Biological Tissue-Inspired Ultrasoft, Ultrathin, and Mechanically Enhanced Microfiber Composite Hydrogel for Flexible Bioelectronics

Abstract

Hydrogels offer tissue-like softness, stretchability, fracture toughness, ionic conductivity, and compatibility with biological tissues, which make them promising candidates for fabricating flexible bioelectronics. A soft hydrogel film offers an ideal interface to directly bridge thin-film electronics with the soft tissues. However, it remains difficult to fabricate a soft hydrogel film with an ultrathin configuration and excellent mechanical strength. Here we report a biological tissue-inspired ultrasoft microfiber composite ultrathin (< 5 μm) hydrogel film, which is currently the thinnest hydrogel film as far as we know. The embedded microfibers endow the composite hydrogel with prominent mechanical strength (tensile stress ~ 6 MPa) and anti-tearing property. Moreover, our microfiber composite hydrogel offers the capability of tunable mechanical properties in a broad range, allowing for matching the modulus of most biological tissues and organs. The incorporation of glycerol and salt ions imparts the microfiber composite hydrogel with high ionic conductivity and prominent anti-dehydration behavior. Such microfiber composite hydrogels are promising for constructing attaching-type flexible bioelectronics to monitor biosignals.

Keywords: Electrospinning; Fiber; Flexible electronics; Hydrogel; Thin film.

Fig. 1

Design of PVA/MF-CH-based bioelectronics. a Schematic structure of biological tissue and PVA/MF-CH. In the human body, the ECM of living tissues mainly comprises of interpenetrating collagen fiber and elastin fiber networks. The PVA/MF-CH presents an ECM-mimicked structure of interpenetrating networks, which consists of PU microfibers (blue) and PVA molecular chains (purple). The PVA hydrogel comprising PVA chains and water are presented in semi-transparent blue. b Schematic preparation procedure of PVA/MF-CH. c SEM image displaying the surface of a freeze-dried PVA/MF-CH. d Schematic illustration of the microfiber embedded structure and bonding mechanisms of PVA molecular chains and PU microfiber matrix. e Digital picture of PVA/MF-CH attached to the skin

Fig. 2

Mechanical properties of PVA/MF-CH. a Force-strain curve of the PVA hydrogel with different freeze-thaw cycles (4, 6, and 8). b Optical images of PU microfiber networks prepared with different electrospinning times (40 s, 2 min, 5 min, and 8 min). c Force-strain curves of PVA/MF-CH composed of PU microfiber networks with different electrospinning times (40 s, 2 min, 5 min, and 8 min) and PU microfiber networks with different electrospinning times of 2, 5, and 8 min. d Schematic thickness change of PVA/MF-CH with the increase of rotation rate. The thickness of the microfiber composite hydrogel (d) is comprised of the thickness of the microfiber network (d₁) and the thickness of the PVA hydrogel over the microfiber network (d₂) which can be controlled by the rotation rate during the spinning-coating procedure. e Thickness of PVA/MF-CHs processed under different rotation rates. f Force-strain curves of the PVA/MF-CH processed under different rotation rates of 0.5, 1.0, 2.0, and 3.0 k rpm. g Schematic anti-tearing behavior of microfiber composite hydrogel. h Digital images of PVA/MF-CH with a cut crack under different strains. i Maximum strains of PVA/MF-CHs and pure PVA hydrogels with and without a crack

Fig. 3

Dehydration behavior and conductivity. a Schematic structure and constituents of PVA/MF/Gly-CH with NaCl. b Mass change of PVA/MF/Gly-CH and PVA/MF -CH. c Digital images of PVA/MF/Gly-CH and PVA/MF-CH exposed in air for different time duration. The inserted picture is the PVA/MF/Gly-CH stored in the air for 7 days, which yet sustains its flexibility. d Optical images displaying the thickness change of PVA/MF/Gly-CH and PVA/MF-CH. e Electrochemical impedance of PVA/MF-CH with different amounts of glycerol and NaCl and corresponding electrochemical impedance after exposure in the air for 12 and 48 h. f Force-strain curve of PVA/MF-CH with different amounts of glycerol and NaCl

Fig. 4

Tunable conformability and flexibility. a Surface roughness of the artificial skins. b Digital image displaying wrinkles generated from the PVA/MF-CH and PET glue tape induced by squeezing the skin. c Schematic wrinkle-generating mechanism of skin covered by PVA/MF-CH and PET glue tape when squeezing. d Schematic diagram of a method to evaluate the softness of a material with a bending diameter (D). e Digital images of bending diameters generated from different materials. Specimen size: 1 cm (L) × 0.5 cm (W). f The diameter of the bending circle generated in different materials. P-P and V-V mean the distance between two peaks and two valleys, respectively. g Young’s Modulus and thickness of different materials in this work and previously published works. PAN: polyacrylonitrile. h Modulus matching range of our PVA/MF-CH with biological tissues and organs. i Digital image of a porcine heart with attaching PVA/MF-CH -based bioelectrode. Scale bar 2 cm. The inserted picture is the PVA/MF-CH -based bioelectrode. Scale bar 1 cm

Fig. 5

Monitoring of EMG biosignals. a Schematic of the equivalent circuit model used for monitoring EMG biosignals. At the electrode level (top three elements): R_d is the charge-transfer resistance, C_d is the double-layer capacitance, and R_cg is the resistance of our composite gel. At the skin level (bottom three elements), R_e and C_e are the epidermal resistance and capacitance, respectively, and R_sub is the resistance of the dermis and deep tissues. b Performance comparison of EMG biosignals collected by the electrode composed of our PVA/MF/Gly-CH and commercial gel. c The background noises of the electrode composed of our PVA/MF/Gly-CH and commercial gel. d Performance comparison of the electrode composed of our PVA/MF/Gly-CH and commercial gel for the monitoring EMG biosignals after 48 h. e Performance of the electrode composed of our PVA/MF/Gly-CH for the monitoring EMG biosignals after 7 d. f EMG biosignals of the forearm are generated from different gestures. g EMG biosignals of the forearm are generated from different gripping forces. h EMG biosignals of the bicipital muscle of the arm lifting the different masses of the object. i A tri-electrode system comprised of the PVA/MF/Gly-CH for the monitoring of EMG biosignals. (Electrode in red rectangle, GND in yellow rectangle, and Ref electrode in green rectangle) j Digital image of the hand with attached a tri-electrode system comprised of the PVA/MF/Gly-CH. k EMG biosignals of the forearm collected by our PVA/MF/Gly-CH-based bioelectrode