Kneading‐Inspired Versatile Design for Biomimetic Skins with a Wide Scope of Customizable Features

Abstract Biomimetic skins featuring customizable functions and human tissue‐compatible mechanical properties have garnered tremendous interest for potential applications in human–machine interfaces, flexible wearable devices, and soft robotics. However, most existing skin‐like materials require complex molecular design or multistep functionalization to achieve various functionalities that match or even surpass the performance of human skin. Thus, simultaneously minimizing production costs and achieving customizable features are still highly desirable yet challenging. Herein, inspired by a well‐known kneading technique that renders a homogeneous mixture of all the ingredients, a versatile method involving two steps of kneading and resting is employed to prepare biomimetic skins with a wide scope of customizable features. Commonly used one‐dimensional (1D), two‐dimensional (2D), three‐dimensional (3D) nanofillers and even solvents are demonstrated to be homogeneously dispersed in the viscoelastic hydrogel matrices by hand kneading, which not only contributes to improved mechanical properties and new functionalities, but also makes full use of raw materials without waste. Furthermore, similar to the combination of “condiments” in kneading dough, the flexible integration of functional fillers offers exciting and versatile platforms for the design of biomimetic skins with tunable application‐specific properties, such as mechanical compliance, sensory capabilities, freezing resistance, 3D printability, fluorescence tunability, etc.

S3 min, and the suspension containing the delaminated MXene flakes was collected after centrifugation at 3500 rpm for 30 min, which termed as delaminated MXene solution. MXene slurry was finally obtained by centrifuging delaminated MXene solution at 9000 rpm for 60min, whose concentration was calculated at about 40 mg/mL.

Preparation of Lanthanide Metal-Organic Frameworks (LnMOFs)
LnMOFs crystals with red fluorescence (EuMOFs) and green fluorescence (TbMOFs) were synthesized according to the previously reported rapid solvent precipitation method. [S3, S4] Specifically, to prepare EuMOFs crystals, 1 mmol Eu(NO 3 ) 3 ·6H 2 O, 1 mmol mellitic acid, and 10 mL deionized water were first magnetically stirred for 10 min. Then rapid crystal nucleation was triggered by the slow addition of 3 mL ethanol solvent (30% in volume). After several minutes, the formed crystals were collected by centrifugation at 5000 r.p.m. for 5 min.

Kneading method
The kneading process is similar to kneading dough. Taking VEH-CNT as an example, CNT paste was first spread on the as-prepared viscoelastic hydrogel (VEH), then our hands acted as an agitator to stir the ingredients. The mixed hydrogel was repeatedly rolled and flattened until its color became uniformly black. Note that the CNT paste should be added in small amounts with many times to achieve better homogeneity. To prepare large-sized VEH-CNT samples, an automatic household dough maker (HMJ-A20E1 2L) was used. For VEH-MXene, the MXene content was 5 wt%. For VEH-GO, the GO content was 5 wt%. For VEH-EuMOF, the content of kneaded EuMOF was 5 wt%. For VEH-TbMOF, the kneaded TbMOF was 5 S4 wt%. For VEH-MX-Gly, the kneaded MXene was 5 wt% and the glycerol content was 15 wt%. VEH-MX-Gly was prepared by kneading MXene first and then glycerol, while VEH-Gly-MX was obtained by kneading glycerol followed by MXene.

Characterization
Fourier transforms infrared (FTIR) spectra were collected by Nicolet Nexus 470 spectrometer.
Rheological properties of hydrogel samples were characterized at 25 o C using a HAAKE MARS modular advanced rheometer with a 25-mm parallel plate. The storage modulus (G′) and loss modulus (G″) were measured in the frequency range from 0.1 to 100 Hz at 0.1% strain amplitude. The temperature sweep measurement of the VEH-MX-Gly was measured with a temperature range of 10 to -25 °C, a fixed oscillatory strain of 0.1%, and a fixed frequency of 1 Hz. Scanning electron microscopy (SEM) images were recorded by field emission scanning electron microscopy (FESEM, Zeiss, Ultra 55), and the element mapping images were taken from the energy dispersive spectrometer (EDS) equipped on a FESEM (S-4800, Hitachi, Japan). The morphology and structure of MXene, GO, and CNT were observed by TEM (JEOL JEM2011 F Microscope). Fluorescent emission spectra were obtained with FLS 1000 at ambient conditions. X-ray diffraction (XRD) data were measured on Bruker D8 diffractometer (Germany) with Ni-filtered Cu Kα radiation (40 kV, 40 mA).
Due to the excellent shape adaptation of VEH-CNT, the volume of VEH and VEH-CNT was measured with the syringe. The Teflon mold used for resistance test was shown in Figure S3, with two flexible conductive tapes adhered onto two sides. The hollow part of the mold was used to hold samples with dimensions of 1 × 1 × 1 cm 3 . Capacitance and resistance signals were measured on an LCR meter (TH2830) with an AC voltage of 1V and a sweeping S5 frequency of 1 kHz. The conductivity (σ) was calculated from the formula, σ = L/RS, where L and S corresponded to the length and cross-sectional area of hydrogels, respectively. The VEH-MX-Gly was printed by a 3D printing system (3D Bio-Architect work station, Regenovo), with a tip diameter of 0.4 mm, extrusion pressure of 0.3 MPa, and printing speed of 4 mm s -1 . The entire printing environment was kept at about 25 o C. Photothermal effect measurements were performed under irradiation by 808 nm diode laser with a spot size of 5×8 mm 2 (Changchun New Industries Optoelectronics Technology Co., Ltd., MDL-III-808R), which was oriented perpendicular to hydrogel samples. And the variation of the temperature was recorded every 5 seconds by a thermocouple thermometer. Figure S1.            Table S1. Comparation of performance of malleable hydrogels.

Materials
Stretchability Freezing tolerance Self-healing efficiency 3D printability Ref