Skin-inspired hydrogel-elastomer hybrids with robust interfaces and functional microstructures

Abstract

Inspired by mammalian skins, soft hybrids integrating the merits of elastomers and hydrogels have potential applications in diverse areas including stretchable and bio-integrated electronics, microfluidics, tissue engineering, soft robotics and biomedical devices. However, existing hydrogel-elastomer hybrids have limitations such as weak interfacial bonding, low robustness and difficulties in patterning microstructures. Here, we report a simple yet versatile method to assemble hydrogels and elastomers into hybrids with extremely robust interfaces (interfacial toughness over 1,000 Jm(-2)) and functional microstructures such as microfluidic channels and electrical circuits. The proposed method is generally applicable to various types of tough hydrogels and diverse commonly used elastomers including polydimethylsiloxane Sylgard 184, polyurethane, latex, VHB and Ecoflex. We further demonstrate applications enabled by the robust and microstructured hydrogel-elastomer hybrids including anti-dehydration hydrogel-elastomer hybrids, stretchable and reactive hydrogel-elastomer microfluidics, and stretchable hydrogel circuit boards patterned on elastomer.

Figure 1. Schematic illustration of the fabrication of robust microstructured hydrogel–elastomer hybrids.

(a) The hybrids are formed by bonding tough hydrogels of interpenetrating polymer networks with elastomers. One polymer network of the hydrogel is first physically crosslinked, while infiltrated with monomer/macromonomer solution of the other polymer network. The physical crosslinking sets the shape and microstructures of the hydrogel. (b) The surface of a cured elastomer with patterned microstructures is treated with benzophenone. (c) The pre-shaped hydrogel and elastomer are assembled together followed by ultraviolet irradiation to chemically crosslink the other polymer network in the hydrogel. (d) After ultraviolet irradiation, the resultant hydrogel–elastomer hybrid forms extremely robust interfaces due to the covalently anchored polymer network in the hydrogel on elastomer surface. The pre-patterned microstructures in elastomers and hydrogels are also preserved in the hybrid. The hybrids can be highly stretched without interfacial failure.

Figure 2. Experimental and simulation results of 90°-peeling tests on hydrogel–elastomer hybrids.

(a) Schematic illustration of the 90°-peeling test (ASTM D 2861) on various hydrogel–elastomer hybrids. A stiff backing is introduced to prevent elongation of hydrogel sheet along the peeling direction. (b) Photos of the hydrogel–elastomer interface during peeling test. The tough hydrogel undergoes a cohesive failure during the peeling test, leaving a thin residual layer of hydrogel ∼0.2 mm on the elastomer substrates. (c) The measured peeling forces per width of the hydrogel sheets for various hydrogel–elastomer hybrids (in as-prepared state). (d) The calculated peeling forces per width of the hydrogel sheets for various hydrogel–elastomer hybrids in finite-element simulation. The simulated interfacial toughness is significantly decreased as dissipative properties is eliminated in the hydrogel (that is, without Mullins effect) while maintaining other parameters the same. Note that inset pictures are snapshots of the 90°-peeling simulation. The contours indicate the energy dissipation per unit area in the material. (e) Summary of measured interfacial toughness of various hydrogel–elastomer hybrids using the proposed method at both as-prepared and fully swollen states. Values in e represent mean and the error bars represent the s.d. of measured interfacial toughness for each elastomer materials (n=3–5).

Figure 3. Hydrogel–elastomer hybrids under uniaxial stretches.

(a) PAAm-alginate hydrogel bonded on Ecoflex elastomer using the proposed method can withstand large deformation (stretch ∼7) without debonding. The robust hydrogel–elastomer bonding is intact even after fracture of the hybrid. (b) PAAm-alginate hydrogel bonded on Ecoflex elastomer untreated by benzophenone detaches from the elastomer under small deformation (that is, stretch ∼1.1) due to weak adhesion. (c) PAAm hydrogel bonded on Ecoflex elastomer using the proposed method fails under large deformation (stretch ∼4) due to crack propagation in the brittle bulk PAAm hydrogel. Note that red food dyes are added into the hydrogels to enhance the contrast between hydrogels and elastomers.

Figure 4. Anti-dehydration hydrogel–elastomer hybrid.

(a) Schematic illustration of the anti-dehydration elastomeric coating for hydrogels. A very thin layer of Ecoflex elastomer robustly bonded to the hydrogel can effectively prevent evaporation of water from the hydrogel. (b) The hydrogel–elastomer hybrid does not show noticeable change in its weight under the ambient testing conditions (24 °C and 50% humidity) for 48 h; whereas hydrogel without elastomeric coating loses most of its water content after 48 h. (c) Snapshots of the hydrogel–elastomer hybrid and hydrogel during the dehydration experiments. Scale bar, 10 mm (c).

Figure 5. Stretchable diffusive and reactive microfluidic chips based on hydrogel–elastomer hybrids.

(a) Schematic illustration of the fabrication procedure for hydrogel–elastomer microfluidic chip. (b) The resultant hydrogel–elastomer microfluidic hybrid supports convection of chemicals (represented by food dye in different colours) in the microfluidic channels and diffusion of chemicals in the hydrogel. (c) The hydrogel–elastomer microfluidic hybrid can maintain functionality under large deformation (for example, stretch ∼2) without debonding failure or leakage thanks to the robust interfacial bonding. (d) The hydrogel–elastomer microfluidic hybrid can be used as a platform for diffusion-reaction study. Acid (pH ∼3) and base (pH ∼10) solutions from two microfluidic channels diffuse in the pH-sensitive hydrogel and form regions of different colours (light red for acid and dark violet for base). The reaction of acid and base solutions in the hydrogel further form a neutral region (pH ∼7, light green colour). Scale bars, 10 mm (b–d).

Figure 6. Stretchable hydrogel circuit board patterned on elastomer.

(a) Schematic illustration of fabrication procedure for conductive hydrogel circuit patterned on flexible elastomer substrate. (b) Ionically conductive PAAm-alginate hydrogel circuit bonded on an Ecoflex elastomer substrate using the proposed method is robust under large deformation without visible failure. (c) The hydrogel circuit board connected with an AC power source can light up LED, and it can maintain its electrical functionality even under severe deformation. (d) A hydrogel circuit bonded on Ecoflex elastomer without benzophenone treatment delaminates and fails under deformation, due to the weak hydrogel–elastomer bonding. Scale bars, 10 mm (b–d).