Synapses play a critical role in the computation of the nervous system. The primary function of synapses includes integrating pre-synaptic circuits and converting them into post-synaptic currents, which are scaled by the synaptic weight [1]. Overwhelmingly, bio-inspired neuromorphic architectures are almost exclusively implemented using physically rigid silicon technology [2]. As such, they are limited to a small area, hard to interface with biological tissues, require expensive fabrication equipment, and need high-temperature fabrication processes. Organic electronics are an alternative to conventional materials and electronics [3]. They can be integrated with low-temperature processes with relatively low-priced fabrication equipment over a large area. Organic electronics and materials also offer flexibility, stretchability, and biocompatibility [4].

Here, we report two biologically realistic synaptic models implemented using flexible complimentary organic materials, including organic log-domain integrator [5] and organic differential-pair integrator synaptic circuits [6]. The synapses are shown to convert input voltage spikes into output current traces with biologically realistic time scales. We characterize the circuits’ response based on the synaptic circuit parameters, including gain and weighting voltages, synaptic capacitance, and circuit response due to inputs of different frequencies. The time constant is estimated using the circuit response and compared to the theoretically calculated one. The results show that the theory confirms the experimental measurements. The effect of flexibility is studied over the organic log-domain integrator. The strain shifts the threshold voltage of p and n-type organic field effect transistors and increases the time constant, a parameter critical in learning neural codes and encoding spatiotemporal patterns. Time constants comparable to those of biological synapses, in excess of tens to hundreds of milliseconds, are necessary for future integration with biological neurons and in processing real-world sensory signals such as speech or biosignals. The estimated time constant for the organic log-domain integrator and organic differential-pair integrator reaches 126 and 2000 ms, respectively. The synaptic capacitors are at the level of nano-Farad, while biologically plausible time constants are achieved by deploying smaller synaptic capacitors. Integration of such organic synaptic with organic somatic circuits [7-9] paves the way for future networks of biocompatible artificial organic neurons.