CONDUCTIVE HYDROGEL-BASED STRAIN SENSORS FOR MEASURING BODY MOVEMENTS

1 . A dual mode conductive hydrogel-based strain sensor having both an ion conductive mechanism and electron conductive fillers, the hydrogel-based strain sensor comprising:

a hydrogel including:

a first crosslinked hydrogel-forming polymer network;

a second crosslinked hydrogel-forming polymer network, the second crosslinked hydrogel-forming polymer network interpenetrating with the first hydrogel-forming polymer network without crosslinking between the first and second hydrogel-forming polymer networks;

a water-based liquid entrained by the first and second crosslinked hydrogel-forming polymer networks in an amount of approximately 50-75 wt. % of the hydrogel, the water including an ionically-conducting salt in an amount of 5-25 wt. % the formed hydrogel;

conductive fillers selected from two or more of graphene, carbon nanotube, and MXene; and

stretchable conductive electrodes formed on the hydrogel, the stretchable conductive electrodes selected from conductive particle-filled elastomers, stretchable metal meshes, and stretchable conductive fabrics.

2 . The dual mode conductive hydrogel-based strain sensor of claim 1 , wherein the first hydrogel-forming crosslinked polymer network includes a polyvinyl alcohol-based polymer and the second hydrogel-forming crosslinked polymer network includes an acrylamide-based polymer or a urethane-based polymer.

3 . The dual mode conductive hydrogel-based strain sensor of claim 1 , wherein the salt is selected from NaCl, CaCl 2 , LiCl, or KCl.

4 . The dual mode conductive hydrogel-based strain sensor of claim 1 , further comprising a protective layer formed over the hydrogel.

5 . The dual mode conductive hydrogel-based strain sensor of claim 4 , wherein the protective layer is selected from silicone or polyurethane.

6 . A strain measure measurement system comprising the dual mode conductive hydrogel-based strain sensor of claim 1 and a wearable flexible strain measurement device connected to the dual mode conductive hydrogel-based strain sensor.

7 . A method for making the dual mode conductive hydrogel-based strain sensor of claim 1 , comprising:

mixing a water-soluble synthetic polymer for the first hydrogel-forming polymer network, a polymerizable monomer for forming the second hydrogel-forming polymer network, and an ion conductive salt solution into a first mixture;

adding the electron conductive filler to the first mixture;

adding a crosslinking agent to the first mixture to crosslink the polymerizable monomer, creating the hydrogel having an interpenetrating network of the first hydrogel-forming polymer network and the second hydrogel-forming polymer network;

cutting a sensor blank from the hydrogel; and

forming electrodes on the sensor blank.

8 . The method for making the dual mode conductive hydrogel-based strain sensor of claim 7 , further comprising adding an initiator to form the second hydrogel-forming polymer network.

9 . The method for making the dual mode conductive hydrogel-based strain sensor of claim 8 , wherein the mixture includes:

5-15 wt % of the water-soluble polymer for forming the first hydrogel-forming crosslinked polymer network;

5-20 wt % of the polymerizable monomer;

5-25 wt % of salt, contributing to the ion conduction,

0.005-3 wt % of the electron conductive filler;

the crosslinking agent;

the initiator for a sol-gel process;

an accelerator for a sol-gel process; and

50-75 wt % of water.

10 . The method for making the dual mode conductive hydrogel-based strain sensor of claim 9 , wherein the crosslinking agent is N,N′-methylenebisacrylamide.

11 . The method for making the dual mode conductive hydrogel-based strain sensor of claim 9 , wherein the initiator is ammonium persulfate.

12 . The method for making the dual mode conductive hydrogel-based strain sensor of claim 9 , wherein the accelerator is N,N,N′, N′-tetramethylethylenediamine.