(591d) Biomimetic Soft-Matter Memristors Composed of Hydrogels

This talk will describe the use of bio-compatible hydrogels as the basis for a new type of memory architecture composed entirely of soft materials that mimic synapse operation.  New types of electronic devices and circuits based on soft materials have potential applications in bio-electronic circuits, artificial neural networks, and brain-machine interfaces.  We will present a new class of diodes and memristors (i.e., memory resistors) composed entirely of soft, liquid-based materials formed by combining a moldable liquid metal and hydrogel doped with polyelectrolytes. Memristors are next generation memory devices that mimic the function of synapses.  The electronic functionality of these soft devices originates from the ability to control the electronic and ionic transport at the interface between the metal and the hydrogel. The metal is a eutectic alloy of gallium and indium (75 % Ga, 25 % In).  At room temperature it is a low viscosity liquid with a high conductivity (σ = 3.4 x 10⁴ S/cm). Its surface is coated with a thin, native skin of gallium oxide, which is a wide band-gap semiconductor. The oxide skin passivates the liquid metal; that is, the oxide does not grow thicker with time without any additional driving force. The electrical resistance through the oxide skin depends on its thickness, which can be controlled using pH and/or electrical bias to oxidize or reduce the skin. The agarose gel, which interfaces with the liquid electrodes, is composed of more than 90% water and offers a soft aqueous media into which polyelectrolytes may be doped to control the local pH. We fabricated soft and quasi-liquid electronic devices that mimic solid-state, semiconductor devices, such as diodes and memristors, by sandwiching two hydrogel films: one doped with polyacrylic acid (PAA) and one doped with polyethyleneimine (PEI, which is basic). We contacted both sides of the film stack with liquid metal electrodes. The PEI-doped hydrogel suppresses the oxidation reaction at the metal electrode interface, whereas the thickness of the metal oxide layer at the interface with the PAA-doped hydrogel can be controlled by applying an electric bias. The direction and the magnitude of the electric bias pre-applied to the electrodes controls the anisotropy of current conductance, resulting in tunable rectification ratios.  The pre-programmed anisotropy is preserved for more than an hour, which enables the fabrication of memory storage devices (i.e., memristors). We will discuss the rectification characteristics of diodes and the on/off switching behavior of memristors with different geometries composed entirely of soft materials.