Emerging Additive Manufacturing Methods for Wearable Sensors: Opportunities to Expand Access to Personalized Health Monitoring

Persistent disparities exist in access to state‐of‐the‐art healthcare disproportionately affecting underserved and vulnerable populations. Advances in wearable sensors enabled by additive manufacturing (AM) offer new opportunities to address such disparities and enhance equitable access advanced diagnostic technologies. Additive manufacturing provides a pathway to rapidly prototype bespoke, multifunctional wearable sensors thereby circumventing existing barriers to innovation for resource‐limited settings imposed by the need for specialized facilities, technical expertise, and capital‐intensive processes. This review examines recent progress in the additive manufacture of wearable platforms for physiological health monitoring. Supported by an initial overview of relevant techniques, representative examples of 3D printed wearable sensors highlight the potential for measuring clinically‐relevant biophysical and biochemical signals of interest. The review concludes with a discussion of the promise and utility of additive manufacturing for wearable sensors, emphasizing opportunities for expanding access to vital healthcare technology and addressing critical health disparities.


Introduction
[3][4] By placing rigorously evaluated empirical evidence above reliance upon DOI: 10.1002/adsr.2023001377][8][9][10] However, even with the undeniable progress in improving healthcare quality, persistent disparities in access to care underscore the necessity for continued efforts toward equitable healthcare distribution and universal access to state-of-the-art medical interventions across diverse populations and settings.
Healthcare inequity manifests in a multifaceted manner, encompassing areas such as advanced technology, vital health information, and medical expertise.While progress in healthcare policy and expanding insurance access have improved overall access to care, such measures are insufficient to bridge the gap between urban centers and remote or rural locations, especially for underserved and vulnerable populations, such as those based on race/ethnicity, gender and sexual orientation, and individuals with disabilities.The ongoing COVID-19 pandemic serves as a poignant reminder that the quality of available medical interventions is intrinsically linked to health outcomes, and the lack of access to best-in-class medical technology profoundly affects the well-being of vulnerable communities. [11][17][18][19] Conventional clinical approaches to healthcare monitoring predominantly rely upon episodic data collection in controlled environments using cumbersome, high-cost monitoring devices. [20]Such methods inherently exhibit limitations in capturing a continuous and comprehensive portrait of the health status of an individual.26][27] However, access to these devices remains a considerable barrier for low-resource settings, such as remote and rural locations, due to capital-intensive fabrication methods and the need for extensive training for personnel.The necessity for specialized tools to fabricate new sensing materials, device form factors, and general sensing platforms constrains place-specific innovation. [28,29]rototyping these platforms requires complex, time-consuming, and expensive fabrication methods, specialized facilities (e.g., cleanroom), and highly-skilled technicians. [30]Low-cost fabrication methods (e.g., screen printing) for flexible electronics offer strategies to reduce costs but present additional challenges on account of resolution, materials libraries, and often inferior performance in comparison to established electronic fabrication methods. [31]Broadening equitable access to both best-in-class medical care and the tools for innovation requires the advent of advanced manufacturing techniques amenable to smaller prototyping methods.
Additive manufacturing (AM), commonly referred to as 3D printing, represents a groundbreaking manufacturing approach that provides unparalleled opportunities for generating bespoke physical objects characterized by precisely tailored geometries, properties, and functionalities uniquely suited to a given application. [32,33]AM describes a process by which digital models created using computer-aided design (CAD) software are transformed into physical objects by a 3D printer that constructs a series of 2D cross-sections in a layer-by-layer manner. [34]AM has emerged as a particularly promising solution to address existing barriers to prototyping wearable platforms and democratize access to state-of-the-art healthcare technology. [35,36]The potential of AM technology lies in the capacity for rapidly producing intricate, customizable structures from an expansive library of materials, thereby streamlining the prototyping process to expedite the development of application-specific wearable sensors. [37]AM also reduces the reliance on specialized facilities and skilled technicians thereby lowering the barriers of entry for developing and producing wearable platforms. [38]Democratizing wearable sensor prototyping and manufacturing processes through AM not only fosters innovation by enabling the creation of platforms to address location-specific needs but also offers a pathway toward broader access to advanced healthcare technology. [39]his review explores the latest advancements in the additive manufacture of wearable sensing platforms, an emerging application area undergoing rapid, continuous technological progression across printing methods, materials chemistry, and potential applications.Drawing parallels with the transformational impact of 3D printing on lab-on-chip diagnostics, which revolutionized the field through low-cost prototyping and widespread innovation, we envision similar potential for wearable sensors.A wide range of recent reviews survey wearable sensor progress in terms of form factors, [40][41][42] sensing approaches, [43][44][45] fabrication methods, [46][47][48] material systems, [49][50][51] and applications, [4,52,53] while others center on additive manufacturing in the context of methods, [38] materials, [32] general applications, [35,36,54,55] and challenges. [56,57]By comparison, this review examines the relatively uncharted intersection of wearable sensors and 3D printing techniques, emphasizing sensors and devices primarily fabricated through additive manufacturing.Specifically, this manuscript contextualizes the current state, benefits, challenges, and opportunities within the field of additive manufacturing for enhancing access to diagnostic technologies through the fabrication of wearable sensing platforms.The introductory section highlights additive manufacturing processes relevant to wearable sensing platforms and the considerations for successfully applying each fabrication method.The sections that follow highlight representative examples of wearable sensing platforms fabricated via additive manufacturing categorized by the primary biophysical and biochemical signals of interest.The review concludes with a discussion of the overall promise and utility of AM for wearable sensor fabrication with a particular emphasis on the opportunities for expanding access to vital cutting-edge healthcare technology.

Overview of Additive Manufacturing Methods
Additive manufacturing broadly encompasses seven distinct printing methodologies: material extrusion, vat photopolymerization, material jetting, binder jetting, powder bed fusion, sheet lamination, and directed energy deposition. [55,58]Each of these processes occupies a specialized niche for specific applications, with material extrusion and vat photopolymerization favored for consumer-based rapid prototyping (e.g., consumer 3D printers).[61] Although AM holds transformative potential for the creation of wearable devices with unprecedented levels of design freedom and customization capabilities, at present only a select subset of these printing methods is compatible with the processing requirements of wearable sensors.
Selecting an appropriate additive manufacturing method necessitates a comprehensive evaluation of these factors to ensure process compatibility and performance optimization, given the inherent limitations in printable materials, printer resolution, and process parameters.Material advances address some of these challenges via the expanding library of printable materials suitable for constructing device components [89][90][91][92][93][94] (e.g., circuit traces, physical structure), such as the development of printable stretchable conductive inks [95] and biocompatible hydrogels. [96,97][100] Material extrusion, material jetting, and vat photopolymerization represent the predominant additive manufacturing technologies of interest for wearable platforms and support novel device architectures unattainable through conventional manufacturing.Moreover, these printing approaches offer distinct advantages and opportunities for prototyping advanced wearable devices in resource-constrained environments, which is critical for addressing fundamental challenges inherent in lab-at-home applications. [32,38]aterial extrusion is a versatile additive manufacturing technique deployed widely in most consumer-grade 3D printers.Here, material extruded through a nozzle in a layer-by-layer manner forms complex 3D architectures. [101]Within this category are two sub-classifications: fused deposition modeling (FDM) and direct ink writing (DIW).FDM employs heated thermoplastic filaments (primarily polylactic acid (PLA), polyethylene terephthalate (PET), and acrylonitrile butadiene styrene (ABS)), which solidify by cooling after deposition to form the desired net structure (Figure 1A). [39,102]Strengths of FDM printing include compatibility with various thermoplastics, cost-effectiveness, and ease of use; however, limitations arise from low print resolution and anisotropic mechanical properties. [59]Conversely, DIW leverages viscoelastic inks that solidify through mechanisms such as solvent evaporation or chemical cross-linking, which in turn enables finer printed resolutions, expanded material versatility, and the capacity for embedding functional materials.The primary appeal of utilizing DIW for wearable sensors resides in its capacity to print a diverse array of materials, provided they can form selfsupporting extruded layers that maintain shape fidelity.The capability spans polymers, conductive carbon, metals, biomaterials, and other custom inks. [103][105] Both FDM and DIW techniques have garnered substantial attention in wearable sensor development with recent reports demonstrating creation of customizable device architectures with integrated functional materials using minimal hardware requirements. [33,55,57,106]aterial jetting is a similarly attractive AM method for fabricating wearable sensors owing to the capability to simultaneously incorporate multiple materials during printing. [107]Here, it is crucial to distinguish material jetting from inkjet printing.Inkjet printing, while employing a similar jetting mechanism, does not constitute a 3D printing process.Rather the inkjet printing process typically coats a 2D planar substrate with a single layer of ink. [34]In contrast, material jetting (i.e., polyjet) describes a process by which droplets of photopolymer resin jet from a printhead and are subsequently cured in a layer-by-layer manner, resulting in a solid 3D object (Figure 1B). [108]This technique offers exceptional printed resolution, rapid printing times, and multi-material capabilities, rendering it well suited for complex structures. [109]However, material jetting is constrained by material property requirements to support deposition and requires extensive post-processing steps to yield a final product.
Aerosol jet printing (AJP) represents an emerging additive manufacturing technique akin to traditional material jetting yet with unique operating principles that are particularly attractive for wearable sensor fabrication. [110,111]Central to the AJP process is the transformation of liquid ink into aerosol droplets, which are then focused and deposited onto a substrate via a gas-assisted stream (Figure 1C). [112]This technique exhibits broad compatibility with a diverse array of inks, including nanoparticle-based and conductive formulations, enabling the creation of intricate patterns and fine features suitable for high-quality electronics. [113]JP is broadly deployed in the manufacture of printed hybrid electronics owing to low-temperature processing requirements and broad adaptability to various substrates. [114]However, AJP is constrained by extended printing times and complex hardware requirements, which hinder efficacy in specific manufacturing contexts.Nevertheless, AJP holds immense, albeit underexplored, potential for fabricating novel device architectures and enhancing the functionality of wearable platforms beyond traditional manufacturing approaches.
Vat photopolymerization describes a process by which controlled light exposure-predominantly within the ultraviolet spectrum-selectively cures photosensitive resins in a layerby-layer manner to form intricate structures characterized by high resolution and exceptional surface finish. [34]This AM process enables the creation of components with elaborate, detailed geometries that cannot be fabricated via alternative AM methodologies. [115]Vat polymerization resins employ a range of photosensitive polymers, including numerous biocompatible materials (e.g., N,N-dimethylaminobenzoic acid ethyl ester, poly(-caprolactone), poly(ethylene glycol)) that are aptly suited for wearable sensors. [34,116]The primary subcategories of vat photopolymerization include stereolithography (SLA), digital light processing (DLP), and continuous liquid interface production (CLIP). [117]Whereas SLA, the most established and perhaps the most well-known, utilizes laser illumination for selective resin curing through tracing, [115] DLP uses a digital projector to simultaneously expose an entire resin layer in (Figure 1D), thereby accelerating the printing process. [32]CLIP is a relatively recent entrant whereby an oxygen-permeable window facilitates a continuous printing process that accelerates printing speed while preserving resolution and optimal surface finish. [118]Vat photopolymerization processes present certain drawbacks that restrict broad adoption (as compared to FDM-based printers), particularly in relation to wearable devices.While the assortment of biocompatible resins has expanded, the availability of flexible resins appropriate for skin-interfaced wearable devices remains comparatively limited.Moreover, the mechanical attributes of cured photopolymer resins may not entirely fulfill the exigencies of wearable devices, and post-processing requirements can prove labor-intensive.However, vat photopolymerization offers compelling capabilities for fabricating the advanced, high-resolution structures required for many wearable sensing applications; particularly with regards to fluid-based systems (i.e., epidermal microfluidics [119] ).
The emergence and rapid maturation of these AM processes for producing hybrid/flexible electronic systems and objects with high-resolution, multiscale features (micro-to-macroscale) has sparked the beginnings of a paradigm shift in our conception of the limits of what is possible for wearable platforms.The op-portunities afforded by the rapid, on-demand, facile manufacture of intricate, sophisticated, and highly customized sensors eliminates traditional cost barriers to device innovation. [32,33,58,59]In the sections that follow, we highlight recent progress in deploying AM to fabricate wearable sensing platforms.We restrict the scope of our focus to exemplars that utilize AM techniques produce a finished, working component, such as a functional sensing module [73,74,76,86,96,120] or the structural/defining elements of a wearable platform [30,72,75] (e.g., microfluidic channels), in contrast to supporting (i.e., soft lithographic molds) or cosmetic elements [77,87] (e.g., nonactive wristbands/straps).

Additive Manufacture of Wearable Platforms for Biophysical Monitoring
Monitoring biophysical signals arising from the natural physiological processes of the body is a fundamental component of assessing the health and wellness status of an individual. [12,21,25,40,69]][123][124][125] However, current methodologies require trained personnel to use expensive, cumbersome equipment via wired interfaces that restrict patient movement to record these physiological signals.
[128] These platforms-spanning numerous form factors (e.g., smartwatches, [129] rings, [130] patches, [131] and bands/straps [132] )support a comfortable, user-friendly mode of data collection.Yet the applicability of these existing wearable technologies in a clinical setting remains restricted due to inherent limitations in measurement capabilities, accuracy, and reliability.Addressing the inherent measurement challenges of collecting the full spectrum of biophysical signals fundamental to critical care from a wearable platform requires the development of novel device architectures and form factors.Moreover, such efforts demand additional innovations in fabrication methods to support sustainability across the device lifecycle and facilitate the creation of bespoke, patient-specific device form factors required for personalized medical care.Recent efforts [32,64] to leverage advances in additive manufacturing to support sophisticated biophysical measurement capabilities in platforms relevant for clinical applications underscore the vast potential for innovation in addressing these critical areas.
Most instances of additively manufactured wearable sensors for biophysical monitoring utilize material extrusion-based printers, a selection largely driven by the technological maturity, broad library of printable materials, the relatively minimal hardware requirements of this class of AM processes.In simplest form, FDM printers can reduce the complexity of a sensor manufacturing process by enabling fabrication of a relatively simple yet architecturally intricate component of a device.[136] The use of a stretchable polyurethane Figure 2. A) 3D printed "earable" core body temperature device.Adapted with permission. [133]Copyright 2017, American Chemical Society.B) 3D printed wearable pressure sensor.Adapted with permission. [137]Copyright 2019, Wiley-VCH.C) 3D printed wearable tactile sensor.Adapted with permission. [106]opyright 2021, Wiley-VCH.D) Representative image of a patient specific 3D printed wearable pulse oximeter.Adapted with permission. [151]Copyright 2020, Wiley-VCH.E) 3D printed self-powered pressure sensor.Adapted with permission. [154]Copyright 2023, Elsevier B.V. F) 3D printed wearable strain sensor for human/machine interfacing.Adapted with permission. [162]Copyright 2023, Springer Nature.
filament to form the structural components of the device facilitates a comfortable and seamless fit within the ear canal, personalized to accommodate size and shape variations among patients.Additionally, liquid metal (Galinstan) microchannels were embedded in the device, rather than less compatible traditional metal wiring, to provide circuit functionality for a wireless modular communication system.In another example (Figure 2B), [137] a dual-nozzle FDM printing process enables fabrication of a wearable sensor for recording heel pressure.Here, a multi-nozzle configuration expands the capabilities of FDM printers to print intricate objects with high fidelity (via removable support materials) or cosmetic features (i.e., multiple colors).In this demonstration a water-soluble support material (PVA) serves as a sacrificial layer to facilitate creation of a microbump array (PLA) with integrated microchannels.Encapsulation of the printed device within a stretchable elastomer (Dragon Skin 10) prior to removal of the PVA by water infiltration forms sealed channels designed to contain a conductive liquid metal (Galinstan).The integration of the 3D printed microbump array enhances the performance of the pressure sensor in a manner comparable to other microbump arrays fabricated using more conventional cleanroom-based approaches. [138]Other reports detail the similar use of a dual nozzle printer to create a soft, compressible, mul-tilayer pressure sensor. [139]Here, PVA serves as a sacrificial scaffold for printed layers of a carbon black-infiltrated TPU filament that, when dissolved, enables the compression of the printed conductive layers.
Rapid innovation within the FDM printing ecosystem has broadened the portfolio of printable materials well beyond the common thermoplastics used in most applications.Recent efforts seek to enable printing soft, lightweight materials that are both non-toxic and biocompatible, which would further expand the opportunities of using FDM-based printing processes for fabricating wearable platforms.However, the growing ubiquity of FDM printing comes with an unfortunate corollary-a sharp increase in plastic waste.This issue can be viewed as a microcosm of a more expansive challenge within the wearable domain as many embodiments are single use.As a consequence, it is critical to evaluate the environmental footprint of a wearable platform throughout the device lifecycle to mitigate any potential ecological impact.In this context, it is of particular interest to utilize materials that are not just recyclable but also biodegradable within a reasonable timeframe and with negligible processing requirements.For example, a recent report [140] details a new dynamically covalent cross-linked thermoset elastomer for use with FDM printers.Demonstrations of using this material to fabricate wearable capacitive pressure sensors and other electronics underscores the potential such materials hold in this capacity.
These considerations-encompassing the necessity for nontoxic, biocompatible materials and solutions to mitigate environmental impact-highlight the continual need for material innovation.However, emerging materials often lack compatibility with a given printing technology.For example, gel-type biopolymers [141] such as hydrogels exhibit properties highly relevant for wearable platforms including a low Young's modulus, biocompatibility, and straightforward pathway for the integration of functional particles.As hydrogels lack compatibility with FDM printers research efforts must leverage DIW systems designed for viscous inks and external solidification.Recent work shows the use of a thermo-reversible material, termed eutectogel, that exhibits ionic-electronic conduction enables printing stretchable conformal electrodes for electrophysical signal monitoring. [142]Another report [143] details a conductive, self-healing hydrogel facilitates printing a self-adhesive wearable strain sensor for activity monitoring.The wide viscosity range accessible by DIW allows incorporating functional additives, such as nanomaterials, into inks to modify sensor performance. [144]In one demonstration, [145] DIW printing with a graphene nanoplatelet/PDMS ink enables fabrication of a skin-mounted resistive temperature sensor with a honeycomb structure that strain-isolates the active sensing element to reduce motion-induced noise artifacts.Additional sensor sophistication is possible through a multimaterial DIW process whereby layers of distinct materials are printed sequentially.One demonstration [106] (Figure 2C) leverages a layer stack comprising thin conductive layers and microstructured dielectric layers to enhance sensing performance of an all-printed tactile pressure sensor.Structured air voids introduced by printing the dielectric layers in a woodpile structure (via a rectilinear infill pattern) increase sensor compressibility and thus overall sensitivity.Resolution limitations remain a key challenge for DIWbased printers arising from materials deposition and form holding constraints. [146,147]Recent examples demonstrate the use of optimized ink formulations to obtain printed soft electrodes [148] and piezoresistive sensors [128] with feature sizes on the order of 50 m.Emerging DIW techniques, such as freeform reversible embedding of suspended hydrogels (FRESH), [149,150] can further enhance printer resolution or expand material compatibility.For example, one recent demonstration [151] (Figure 2D) utilizes FRESH-based printing to fabricate pulse oximeter cuffs from PDMS with 50 m feature resolution.
Alternatively, DIW has been employed for other components necessary to embody a fully integrated wearable system.For example, the creation of wearable piezoelectric [152,153] and triboelectric [154,155] nanogenerators (PENG and TENG) enables fabrication of self-powered wearable platforms to eliminate the requirement of a battery.A recent report details a power-efficient TENG that combines 3D printed MXene ink with DIW technology to produce a self-powered, battery-free, and wireless system that facilitates the direct transmission of pressure sensor data to a smartphone (Figure 2E). [154]Ongoing advances in printer capabilities via such techniques will continue expanding material extrusion methods for sensors and devices with personalized form factors.
Although material extrusion-based printing is inherently limited in resolution, vat photopolymerization methods excel in fabricating highly complex, high-resolution architectures ideal for on-body sensing. [156,157]Yet, the comparatively high modulus of resin-based objects has historically restricted conformal skin interfacing.Recent innovations in both materials and printing processes [158] offer strategies for addressing this inherent stiffness mismatch.For example, one recent work [159] demonstrates the rapid fabrication (<30 s) of a multifunctional movement sensor comprised of a 3D printed solid-state low-modulus conductive ionogel by a DLP-based process.In a similar manner, [160] optimization of a photocurable ionic liquid/acrylate resin formulation enables fabrication of a porous, flexible ionogel sensor tailored for human motion monitoring.Tuning the resin material properties and sensor geometry (i.e., lattice structure) increases sensor sensitivity while reducing hysteresis, allowing long-term acquisition of high-fidelity signals.The addition of functional materials to a resin (e.g., graphene) can impart new properties (i.e., conductivity); however, uniform dispersion is critical.Other work [161] fabricated electrophysiological sensors by leveraging DLP printing and a customized graphene-based resin that incorporated an amphiphilic dispersant reduce particle aggregation resulting in a material with enhanced conductivity and flexibility.Additional innovative approaches lie in the advancement of printing techniques themselves, as with the recently reported [162] single-vat single-cure grayscale DLP method that leverages modulated light intensity to dictate local transitions from soft, stretchable organogels to rigid thermosets within a single printed layer (Figure 2F).
While such advances in resin formulations, functional composites, and printing techniques expand the opportunities for using vat photopolymerization to fabricate wearable sensors, additional innovations in multimaterial printing and performance optimization are critical to fully realize the promise of this manufacturing approach.
Other additive manufacturing approaches demonstrate emerging potential for wearable physiological sensors.For example, powder bed 3D printing enables fabrication of scaffolds that can be injected with functional materials, such as SWCNTcoated elastomers, to produce inexpensive electrophysiological sensors. [125]This approach, in combination with high-resolution 3D body scans, enables a workflow to produce personalized, highly conformal sensors on demand.Material jetting is another emerging alternative technique that offers superior fabrication speed but deployment has been hindered by a limited library of printable materials suitable for wearable sensors.Recent work [163] highlights efforts to address these challenges demonstrating the fabrication of flexible piezoresistive sensors at 5x the speed of a similar extrusion-based process.Similarly, crosstalk between sensing nodes was eliminated in a wearable fingertip pressure sensors by leveraging the precise control and resolution of aerosol jet printing to improve spatial distribution. [164]While extrusion and vat-photopolymerization-based printing methods currently dominate the field, the rapid pace of innovation continues to expand the suite of available fabrication methods for producing wearable biophysical sensors with each offering distinct advantages in terms of resolution, customization, and material versatility.

Additive Manufacture of Wearable Platforms for Biochemical Monitoring
A comprehensive assessment of physiological health status requires measurement of a broad spectrum of biochemical analytes.These targets include nutrients and metabolitesencompassing vitamins, minerals, amino acids, glucose, and lactate-alongside pH, various electrolyte ions (specifically, sodium, potassium, calcium, and chloride ions), and diseaseassociated biomarkers (e.g., cytokines). [6,16,23,67,122,165]Bloodbased analysis represents the traditional pathway such biochemical monitoring; however, invasive sampling procedures, associated discomfort for the patient, and a requirement for sophisticated, high-cost equipment for analysis present considerable limitations for continuous health monitoring beyond specific, wellestablished use cases (e.g., diabetes).
Wearable sensing platforms offer significant promise for mitigating such challenges by utilizing a wide range of non-invasively sampled biofluids-including sweat, [12,26,30,[166][167][168] saliva, [169] tear, [170] and interstitial fluids [171] -to monitor biochemical markers indicative of health status.By potentially resolving the discomfort and inconvenience of traditional bloodbased analysis, these platforms offer a pathway for real-time, continuous, long-term biochemical health monitoring.As with biophysical sensors, advanced manufacturing techniques enabling rapid fabrication of customized, patient-specific devices would enable new opportunities within the context of developing personalized therapeutic interventions for an individual.When considering technologies within this context, additive manufacturing emerges as a powerful approach.Examples of fully 3D printed wearable devices incorporating all essential components of wearable chemical sensors remain rare, which is largely attributed to the complex nature of such devices.Successful efforts must leverage the unique strengths of various 3D printing techniques for fabricating individual components (e.g., microfluidic networks, [172][173][174] electrical circuits, [140,175,176] sensing modules [79,[177][178][179][180] ) and subsequently integrating these disparate elements into a single, unified platform.In the following discussion, we focus on exemplars of 3D printed wearable biochemical sensors with demonstrated on-body measurement capabilities to emphasize the rapidly increasing sophistication of such sensing platforms.
As with biophysical sensing platforms, material extrusionbased 3D printing offers a broadly accessible pathway for fabricating wearable biochemical sensors via additive manufacturing.Realizing such sensing platforms requires printing key functional components including substrates, electrodes, sensors, and fluid routing elements as an integrated system.For example, one recent report [130] (Figure 3A) details using a dual extrusion FDM printer to fabricate an electrochemical ring for noninvasive monitoring of sweat glucose concentration.Dual extrusion enables layer-by-layer printing of the flexible, nonconductive ring structure and carbon composite electrodes as a single monolithic unit.Other demonstrations include fabricating epidermal sweat platforms with integrated sensors [30,181] and finger-based electrochemical sensors for detecting sedation drugs in alcoholic beverages. [175]Leveraging emerging extrusion techniques (e.g., FRESH) offers additional design capabilities as demonstrated by the fabrication of a silk-based wearable sensing platform [182] Figure 3. A) 3D printed electrochemical ring (e-ring).Adapted with permission. [130]Copyright 2021, American Chemical Society.B) 3D printed wearable, multifunctional wound dressing.Adapted with permission. [183]Copyright 2017, Wiley-VCH.C) 3D printed wearable sweat lactate sensors.Adapted with permission. [166]Copyright 2022, Elsevier B.V. D) 3D printed epidermal microfluidic device for sweat analysis.Adapted with permission. [119]Copyright 2023, American Association for the Advancement of Science.
with an integrated microfluidic network via use of a sacrificial support structure.Another study utilized high-throughput DIW-based 3D printing technology to fabricate a multifunctional dressing that detects bacterial infection through printed colorimetric pH sensing fibers and releases antibiotic agents at wound sites to manage chronic and acute injuries caused by trauma, surgery, or diabetes (Figure 3B). [183]In aggregate, these demonstrations exemplify the potential for fabricating personalized, scalable biochemical sensing platforms in a straightforward, user-customizable manner via additive manufacturing.
While extrusion-based techniques offer readily accessible pathways to fabricate wearable biochemical sensors, vat photopolymerization methods (e.g., SLA, DLP) provide additional advantages in terms of resolution, feature size, and device sophistication.Most demonstrations utilize vat photopolymerization techniques to fabricate structural housings for sensing components. [184]One example [166] (Figure 3C) utilizes a highresolution SLA printing process to fabricate a case with integrated fluidic network to route sweat from the epidermis to sensing zones for the simultaneous monitoring of sweat rate and lactate concentration.By comparison, one recent work [119] (Figure 3D) highlights the potential for vat photopolymerization to create functional epidermal microfluidic platforms for sweat analysis.Here, high-resolution DLP printing enables fabrication of epifluidic devices with complex, 3D microfluidic network geometries and integrated colorimetric sensors.
Although these demonstrations highlight the limited number of examples of resin-printed noninvasive sensing platforms, there exists a substantial body of work [48,185,186] highlighting the utility of vat photopolymerization for fabricating minimally invasive microneedle-based sensing platforms.While a broader discussion of such minimally invasive sensors is beyond the scope of this work, microneedles require small, sharp geometries that are challenging to fabricate without a reliance upon cleanroom-based processes. [171]The use of high-resolution SLA printing enables fabrication of hollow microneedle arrays [187] that support collection and analysis of glucose and lactate in interstitial fluid (Figure 4A) or microneedle patches [186] that integrate with electrochemical sensors for the continuous monitoring of apomorphine levels for Parkinson's disease (Figure 4B).These examples highlight the transformative capabilities of vat photopolymerization, and additive manufacturing in general, for facilitating complex, integrated biochemical sensing platforms (Figure 4C). [188]ther additive manufacturing approaches, such as material jetting, demonstrate emerging potential for producing wearable biochemical sensors.[113] With few processing limitations beyond aerosolizability, AJP printable inks include all materials necessary for fabricating functional sensors (e.g., conductive, insulating, dielectric, and sensing materials) [189] as highlighted by a recent demonstration of a wearable electrochemical biosensor for continuous noninvasive monitoring of sweat lactate levels. [111]While these few examples highlight promising capabilities for manufacturing wearable biochemical sensors, continued improvements in resolution, scalability, and integration are required to enable practical manufacturing of sophisticated, miniaturized sensor systems in a manner similar to material extrusion or vat photopolymerization methods.

Pathway toward Fully 3D Printed Multimodal Wearable Platforms
Although a rapidly evolving field, there exist relatively few examples of additively manufactured integrated systems that support multimodal collection of biophysical and biochemical signals.Material extrusion techniques (e.g., FDM, DIW) appear most promising and prevalent among the few published examples.For example, one recent work [123] utilized FDM to 3D print electronic eyeglasses capable of continuous physiological monitoring including electrophysiological signals, ultraviolet light detection, and motion tracking with wireless data transmission (Figure 5A).Another recent work [190] utilized FDM-based 3D printing to create a conformal, adhesive-free mesh platform capable of continuous, wireless, and battery-free multimodal sensing (Figure 5B).Such systems highlight the tremendous potential for fabricating fully integrated wearable systems via additive manufacturing; however, significant challenges restrict continued progress.
Material availability represents a fundamental challenge as sensor functionality depends intrinsically on the library of printable materials.AM process-specific requirements, such as the glass transition temperature for FDM printable materials or the viscosity of ink for AJP, can be incompatible with fabrication of a particular sensor type.For instance, biological materials frequently act as biorecognition components (e.g., antibodies, enzymes) and may impose additional printing considerations (e.g., temperature, shear rate) to avoid degrading functional performance. [195]Other difficulties center on manufacturing factors including part production throughput or unit expense.Compared to conventional high-volume fabrication methods such as roll-to-roll processing and injection molding, additive manufacturing is inherently a batch technique with objects generated either individually or in small batches.While objects made through conventional pathways exhibit limited variation between parts (functionally indistinguishable), additive manufacturing process variability arising from printer hardware (e.g., wear on drive components), material (e.g., voids in FDM filament, localized heterogeneity in resin composition), environment (e.g., temperature, humidity), or operator handling (e.g., user-specific variations in post-processing) yield parts that are fundamentally unique. [196]he presence of weak interlayer bonds or residual stresses introduced during printing further necessitate post-processing to ensure a functional end product.Absent real-time process monitoring innovations, the inherent variability in AM currently restricts potential applications to non-critical roles (e.g., cosmetic rather than structural).Emerging machine learning and AI-based process monitoring represent promising methods for addressing such challenges through the rapid detection of print defects during fabrication. [197]For wearable platforms, such variability mandates extensive sensor quality control to ensure reliable, repeatable, uniform performance.
Hybrid manufacturing paradigms integrating AM with conventional subtractive, or mass production techniques offer strategies to address some of these challenges. [36]Recent process innovations enabling printing systems with high-speed, multinozzle arrays can substantially reduce manufacturing time,  [187] Copyright 2023, Elsevier B.V. B) 3D printed wearable electrochemical microneedle sensing patch for continuously monitoring apomorphine levels in interstitial fluid.Adapted with permission. [186]Copyright 2021 Elsevier B.V. C) Aptamer-functionalized microneedles for pharmacokinetic measurements in interstitial fluid.Adapted with permission. [188]Copyright 2024, Elsevier B.V.

Figure 5.
(A) 3D printed wearable smart E-glasses.Adapted with permission. [123]Copyright 2020, American Chemical Society.B) 3D printed wearable biosymbiotic device for multi-site, long-distance, wireless, and battery-free measurement of physiological signals.Adapted with permission. [190]Copyright 2021, American Association for the Advancement of Science.C) 3D printed colorimetric UV sensors.Adapted with permission. [191]Copyright 2023, Wiley-VCH.D) A multi-material 3D printed device containing conductive parts.Adapted with permission. [192]Copyright 2023, The Royal Society of Chemistry.E) 3D printed soft, wearable braille display.Adapted with permission. [193]Copyright 2023, Elsevier B.V. F) Epifluidic elastic electronic skin (e 3 -skin) with multimodal physiochemical sensing, energy management, and machine learning capabilities.Adapted with permission. [194]Copyright 2023, American Association for the Advancement of Science.
[200] Such efforts are essential to enabling new classes of functional printable materials, as diverse analyte detection necessitates tailoring printed components with included functional materials [191] (Figure 5C).Other promising innovations lie in process advancements that expand the capabilities of multi-material printing [192] (Figure 5D) that expand the range of printable, functional components as well as print processes that enable the fabrication of multiscale, microarchitected composites with programmed features (e.g., controlling architecture from nanoscale to macroscale). [201,202]Commercial demonstrations highlight the potential of multi-material printing via the production of complete printed circuit boards (PCBs) which incorporate both insulating and conducting materials, integrated circuits (ICs), and other electronics within a fully-integrated package. [203,204]When coupled to process simulations or patientspecific data, the resulting data-driven models can optimize the print process in a layer-by-layer or voxel-by-voxel manner to precisely control material properties of a highly customized printed object [193] (Figure 5E).For example, a recent report [194] details the use of additive manufacturing to establish a fully integrated electronic skin (e-skin) platform.E-skin systems are multimodal platforms that establish a conformal, seamless skin interface to support the active monitoring of relevant physiological health parameters via on-board sensors. [205,206]Figure 5F highlights this epifluidic elastic electronic skin (e 3 -skin) fabricated almost entirely by DIW, including the biophysical and biochemical sensors, channels for sweat sampling, and an energy management system. [194]Moreover, the integration of machine learning algorithms to assess collected data and enable an accurate prediction of behavioral responses indicative of impairments following alcohol consumption.Collectively, such innovations will permit additive manufacturing of not just individual sensors, but fullyintegrated wearable systems combining microfluidics, flexible electronics, and sophisticated sensing modules in a unified platform with performance indistinguishable from platforms created via conventional manufacturing methods.

Outlook
Realizing the full potential of additive manufacturing for producing fully integrated, customized wearable systems will require sustained collaborative efforts across a diverse array of fields spanning academia, industry, and medicine.Substantial innovation in printable materials is essential to expand the library of functional, biocompatible printable inks tailored for sensing the diverse range of target analytes.Manufacturing processes must be enhanced to improve resolution, speed, and scalability while maintaining print quality.Harnessing the innovations derived from the on-going revolution in artificial intelligence (AI) and machine learning will be essential for advancing 3D printed wearable sensor development by enabling new approaches to on-body analysis via rapid, real-time processing and interpretation of biological signals.Moreover, AI algorithms offer compelling opportunities to accelerate the optimization of device designs, [207,208] 3D printing parameters, [197,209] and material innovation [210,211] for rapid platform development.Seamless integration of heterogeneous printed components into unified platforms is crucial to enable integration within the device ecosystem of an individual to enable real-time personalized health monitoring.Such concerted innovation efforts will in aggregate establish additive manufacturing as a transformative approach to the development and manufacture of the next generation of personalized wearable sensors.

Figure 1 .
Figure 1.Schematic illustrations of the primary additive manufacturing techniques utilized for fabricating wearable devices: A) fused deposition modeling (FDM) printer, B) material jetting, C) aerosol jet printing (AJP), and D) vat photopolymerization.