Photoinitiated chemical vapor deposition (piCVD) of composition tunable, ionically conductive hydrogels on diverse substrates

Abstract

Ionically conductive hydrogels are finding prominence in a wide range of emerging devices and applications, including biopotential sensors, organic field effect transistors, biomedicine, and soft robotics. Traditionally, these gels are synthesized through solution-phase polymerization or solvent based swelling of a polymer network and then cast in place or adhered to an intended substrate after synthesis. These fabrication approaches place artificial limitations on the accessible chemical composition and ionic conductivity of the gels, and limit deployment of ionically conductive hydrogels in complex platforms. Here we present a modular method to create ionically conductive hydrogels on a variety of rigid, flexible, or filamentary substrates through a photoinitiated chemical vapor deposition (piCVD) process. First, a viscosity tunable precursor mixture of desired ionic composition and strength is created and coated onto a target substrate. Next, an acrylate film is grown directly on these coated substrates via piCVD. Since both the monomer and photoinitiator used during the piCVD process are miscible in the aqueous precursor mixture, polymerization occurs at both the surface of and within the precursor layer. Using this two-step strategy, we isolate a robust composite hydrogel with independently tunable ionic properties and physical structure. This method is compatible with most substrates and results in a conformal, persistent gel coating with excellent rehydration properties. Gels containing a variety of biocompatible salts can be accessed, without concomitant changes in physical structure and morphology. Ionic conductivities can be tuned between 1 × 10−5–0.03 S cm−1 by changing the ionic strength of the precursor mixture. Additionally, we show that the material retains its ion concentration and conductivity after washing. Finally, we deploy this material onto several different substrates and show that through this method the same gel can be manufactured in-place regardless of the intended substrate.