Polyvinyl Alcohol artificial synapses bio-integrated devices bottom-up approaches brain-like intelligence conformal design design metrics droplet circuits dry/wet conditions durability electron transport electronic measurements epidermal electronics epidermal sensors feedback control flexible electronics flexible organic electronics hydrophobic paper ionic conduction island-bridge liquid metal long-term plasticity low-cost manufacture microwave photonics nano-fabrication neuromorphic computing non-developable surface paper electronics quantum computing quantum tunneling effect