thermophoresis mechanism, and finally, release therapeutic agents through the rupturing of the platelet membrane induced by NIR irradiation.As shown in Figure 9B, the light-driven nanorobot comprised mesoporous/macroporous silica (MMS), platinum NPs, and platelet membranes.The large thrombolytic drug urokinase (width: approx. 2 nm, length: 30 nm) can be encapsulated by the macroporous structure of silica, while the anticoagulant drug heparin (width: 1 nm, length: several nanometers) can be loaded into the mesoporous architecture of silica.Such a deliberate design for platelet membrane-camouflaged nanorobots can achieve a fast release of UK (3 h) to dissolve blood clots in the early stage of thrombus treatment and a slow release of heparin (>20 days) to prevent the long-term recurrence of thrombi by regulating NIR-induced membrane rupture.

Cancer cell membrane-cloaked active micro/nanorobots
Compared with the blood cells mentioned previously, cancer cells can bypass immune surveillance and play a vital role in targeting homotypic tumor tissues due to the presence of specific receptors and proteins on their surfaces. [266,354,355]ence, the use of cancer cell membranes as biological stealth coatings can extend the circulation time of micro/nanorobots in the human body by allowing them to escape immune clearance and endowing them with homologous targeting ability for precise tumor treatment.He and coworkers [334] developed a cancer cell membrane-camouflaged core-shell (CaCO 3 -Au) structured microrobot, the locomotion of which was powered by an external ultrasonic field.Due to inheriting the biological characteristics of cancer cells, biomimetic microrobots can actively target tumor sites through the so-called  7) PL-robots.Reproduced with permission. [353]Copyright 2018, the Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science.(B) A representative nanorobot camouflaged with platelet membrane for thrombus targeting and thrombolytic therapy.MS, mesoporous silica; MMS, mesoporous/macroporous silica; MMNM, mesoporous/macroporous silica (MMS)/platinum nanorobot; MMNM/Hep/UK nanorobot, MMNM nanorobot loaded with Hep and urokinase drugs; MMNM/Hep/UK/PM, MMNM/Hep/UK nanorobot coated with platelet membrane (scale bar: 100 nm).Reproduced with permission. [333]Copyright 2020, the Authors.
"homotypic binding effect" and function as an inactivated antigen to modulate immune activity, as confirmed by in vivo animal experiments. [334]

Fused cytomembrane-cloaked active micro/nanorobots
Researchers have mainly used a single type of cell membrane to cloak micro/nanorobots and to endow them with the specific biological functions of source cells, leading to limited functions.Integrating different membrane functions from multiple types of cells into a single motile micro/nanorobot may lead to wider and more powerful uses of micro/nanorobots to carry out multiple complicated medical tasks in a single therapy.For instance, an ultrasoundpropelled biomimetic nanorobot has been developed by cloaking gold nanowires with a hybrid cytomembrane derived from human RBC and platelets. [353]The hybrid cytomembrane inherits various functional proteins from the parent cells and provides the nanorobot with many biological functionalities, such as attaching and binding to plateletadhering bacteria and neutralizing pore-forming toxins.Such biomimetic nanorobots exhibit fast, prolonged ultrasound propulsion in whole blood without significant biofouling.Collectively, these hybrid cytomembrane-cloaked nanorobots have demonstrated their favorable capacity to efficiently isolate and concurrent remove pathogens and toxins.

Active microrobots cloaked with living cells
For cytomembrane-based micro/nanorobots, synthetic NPs (e.g., PLGA NPs) or drug molecules (e.g., DOX) can be loaded onto the cytomembranes or into the NPs; living cell-based microrobots can also load these cargos into the intracellular spaces of living cells.Due to the presence of cytomembrane-coated synthetic NPs, cytomembrane-based micro/nanorobots can facilitate their active interactions with specific cells through the ligands and receptors on cytomembranes.In addition to membrane ligand-receptor interactions, living cell-based microrobots can take full advantage of the intrinsic biological functions of the used living cells, such as secretion of biologically active substances (e.g., growth factors, cytokines, and chemokines), chemotaxis, and cell homing ability.It is envisioned that living cell-based microrobots are expected to bridge the gap between the biological world and synthetic micro/nanorobots.These biohybrid microrobots can exploit the natural locomotion of living cells as propulsive forces.In addition to serving as bio-engines, such mobile living cells can act as transporters of therapeutic cargos.Taken together, the exploitation of the unique characteristics of living cells to generate controllable movement in artificial micromachines represents an emerging research direction in the micro/nanorobotics field; their coupling has led to cell-based, motile micromachines with largely reinforced functionalities and performances.
Although living erythrocytes [335,356] and platelets [340] have been engineered into biohybrid microrobots, this section mainly focuses on discussing the development of immune cell-based microrobots.As its fundamental feature, the human immune system can discriminate between self and nonself to attack and clear out invading viruses, bacteria, fungi, parasites, cellular debris, damaged, diseased, or senescent cells, and other foreign matter. [357,358]This innate immunity is the first line of defense against pathogen exposure, implemented by phagocytes, including macrophages, DCs, NK cells, and granulocytes (basophils, eosinophils, neutrophils, and mast cells).Moreover, immune cells can secrete various cytokines (e.g., interleukins, interferons, the tumor necrosis factor superfamily, colony-stimulating factors, chemokines, and growth factors), acid hydrolases, neutral proteases, lysozymes, and antibodies.Taken together, immunocytes have many unique intrinsic properties that can allow synthetic micro/nanorobots to acquire functionalities for accomplishing complex medical tasks.Therefore, researchers have engineered synthetic nonliving micro/nanorobots with various immune cells to generate multiple living immunocyte-based microrobots (immunobots) with the natural properties of source immune cells.
In this section, we introduce the CCTs of micro/nanorobots fabricated via the second route, focusing on the use of phagocytes.Living cell-based microrobots are created by the engulfment process of phagocytes (e.g., macrophages, monocytes, and neutrophils).

4.3.1
Macrophage-cloaked active microrobots Synthetic micro/nanorobots, which are alien to the host body, run the risk of being eliminated by the innate immune defense.Extensive nanomedicine research indicates that the surface properties of artificial micro/NPs (including micro/nanorobots) can be regulated to either circumvent recognition by immune cells and minimize unwanted immunogenic responses or even specifically boost immune responses.Yasa et al. [359,360] unveiled morphologydependent interactions between double-helical magnetic microrobots and immune cells.Metin Sitti's [361] group has developed zwitterionic nonimmunogenic microrobots that can escape recognition by immunocytes and neutralization by phagocytosis (Figure 10A).Fabricated from fully zwitterionic photoresists using two-photon polymerization 3D printing, these hydrogel microrobots possess versatile functions, such as adjustable mechanical, antibiofouling, and nonimmunogenic properties; magnetic actuation capacity following functionalization; encapsulation of biomolecules into hydrogel matrices; and surface modification of microrobots for targeted drug delivery.Collectively, such versatile zwitterionic materials provide a toolbox for designing nonimmunogenic medical microrobots and eradicate a major roadblock to developing biocompatible microrobots.[364][365] For instance, a monocyte-camouflaged microrobot that uses chemotaxis-guided locomotion has been developed for tumor theragnosis. [336]][368][369] Han et al. [370] fabricated a macrophage-camouflaged microrobot for cancer treatment through the direct swallowing of Fe 3 O 4 NPs and anticancer drug (docetaxel-loaded) PLGA NPs by macrophages (using mouse macrophage cell line J774A.1).The inclusion of magnetic NPs allows macrophage-cloaked microrobots to be driven and navigated toward tumor sites by an external electromagnetic actuation system.Moreover, macrophagecloaked microrobots tend to accumulate around the tumor spheroid because of the intrinsic tumor infiltration characteristics of macrophages.Hence, macrophage-based microrobots can be used as efficient DDSs to actively transport therapeutic agents for targeted disease therapy.Recently, Nguyen et al. [338] manufactured a macrophage-camouflaged microrobot as an active DDS via the direct ingestion of citric acid-modified magnetic NPs and anticancer drug DOXloaded thermo-responsive nanoliposomes (LPs) by RAW 264.7 macrophages.As shown in Figure 10B, drug-loaded microrobots can be guided to specific disease sites by both the intrinsic ability of macrophages to target tumors (i.e., chemotaxis) and the magnetic response properties of CA-MNPs.In addition to tumor targeting, microrobots can infiltrate tumors when propelled by an external magnetic force, facilitating more precise and efficient cancer treatment.When exposed to NIR irradiation, photothermal-responsive microrobots can release DOX drugs and exosomes to kill tumor cells.

4.3.2
Neutrophil-cloaked active microrobots Neutrophils, as phagocytes, have also been employed to construct biomimetic biohybrid microrobots via phagocytosis.To the best of our knowledge, Qiang He's group first engineered neutrophil-camouflaged microrobots by coculturing neutrophils with bacterial membrane-cloaked MSNs loaded with anticancer drugs. [327]The introduction of the Esxherichia coli bacterial membrane enhanced the uptake of drug-loaded MSNs into neutrophils and prevented the leakage of drugs (especially water-soluble ones) from the MSNs.Inheriting the intrinsic chemotaxis capability of neutrophils, biohybrid microrobots can actively search for inflammatory sites, then release drugs for disease treatment.Using a similar design concept, Qiang He's group [128] also developed dualresponsive neutrophil-camouflaged microrobots through the engulfment of E. coli membrane-coated drug-encapsulated F I G U R E 1 0 (A) Nonimmunogenic stealth microrobots: (a) interaction of two representative microrobots with macrophages (after inspection, control microrobots are captured and swallowed by activated macrophages, while undetected stealth microrobots can escape from phagocytosis); (b) time-lapse tracking images demonstrating the inspection, capture, and phagocytosis of PEG microrobots (control); and (c) time-lapse tracking images demonstrating the inspection, release, and stealth process of a microrobot.Reproduced with permission. [361]Copyright 2020, the Authors.Published by Wiley-VCH GmbH.(B) A representative microrobot camouflaged with macrophage cells for effective cancer therapy via NIR-triggered drug release.Reproduced with permission. [338]opyright 2021, American Chemical Society.magnetic nanogels by neutrophils (Figure 11).The locomotion of biohybrid microrobots can be controlled using an external magnetic field and chemical gradients due to the existence of magnetic nanogels and neutrophil camouflaging.Consequently, such an immunocyte-based microrobot is capable of transporting therapeutic agents toward tumor sites (such as brain malignant glioma) and penetrating the BBB to achieve complete, efficient, and precise cancer therapy (Figure 11).This CCT provides a novel design concept for medical micro/nanorobots, which has great potential as a next-generation theranostic platform.

Active microrobots based on different cells
In addition to the aforementioned living cell-cloaked microrobots driven by external fields (or combined with taxi locomotion), biohybrid microrobots, which are created by integrating natural microorganisms that are capable of swimming, such as sperm, microalgae, and bacteria, with synthetic NPs, have received increasing attention in recent years as biofunctional microdevices with novel, reinforced capacities. [372]For example, sperm cells have been exploited to develop sperm-based microrobots and act as a driving force for effective locomotion. [341,373]These sperm-based microrobots are promising applications for targeted drug delivery. [113,126]One of the advantages of sperm-based microrobots is their adaptation to swimming in high-viscosity media, such as serum.Motile microalgae have also been integrated into biohybrid microrobots for on-demand delivery of drugs, such as antibiotics and DOX. [130,344,374,375]][377][378]

CONCLUSION AND FUTURE OUTLOOK
This review provides a summary of advances in the development of cytomembrane-camouflaged micro/nanorobots using cell membranes or living phagocytes as functional biological F I G U R E 1 1 Illustration of neutrophil-camouflaged microrobots capable of transporting therapeutic drugs across the BBB to treat malignant gliomas.Reproduced with permission. [371]Copyright 2021, the Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science.stealth coatings to bridge the gap between cytomembranecamouflaged micro/nanorobots and cytomembrane-cloaked nanomaterials.For practical clinical applications, medical micro/nanorobots must overcome many biological barriers (including but not limited to the endothelium, the BBB, and mucous membranes) before reaching the assigned sites to carry out specific tasks (e.g., tumor killing or thrombus removal).On their journey toward the target areas, these mission-focused micro/nanorobots must be "invisible" in the physiological environment to prevent them being detected as threats by the human immune system.The presence of outermost cell membranes with complete biological substances allows micro/nanorobots to escape immune clearance and provides new characteristics (e.g., chemotaxis for disease identification and targeting). [379]Despite their great biomedical application potential, such as for cancer immunotherapy, infection targeting and treatment, inflammation resolution, tissue regeneration, and detoxification, the development of cytomembrane-cloaked micro/nanorobots is still in its infancy, and many challenges lie ahead.Herein, we recommend that researchers conduct investigations to address the following issues: (1) the rational design of cytomembrane-camouflaged micro/nanorobots that possess controlled locomotion capacity, predefined therapeutic or diagnostic functionality, and immune evasion ability; (2) the actuation and navigation of medical micro/nanorobots in complex biofluids and physiological environments, such as blood flow, gastrointestinal tract, abdominal cavity, female reproductive system, brain, and skin; and (3) the interaction of cytomembrane-camouflaged micro/nanorobots with target cells/tissues or even immune system cells.

Design and fabrication of cytomembrane-cloaked micro/nanorobots
Nowadays, various fabrication techniques are used to manufacture functional micro/nanorobots, such as 3D printing (additive manufacturing), [319,380,381] material assembly, [382] capillary micromolding, [383] biohybrid engineering, [374,376,384] template electrodeposition, [385][386][387] electroless plating, [388] photolithography, [389] soft lithography, [390] strain engineering, [391,392] microfabrication, [393] electron beam deposition, [82,394,395] and physical vapor deposition. [396]Compared with other manufacturing techniques, 3D printing has emerged as a powerful method of fabricating sophisticated micro/nanorobots because of its prominent advantages: [319,[397][398][399] (1) The 3D printing method is highly flexible and less wasteful of raw materials and processes; therefore, it can save time and costs and is suitable for the high-throughput manufacturing of 3D objects with predefined properties and functionalities.(2) Supported by CAD/CAM, 3D printing facilitates the hierarchical inclusion of finer substructures to create complex or compound micro/nanorobots.(3) 3D printing can facilitate integrated design of the geometric structures, chemical compositions, physiochemical properties, and biological functions of micro/nanorobots in a digitally predefined manner.(4) The 3D printing technique facilitates the precise design and creation of functional customized micro/nanorobots to meet patients' individual requirements, thereby having great potential for personalized, minimally invasive medicine.To unlock the potential of 3D printing for fabricating cytomembrane-camouflaged micro/nanorobots, three design approaches should be considered: (1) New inks formulated with synthetic materials and cell membranes for 3D printing of cytomembrane-camouflaged micro/nanorobots, (2) 3D-printed micro/nanodevices coated with cell membranes to produce cytomembrane-camouflaged micro/nanorobots, and (3) 3D-printed micro/nanodevices encapsulated into living cells to generate cytomembranecamouflaged micro/nanorobots.Material design plays an important role in endowing medical micro/nanorobots with versatile functionalities. [120,400]For example, stimuliresponsive materials can be used to develop functional micro/nanorobots that will respond to various external forces (e.g., magnetism, ultrasound, light, electricity, and/or heat) or internal microenvironmental signals (e.g., pH, ionic strength, cytokines, chemokines, and/or infection). [318,401]

Clinical translation of cytomembrane-cloaked micro/nanorobots
Despite the advances researchers have achieved in the development of micro/nanorobots for biomedicine, the efficacy of current micro/nanorobots for conducting predefined medical tasks in vivo remains extremely limited, and their clinical adaptation and application are still in their infancy.Studies have focused on preclinical research, and no clinical trials have yet been carried out on cytomembrane-cloaked micro/nanorobots.Therefore, more efforts should be made to conduct clinical trials in the future and to explore the potential of cytomembrane camouflaging for diverse applications.
Because cell membranes and living cells are integrated into cytomembrane-cloaked micro/nanorobots, long-term storage and stability is a key factor in determining their therapeutic efficacy against diseases (such as cancer, infection, and inflammation).Therefore, appropriate storage measures and instruments need to be developed in future research.Likewise, transportation is an issue to be tackled from the perspective of the storage, stability, and treatment efficacy of cytomembrane-cloaked micro/nanorobots.Additionally, the costs of production and storage should be considered and controlled.Although various current fabrication methods can be exploited to manufacture cytomembranecloaked micro/nanorobots, more work is required to achieve their mass production in useful quantities to meet clinical needs; meanwhile, strict quality standards must be consistently set and met to guarantee the safety and effectiveness of cytomembrane-cloaked micro/nanorobots.Since many metallic, inorganic, and polymeric NPs have reached the late-phase clinical trial stage or have been approved for clinical use, [435] a major focus should now be on acquisition of and camouflaging with cell membranes and living cells to enhance the manufacturability of cytomembrane-cloaked micro/nanorobots.
Among the issues to be tackled are controlled actuation and navigation in complicated biological fluids, real-time imaging and tracking in vivo, fully biocompatible material designs, evasion of immune clearance, and regulation of the immune microenvironment.Regarding the circulation time of medical micro-and nanorobots, cytomembrane camouflaging is an effective strategy for overcoming immune-related barriers (especially phagocytosis and clearance by phagocytes) and prolonging their circulation in vivo.Furthermore, with rational design, these cytomembranecamouflaged micro/nanorobots can cooperate with immune molecules, immunocytes, and the immune system to constitute promising immunomodulatory applications in biomedical fields, such as osteoimmunology, cancer immunotherapy, infection immunotherapy, and inflammation regulation and resolution.
To date, research on medical micro/nanorobots in vivo remains limited to several specific regions of the body, including the gastrointestinal tract, [436][437][438][439] abdominal cavity, [440] bladder, [72] brain, [128] lungs, [130] and skin. [429,441]The target sites that medical micro/nanorobots can reach should be extensively investigated.Moreover, among the current studies, detailed, long-term immune observations and investigations of local tissues and organs, or the whole body, are required, especially in the dynamic physiological environment.Overall, the elucidation of general interactions between the material design of medical micro/nanorobots and their immunological properties will significantly support the development of novel medical micro/nanorobots with minimal host toxicity and optimal immune responses.To this end, cytomembrane-camouflaged micro/nanorobots can serve as an efficient research model for exploring and answering relevant challenging questions.Moreover, for further bench-to-bedside adaptation for clinical applications, longlasting therapeutic efficacies and immune reactions should be surveilled in animal models beyond the commonly used mouse models.
In preclinical and clinical studies, several key tests should be carried out to evaluate the comprehensive performance of cytomembrane-cloaked micro/nanorobots, including but not limited to: (1) the locomotion performance of cytomembranecloaked micro/nanorobots in vivo; (2) their real-time imaging and tracking in vivo; (3) their systemic distribution within tumor tissues, infected tissues, inflamed tissues, and diseased organs in vivo; (4) their biodegradability and safety in vivo; and (5) their pharmacokinetics in vivo.It is worth mentioning that the principles underpinning conventional DDSs can provide valuable guidance for the evaluation of cytomembrane-cloaked micro/nanorobots in preclinical and clinical studies.Although there is a long way to go before adapting these state-of-the-art platforms for clinical application, such biomimetic micro/nanorobotics, integrating cytomembrane-based biostealth techniques, controlled locomotion, imaging-assisted navigation, disease targeting, and active drug delivery are extremely promising for the development of next-generation theragnostic platforms at the micron and nanometer scale.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflicts of interest.

F I G U R E 4
Representative nanomaterials cloaked with cancer cell membranes.(A) Cancer cell membrane-coated adjuvant NPs for effective anticancer vaccination: (a) fabrication process for producing a nanovaccine by coating immune-adjuvant PLGA NPs with cancer cell membranes; (b) TEM image of NP@M NPs; (c) confocal laser scanning microscopy images showing the cellular uptake of nanovaccine by BMDCs; and (d) an ex vivo fluorescence image of isolated lymph nodes taken at 6 h after intradermally injecting NPs, demonstrating lymph node retention of nanovaccines.Reproduced with permission.

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I G U R E 7 (A) RBC membrane-cloaked nanomotors: (a) Schematic diagram of the fabrication of ultrasound-propelled nanomotors by coating gold nanowire with RBC membranes; (b) SEM image of a prepared nanomotor; and (c) fluorescent image of a nanomotor, the RBC membrane of which is dyed with Rhodamine B. Reproduced with permission.

A
C K N O W L E D G M E N T SJinhua Li and Huaijuan Zhou contributed equally to this work.J. Li sincerely acknowledges the financial support from the Beijing Institute of Technology Teli Young Fellow Program (3320012222218).H. Zhou thanks the financial support from the Beijing Institute of Technology Research Fund Program for Young Scholars (1750023022215).C. Liu acknowledges support from the National Natural Science Foundation of China (32101062), Guangdong Basic and Applied Basic Research Foundation (2019A1515110005, 2022A1515012607), and the Fundamental Research Funds for the Central Universities, Sun Yat-sen University.X. Zou acknowledges support from the National Natural Science Foundation of China (32071341).
Data of this paper are available by emailing lijinhua@bit.edu.cn.O R C I D Jinhua Li https://orcid.org/0000-0003-1110-7471Huaijuan Zhou https://orcid.org/0000-0002-9289-3613R E F E R E N C E S How to cite this article: J. Li, H. Zhou, C. Liu, S. Zhang, R. Du, Y. Deng, X. Zou, Aggregate 2023, 4, e359.https://doi.org/10.1002/agt2.359A U T H O R B I O G R A P H I E S Dr. Jinhua Li is a Professor at Beijing Institute of Technology (BIT), Fellow of Alexander von Humboldt Stiftung, Beijing Institute of Technology Teli Young Fellow, and Vebleo Fellow.He is Responsible Professor of Materials and Regenerative Medicine Engineering of BIT.He obtained his Ph.D. in Materials Science from the University of Chinese Academy of Sciences and was Awardee of Marie Skłodowska-Curie Individual Fellowship.The research interests of his group encompass micro/nanorobots, biomaterials, 3D printing, tissue engineering, regenerative medicine, and intelligent drug delivery.Dr. Yulin Deng is currently the Chief Professor of School of Life Science, Director of the Institute for Space Biology and Medical Engineering at the Beijing Institute of Technology (BIT), and member of the International Academy of Astronautics.In 1993, he had the honor to obtain a national scholarship from the Ministry of Education of Japan, and undertook a Ph.D. degree at Nagoya Institute of Technology in Japan.After his postdoctoral training, he has worked as a Senior Scientist in Neurology Research Unit of College of Medicine at University of Saskatchewan.He has been promoted to a Full Professor in Biomedical Engineering at BIT of China since 2002.He served as dean of the School of Life Sciences at BIT for 14 years.In 2015, he was nominated to be a member of International Academy of Astronautics.His research interests are on neuroscience, as well as space life sciences and their payload technology.As PI, he has done for a number of national key research projects including the national major scientific instruments projects, key projects of the Natural Science Foundation of China.Dr. Xuenong Zou received his Ph.D. from Århus Universitet in 2003 and subsequently worked as a postdoctoral fellow and senior researcher at the Department of Orthopaedics E, Århus University Hospital in Denmark.Prof. Zou started his career as a professor and chief physician at The First affiliated hospital of Sun Yat-sen University in 2008.Currently, Prof. Zou served as the Director of the Orthopedic Research Institute and Guangdong Provincial Key Laboratory of Orthopedics and Traumatology.His research focuses on biomaterials, stem cell and tissue engineering, and degenerative bone and joint diseases.
TA B L E 1 A comparison of cytomembrane-camouflaged micro/nanorobots and conventional nanoparticle delivery systems.
tactic (CCT):(1)the exploitation of cytomembranes derived from living cells to cloak traditional passive nanomaterials or intelligent active micro/nanorobots, and (2) the direct use of living cells to phagocytose passive nanomaterials or active micro/nanorobots and produce cellularized micro/nanosystems.
Summary of the biomarkers, properties, and functions of different cell types.
TA B L E 2 Summary of representative examples of platelet membrane-camouflaged nanomaterials.
TA B L E 4 Summary of fused cytomembrane-camouflaged NPs for diverse biomedical applications.
TA B L E 6 Summary of the intrinsic properties of erythrocytes, leukocytes (monocytes, macrophages, eosinophils, neutrophils, basophils, lymphocytes, and DCs), and platelets in the human body.
TA B L E 7 Summary of cytomembrane-cloaked active micro/nanorobots.
TA B L E 8