| Polymers, as the active materials in organic devices such as OTFT, have attracted significant scientific interest for the e-skin applications. Previous works on e-skin have focused on how to achieve high sensitivity to emulate the sensory properties of skin neurons. However, the recognition and readout of external stimuli is just the initial and primary response of natural skin. The sensory signals are extracted, transmitted and processed during the feature detection process through synaptic contacts between sensory neurons to high-level neurons in neural networks. The processing elements with synaptic adaptation ability are always missing when mimicking the natural skin because of the lack of flexible electronic synapses. A generalized concept of e-skin systems composed of not only sensors but also synapses is envisioned to enable multifunctionality for robotic, medical and wearable electronic applications. Polymer electronic synapses with inherent activity-dependent plasticity and mechanical flexibility can provide new design principles to implement the e-skin systems. Although organic resistive memory enables the development of flexible data-storage devices, it is still a challenge to achieve flexible polymer synapses at this stage because of the restrictions, such as the difficulty in achieving memory beyond the binary storage, the deficiency of reliability and degradation in performances under the bended/stretched conditions.In this work, flexible polymer synapses with Polyvinyl Alcohol (PVA) as the active material are demonstrated. The polymer synaptic device in this work has a two-terminal cross-aligned structure instead of the traditional source-channel-drain structure in the reported synaptic transistors. A key polymer, PVA, was employed to construct the flexible polymer synapses. Most of the polymers reported in organic resistive memories are aromatic heterocyclic organic compounds consisting of benzene rings or having a similar aromatic-ring configuration of atoms, such as Carbazole based polymers, which are unsuitable for the e-skin applications. PVA is a well-known polymer with non-toxicity, biocompatibility, high film forming ability and flexibility. The most important reason that PVA stands out from other polymer candidates is its field-controlled dipoles that enable the activity-dependent plasticity. In this work, the direct identification of the tunable polarization in PVA, which comes from the field-induced alignments of dipoles, is reported for the first time.The main results are summarized as follows:First, we fabricated a series of PVA films via spin-coating methods based on different parameters. The structure, composition and surface morphologies of the PVA thin films were investigated by using FTIR and AFM as deposited. This work provides a feasible way to optimize the fabrication parameters and hence the performance of the synaptic device. The optimized parameters are as followed: concentration of PVA aqueous solution is 12 mg/ml, with rotation speed as 2000 rpm and rotation time as 40 s.Second, we present a new mechanism responsible for the synaptic plasticity in full organic devices. The most widely used memory mechanisms in polymer resistive memory devices are charge transfer, conformational change, SCLC, filamentary conduction and redox. The field-controlled dipoles in PVA supply the polarization charges at the contact interface, which modify the tunneling barrier for the transport electrons and consequently result in the activity-dependent plasticity. This provides a principle to overcome the limitation on memory states in polymer devices.Third, we applied the polymer synapses to neural stimulations, using pulse tests to mimic the synaptic plasticity under external stimulations. The rehearsal-dependent decay processes in PVA polymer synapses were investigated, where STP is achieved in the case of limited stimulations of shorter duration, while LTP is consolidated through repeated stimuli of longer duration. In biological systems, the connections between neurons during the signal transmission and adaptation are sensitive to the intensity, the duration and the repetition frequency of the stimulation impulses, which can be mimicked to perform the bio-inspired computing in electronics. The PVA polymer synapses also show the similar behaviors owing to its field-controlled dipoles, which can be employed for the signal transmission and adaptation processes in e-skin systems.At last, we validate the identical synaptic rules of PVA polymer synapses under different bended conditions. In this work, we simulate three representative situations, which are closely analogous to the thumb, the thenar eminence and the wrist of human hand. The success in implementing synaptic functions under these bended conditions promises the advanced functionalities in e-skin systems, such as haptic perception and physiological signals processing.The presented polymer materials, mechanisms and performances open up a new route to develop flexible neural networks for e-skin systems. |
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