Stretchable Hydrogel Electronics and Devices

Abstract

Stretchable hydrogel electronics and devices are designed by integrating stretchable conductors, functional chips, drug-delivery channels, and reservoirs into stretchable, robust, and biocompatible hydrogel matrices. Novel applications include a smart wound dressing capable of sensing the temperatures of various locations on the skin, delivering different drugs to these locations, and subsequently maintaining sustained release of drugs.

Keywords: biointegrated electronics; drug delivery; flexible electronics; hydrogels; wearable devices.

Figure 1. Schematic illustration of the design of stretchable hydrogel electronics and devices

a) Functional electronic components such as conductors, microchips, transducers, resistors, and capacitors are embedded inside or attached on the surface of the hydrogel. Drug-delivery channels and reservoirs are patterned in the hydrogel matrix, and they can diffuse drugs out of the hydrogel to give programmable and sustained release of drugs. b) As the hydrogel electronic device is stretched, flexible electronic components can deform together with the device but rigid components will maintain their undeformed shapes, which requires robust interfaces between electronic components and hydrogel matrix.

Figure 2. Integration of wavy titanium wires in tough hydrogel matrix

a) Schematic illustration of the transparent, highly stretchable and robust hydrogel electronic (DC) conductor. Long-chain polymer network of tough hydrogel matrix is chemically anchored onto silanized titanium surface via covalent crosslinks. b) The wavy wire can be highly stretched together with hydrogel matrix without fracture or debonding due to robust adhesion between wire and hydrogel matrix. Finite-element simulation shows maximum principal strain of the hydrogel matrix. c) The calculated λ_max as a function of A/L in comparison with experimentally measured maximum stretches in hydrogel conductors that contain titanium wires with and without silanized surfaces. d) The hydrogel conductor (A/L=0.72, diameter D=0.08mm) can sustain multiple cycles (i.e., 10,000) of high stretch (i.e., 3), while maintaining constant resistance.

Figure 3. Integration of rigid chips on the surface of (or inside) tough hydrogel matrix

a) Schematic illustration of a rigid PDMS chip bonded on the surface of hydrogel. Glass slide is used to form stable and robust adhesion layer between PDMS chip and tough hydrogel. Oxygen plasma treated PDMS and glass slide surface are covalently bonded through siloxane bond, while silanization of the glass slide gives tough covalent bonding to hydrogel. b) Chemically anchored Glass/PDMS chip is robustly bonded on hydrogel matrix even when pulled by a tweezer. c) Physically attached Glass/PDMS chip is easily debonded from hydrogel matrix. d) Chemically anchored Glass/PDMS chip doesn’t debond from hydrogel matrix even under high stretch (up to 3 times) due to robust adhesion between adhesion layers. e) The calculated G/µL from the finite-element model is plotted as functions of λ and S/L. The typical values of interfacial toughness between chips and hydrogels with and without silanized interfaces are also given for comparison (1000 J/m² and 20 J/m² respectively). f) A sheet of hydrogel (thickness ~ 1.5 mm) with multiple patterned chips can conformably attach to different regions of human body and survive deformation due to body movements. g) A hydrogel electronic device that encapsulates an array of LED lights connected by stretchable silanized titanium wire. The device is transparent, and robust under multiple cycles of high stretch.

Figure 4. Integration of drug-delivery channels in tough hydrogel matrix

a) Schematic illustration of diffusion of drug solution inside hydrogel from the drug-delivery channel. b) Experimental snapshots of drug diffusion in the undeformed hydrogel. c) Experimental snapshots of drug diffusion in the deformed hydrogel. d) Normalized one-dimensional diffusion of mock drug inside undeformed hydrogel channel. e) Normalized one-dimensional diffusion of mock drug inside deformed (λ = 1.6) hydrogel channel. f) Experimental snapshots of the diffusion of multiple mock drugs in a hydrogel matrix under high stretches.

Figure 5. A smart wound dressing based on stretchable and biocompatible hydrogel device

a) The temperature sensors are patterned into a 3 by 3 matrix with a drug-delivery reservoir next to each of them. The smart wound dressing can give programmable and sustained deliveries of different drugs at various locations over human skin according to the temperatures measured at those locations. b) The temperatures at different locations on the skin are measured via wireless temperature sensor over time. c–f) When the temperature at a location goes above a certain level (e.g., T_c = 35 °C), a mock drugs is injected through non-diffusive channels into the corresponding reservoir to diffuse out over time. Sustained releases of various drugs are achieved at different locations according to the temperatures measured at different times.