Fully Rubbery Stretchable Artificial Synapse Devices for Neurologically Integrated Soft Engineering Systems

Significant progresses have been made in developing soft machines, robotics or engineering systems to mimic soft animals. Neurologic functions, however, have never been realized in these artificial systems. Synapses are unique and critical biological structures that allow for the transmission of electrical or chemical signals thus to enable neurons to communicate with each other and are responsible for encoding the sensations, thoughts, etc. Embodied within human or animals, the synapse is usually soft and able to accommodate various forms of mechanical deformations. In particular, some soft bodied animals have very stretchy synapses that can elongate by significant extent. For instance, earthworms have very stretchable (by 50-100%) nerves that elicit different responses based on the interactions with the surrounding environment. The understandings in biology and advances in materials and electronics technologies have led to the recent development of artificial synaptic devices, mostly in rigid or flexible formats, to emulate the biological counterparts towards emerging applications, such as parallel processing, low power computing, neuroprosthetics, etc. Artificial synapses that are very stretchable like those appear in soft animals, are key to enabled neurological functions in soft machines and many other applications, such as neuroprosthetics. However, elastic stretchable synaptic device, in particular based on intrinsically stretchable materials, has never been reported before.

Here we report an elastic stretchable synaptic transistor and its neurologically integrated devices fully from rubbery materials. Specifically, we developed the fully rubbery synaptic transistor by employing rubbery semiconductor from composite of P3HT nano-fibrils percolated in polydimethylsiloxane (P3HT-NFs/PDMS), rubbery conductor from percolated composite of Au nanoparticles decorated silver nanowires in PDMS (AuNPs-AgNWs/PDMS), and elastic gate dielectric from ion gel. The synaptic transistor exhibits similar functions as those of biological synapses, including excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF), short-term memory (STM), long-term memory (LTM), and filter characteristics. Upon mechanical stretching by 50%, these characteristics still remained. In addition, we developed a neurologically integrated tactile sensory skin in fully rubbery format based on an array of mechanoreceptors of pressure sensitive rubber and fully rubbery stretchable synaptic transistors. The mechanoreceptors respond to physical touches by generating presynaptic pulses, which excite the synaptic transistors to render postsynaptic potentials, which can be potentially interfaced with biological nerves or engineering counterparts. Postsynaptic potential mapping from the arrayed sensory pixels was achieved even when the skin is stretched. We further created a soft adaptive neurologically integrated robot, i.e. neurorobot, which is constructed based on a soft pneumatic robot covered with functional elastic skins of the triboelectric nanogenerators (TENGs) and synaptic transistors. The robot senses physical tapping and locomotes adaptively in a programmed manner through synapse memory encoded signals. Systematic studies of the device construction and operation mechanisms components and neurologically integrated systems reveal all the fundamental aspects and also suggest their future usages.