# Recent Advances in Flexible Resistive Random Access Memory
## 3 Material System of Flexible Resistive Memory
### Flexible Dielectric Materials
#### 3.1.2 Flexible Organic Materials  
In order to make flexible RRAM more flexible, organic materials with excellent mechanical properties are increasingly being used as resistance dielectric layers. Flexible organic materials have excellent flexibility due to the strong cross-linking between their molecules. Moreover, they have become important candidates for flexible electronic devices due to their characteristics of high flexibility, large-area film forming and light weight. This material relies on functional groups to produce ions to form conductive filaments. To date, many materials have been used in flexible RRAM, including polymer materials (chitosan [53], lignin [54], polyparaxylene [55], polyimide (PI) [56], polyethylene glycol dimethacrylate (pEGDMA) [57], poly (1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane) (pV3D3) [58], starch [59], nitrocellulose (NC) [60], conjugated polyrotaxane (CPR1) [61], etc.) and organic composite materials (P3 HT:OD [62], PMMA:P3 HT [63], PMMA:PCBM [64; 65], PI:PCBM [66], PS:PCBM [67], PAA:PEI [68], Rhodamine B:Rhodamine 6G (Rh B:R6 6G) [69], etc.). The performance comparison is shown in Table 2: \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Organic Materials** & **ON/OFF Ratio** & **Durability/Times** & **Retention Time** &
\begin{tabular}{c} **Number of** \\ **Bending Cycles** \\ \end{tabular} & **Bending Radius** \\ \hline chitosan [53] & \(10^{2}\) & \(10^{2}\) & \(10^{4}\) s & \(10^{3}\) & 5 mm \\ lignin [54] & \(10^{3}\) & \(10^{2}\) & \(10^{3}\) s & \(10^{2}\) & 15 mm \\ polyparaxylene [55] & \(10^{4}\) & \(5\times 10^{2}\) & \(2\times 10^{5}\) s & \(5\times 10^{2}\) & 10 mm \\ PI [56] & \(10^{6}\) & \(10^{3}\) & \(4\times 10^{3}\) s & \(10^{3}\) & 5 mm \\ pEGDMA [57] & \(10^{2}\) & \(5\times 10^{2}\) & \(10^{6}\) s & \(10^{3}\) & 4 mm \\ PV3D3 [58] & \(10^{7}\) & \(10^{5}\) & \(10^{5}\) s & \(10^{3}\) & 3.8 mm \\ starch [59] & \(10^{3}\) & \(10^{2}\) & \(10^{4}\) s & \(10^{3}\) & 5 mm \\ NC [60] & \(10^{3}\) & \(10^{2}\) & \(10^{4}\) s & \(5\times 10^{2}\) & 10 mm \\ CPR1 [61] & \(10^{8}\) & \(10^{3}\) & \(3\times 10^{7}\) s & \(10^{3}\) & 3 mm \\ P3 HT:OD [62] & \(5\times 10^{2}\) & \(10^{2}\) & \(10^{4}\) s & 25 & 50 mm \\ PMMA:P3 HT [63] & \(10^{5}\) & \(80\) & \(8\times 10^{3}\) s & N/R & 10 mm \\ PMMA:PCBM [64; 65] & \(10^{3}\) & \(10^{5}\) & \(1.2\times 10^{4}\) s & \(10^{4}\) & 10 mm \\ PI:PCBM [66] & \(10^{4}\) & \(2\times 10^{2}\) & \(10^{4}\) s & \(10^{2}\) & 10 mm \\ PS:PCBM [67] & \(10^{4}\) & \(43\) & \(10^{3}\) s & N/R & 4 mm \\ PAA:PEI [68] & \(50\) & \(2\times 10^{4}\) & \(10^{4}\) s & \(10^{4}\) & 2 mm \\ Rh B:R6 [69] & \(10^{3}\) & WORM & \(5\times 10^{3}\) s & \(10^{3}\) & 8.8 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of properties of organic dielectric materials. Figure 5: Nano porous (NP) structure of WO\({}_{3}\): (**a**) the current-voltage characteristics; (**b**) change of ON/OFF ratio after multiple bending cycles [54]. Compared with flexible inorganic materials, flexible organic materials show a little weakness in electrical properties, but better flexibility. An Al/CPR1/ITO device prepared by [PERSON] [61] has higher flexibility due to the hydrogen bond of CPR1. The favorable solubility and intrinsic flexibility of CPR1 allow for the large-scale fabrication of flexible CPR1 RRAM device arrays by full-printing technology with an endurance of \(10^{3}\) bending cycles at a minimum bending radius of 3 mm and a high ON/OFF ratio of \(10^{8}\). The Cu/pEGDMA/ITO/PET device prepared by [PERSON] [57] has a typical I-V characteristic curve (Figure 6a--1). Its retention time exceeds \(10^{6}\) s and its ON/OFF ratio reaches \(10^{2}\) (Figure 6a--2). It also maintains good storage performance at the bending radius of 4 mm (Figure 6a--3).