To develop a PVA based ECM memristor with mechanical flexibility, we fabricated the vertical-type device with Mw = 10000 gmol-1 on a flexible substrate of polyethylene naphthalate (PEN), as shown in Figure 3a. Figure 3b shows the IV properties of the flexible memristor at the bending states. Before the measurement, the electroforming process was performed to initiate the CF growth (see Figure S14). We applied the tensile and compressive stresses to the device by conducting the negative and positive bending tests, respectively. The bending radius for the mechanical stresses was 5 mm. The stable resistive switching behaviors which are similar to those in the rigid device (see Figure 3b) were observed in the flexible device at the bending states. Under the positive bending state with a radius of 5 mm, the resistance state of the device was reversibly changed from the HRS to the LRS by the repeated voltage sweeps, as shown in Figure 3c. Furthermore, the device showed the stable resistive switching behaviors under the repeated 5×103 pulse cycles \cite{Lanza_2021} (see Figure S15).
To further explore the mechanical durability of the flexible device, we investigated the effect of the repeated mechanical stresses on the memory retention performance of the device (see Figures 3d and 3e). As shown in Figure 3d, the positive and negative bending processes were performed sequentially in the device, during the memory retention test. The memory states (HRS and LRS) of the device were not deteriorated by the mechanical stresses, which means that the developed memristor can be used as an essential component of flexible neuromorphic systems. Figure 3e shows the results for the bending cycle tests in the device. The device showed a constant conductance at each memory state during the repeated cycles consisting of the positive and negative bending deformations with the same curvatures as those in Figure 3b. Moreover, the device was stably operated as a reversible resistive switching memory after the bending cycle tests (see Figure S16). Considering that mechanical flexibility is a critical requirement for realizing wearable systems \cite{Zhu_2019}, the developed memristor with high endurance for bending stresses can be useful in wearable intelligent electronics.
For confirming the transient characteristics of the developed organic memristor, we immersed the device in DI water at a room temperature of 300 K (see Figure 3f) and observed the dissolution state of the device according to the immersion time (see Figure 3g). The device was gradually dissolved in DI water, and completely vanished after 30 s. This transient behavior of the device is superior to those of previously reported transient memory devices \cite{He_2016,Wang_2016a,Wu_2016,Sun_2018,Song_2018,Ji_2018,Xu_2018,Lin_2019}. Note that PVA is known to be a carbon–carbon backbone polymer that is biodegradable under both aerobic and anaerobic environments \cite{Zhang_2020}. In this regard, the PVA based memristor can be used in a wide range of biodegradable neuromorphic applications.