Flexible, self-healing, and degradable polymeric dielectrics cross-linked through metal–ligand for resistive memory device

The ability to self-heal is a crucial feature in nature, where living organisms can repair themselves when subjected to minor injuries. With an increasing emphasis on environmental sustainability, the concept of biomimetic self-healing polymeric materials has emerged as a prominent trend, promising to significantly extend the lifespan and reliability of products. Studies have shown that one-third of proteins in living organisms require metal cofactors to function properly. It is known that protein-metal interactions can enhance the performance of certain biomaterials, and different choices of metals and ligands can create diverse material properties, influencing characteristics such as hardness, toughness, adhesion, and self-healing abilities. Gelatin is a natural polymer derived from the hydrolysis of collagen, and its unique amino acid structure has led to a wide range of applications. In this research, by introducing aluminum ions that form metal coordination complexes with the carboxyl groups in gelatin, an elastic network with self-healing properties was constructed. This gelatin-based material was utilized as an insulating layer in resistive switching devices. Furthermore, by employing a gelatin substrate of the same composition, the device demonstrated strong interfacial adhesion. The device based on the self-healing gelatin film exhibited excellent electrical performance and mechanical properties. Even after self-healing, it maintained a high ON/OFF ratio of up to 105 and a concentrated distribution of switching parameters. Supported by compelling physical and electrical evidence, this study showcases significant development opportunities for biomimetic materials in green electronic devices.


Introduction
Wearable technology with entertainment and healthcare functions shows great potential to change people's lifestyles.The application of wearable technology brings a wealth of new market opportunities for electronic products, including physiological parameter monitoring and electronic clothing that can dynamically adjust its material properties according to the temperature, humidity, and the needs of the wearer [1].Since many of these applications also require computer memory for data storage, flexible nonvolatile memory devices are more and more desirable in wearable devices.
Although the research of flexible resistive switching random-access memory (RRAM) devices has made great progress, to meet the higher requirements of high flexibility, environmental sustainability, and high durability, there is an urgent need to design biodegradable and self-healing materials with better flexibility, due to material cracks are the main cause of device failure under severe bending conditions.
Self-healing materials that can restore their physical properties after large deformations caused by long-term use can improve the lifetime of flexible electronics.Currently, self-healing property has been integrated into various electronics [2].Haick and Huynh designed self-healing chemical-resistor to sense pressure, temperature, and VOC.Although this self-healing chemiresistive electrode heals within 10 min and achieves complete mechanical healing after 30 min, the conductivity cannot achieve 100% recovery [3].Susan A Odom and her team separately encapsulated non-conductive charge-transfer salt precursor solutions, namely tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ), into poly(urea formaldehyde) core-shell microcapsules.When mechanical damage occurs, both the TTFcontaining and TCNQ-containing microcapsules rupture, allowing the repair agents to mix within the damaged area.This interaction results in the formation of high-conductivity crystalline salts, effectively restoring the material's electrical conductivity.However, the microcapsule-based self-healing mechanism typically allows for only one healing event.
In addition, currently, self-healing electronic devices are manufactured and operated in specific experimental environments.However, many polymers, during actual operation, may face various environmental influences leading to internal short circuits and corrosion, resulting in poor electrical performance.Therefore, new design concepts for selfhealing electronic will be necessary to improve their electronic properties.
In this research, the introduction of dynamic non-covalent bonds into the gelatin polymer matrix imparts a self-healing capability to the cross-linked network.These reversible dynamic non-covalent bonds are known as metal coordination bonds and are typically composed of amino acid residues and coordinated metal ions.Metal coordination bonds occur when one or more chemical groups act as ligands by providing lone pair electrons to the vacant orbitals of metal ions [4].These protein-metal interactions exhibit significantly greater bond strengths than typical non-covalent interactions such as hydrogen bonds and van der Waals forces.However, they remain considerably less stable, possessing rapid bond kinetics that render them more unstable than covalent bonds [5].Due to the kinetic instability of metal-ligand bonds, they allow for rapid reformation within a short period.This dynamic and reversible cross-linking approach enables the unique material self-healing properties.
This self-healing mechanism not only enables multiple healing cycles but also research indicates that electronic devices after healing exhibit excellent electrical performance and can achieve nearly complete recovery, opening up new applications for chemically modified biopolymer materials.

Substrate preparation
To prepare a gelatin solution with a concentration of 25% (extracted from porcine skin, referred to as GP), 13.33 g of gelatin powder were added to 40 ml of deionized water.The mixture was thoroughly dissolved using a magnetic stirrer heated at 60 • C and then allowed to stand to remove bubbles.The gelatin solution was poured into polystyrene Petri dishes and placed in a refrigerator until it solidified.Subsequently, after approximately one day of cooling, a flexible film could be obtained.This film is characterized as firm, smooth, and exhibits excellent optical transparency.

Device preparation
The GP substrate was cut into sizes of 2 cm × 3 cm.Subsequently, we utilized conductive silver paste as the bottom electrode, sourced from SPI.Initially, it was diluted in alcohol and then uniformly coated onto the prepared GP substrate.A 2% gelatin solution was prepared using gelatin powder provided by Sigma.Aluminum nitrate powder (purchased from Kodak) was added to the gelatin solution in two different concentrations, 0.1 and 1 M, by adding 0.375 and 3.75 g of powder, respectively, calculated on a molar basis.Then, pure gelatin film (PG) and aluminum nitrate-gelatin films (ACG01, ACG1) were deposited via the spin-coating method, and the samples were placed in a cyclic oven and baked at 60 • C for 30 min.Finally, indium was cut into approximately 2 mm × 2 mm indium beads, melted with a soldering iron, and attached to the device as the top electrode, forming the In/PG/Ag/GP, In/ACG01/Ag/GP, In/ACG/Ag/GP structures.

Instrumentation
The transparency of the samples was confirmed by Ultraviolet-Visible Absorption Spectroscopy (UV-vis) using the Perkin-Elmer Lambda 35.Nanoindentation analysis was conducted using Nano-Indentation System I, MTS XP.Fourier Transform Infrared Spectroscopy (FTIR) was carried out with the Thermo Scientific Nicolet 6700.The electrical properties were characterized by a Keithley 2636B system source meter.

Results and discussion
Figures 1(a)-(c) presents UV-vis spectra analysis of PG, ACG01, and ACG1 films.It can be observed that, in the wavelength range of visible light, approximately 380-760 nm, the maximum transmittance of all three films exceeds 90%.This indicates that the implantation of aluminum ions does not alter the original transparency of the films, as aluminum ions are transparent and colorless.
FTIR spectroscopy is an essential tool for studying functional groups and molecular structural changes and was therefore employed to confirm the functional groups in PG and ACG films.In figure 2, characteristic peaks at 667 and 1081 cm −1 are attributed to primary amine groups [6].Additionally, with an increase in the added aluminum ion quantity, the absorbance of carboxyl groups at 1715 cm −1 decreases, while the absorbance of carboxylate salts at 1600 and 1380 cm −1 increases.The peaks split into two at 1600 and 1380 cm −1 , corresponding to the asymmetric and symmetric stretching vibrations of carboxylate groups, indicating that carboxylate  groups coordinate with aluminum ions in a monodentate manner, involving only one oxygen atom, rather than in a bidentate or chelating manner [7].This characteristic peak is more pronounced in the ACG1 film, suggesting that most carboxyl side groups are deprotonated to carboxylate salts.These findings indicate that the self-healing properties of ACG01 and ACG1 films are primarily based on metal-carboxylate bonds between aluminum ions and carboxylate salts.Metallic carboxylates were formed when metal atoms were substituted for hydrogen atoms in the carboxyl group.These reversible dynamic non-covalent bonds are known as metal coordination bonds and usually consist of an amino acid residue and the coordinating metal ion.Due to the coordination complexes being products of neutral molecules or anions (called ligands) forming coordination bonds with central metal atoms (or ions), these could be further categorized as hard, soft, or borderline acids.In this research, the aluminum ions that were used were classified as hard bases, while the carboxylate ions were identified as soft acids.Nevertheless, the kinetically unstable nature of such bonds were allowed them to recombine rapidly in a short time, which enabled to possess material self-healing properties [8].
Figures 3(a   contrast, ACG01 and ACG1 memory devices with added aluminum ions display markedly higher resistance switching, with both ON/OFF ratio exceeding 10 3 .It is worth noting that ACG1 film-based device shows current value instability during the high resistance state (HRS), which affects the ON/OFF ratio.According to the statistical chart results, the ON/OFF ratio of ACG1 device exceeding 10 3 is only about 61.43% yield, while that of ACG01 device is as high as 88.89%, as shown in figure 3(d).
Studies in this domain have revealed a discernible association between the ON/OFF ratio and the proportion of carbon sp 3 to carbon sp 2 within the insulating layer.A higher sp 2 /sp 3 ratio leads to higher current and lower overpotential, thereby affecting the ON/OFF ratio [9,10].This results in ACG01 exhibiting superior electrical performance.XPS analysis was employed to investigate the chemical bonds in PG and ACG films, as depicted in figure 5.The C 1s spectra of PG and ACG films with different concentrations were analyzed, and the spectra were divided into four component peaks corresponding to C=C, C-C, C-O, and C=O.In particular, a C-C single bond is composed of σ bond, while a C=C double bond is composed of one σ bond and one π bond.Generally, the π bond in the C=C double bond is weaker than the σ bond because its electron cloud is further away from the positively charged atomic nucleus.This results in the C-C bond having higher bond energy than the C=C bond.Therefore, the presence of sp 3 bonds in carbon (C-C) can yield high-resistance materials, while the presence of sp 2 bonds in carbon (C=C) can lead to low-resistance materials [11].The observed C-C/C=C ratios for PG, ACG01, and ACG1 films were 1.07, 1.96, and 1.66, respectively.Thus, the lower current in the HRS contributes to ACG01 having a larger ON/OFF ratio.
To further investigate the influence of the metal ions concentration on the flexibility of the films, we have performed bending tests on the PG, ACG01, and ACG1 memory devices and examined the time taken for that self-healing.As shown in figure 6, the self-healing of the ACG01 device proves significantly more efficient than that of the other devices, with that device can recover to ON/OFF ratio of 10 5 after 40 min.In contrast, the ACG1 resistive device shows the ON/OFF ratio of less than 10 2 for the same self-healing time, and that of PG lacks selfhealing properties, which is attributed to the fact that a suitable increase in the ionic concentration could significantly impact the electrical performance.The dominance of ionic clusters in the material reduces molecular mobility, which markedly inhibits the healing response [12].In ballistic self-healing experiments conducted by Kalista et al [13], materials with high content of ions exhibited inconsistency, suggesting that high concentrations of ions lead to efficient coordination of metal ions, which could be detrimental to healing.Furthermore, as mentioned in other studies [12], the intercalation between polymers and dopants might be compromised because of strong interactions in the polymer main chain, which leads to self-aggregation of the dopants and thus hinders charge transfer.However, the interference of small amounts of dopants in the doping reaction with the polymer molecular stacking is negligible.Therefore, appropriate ion concentration could enhance the self-healing capability, whereas excessive  concentration could obstruct the self-healing process.
Figures 7(a) and (c) present voltage-current curves for PG and ACG01 devices measured at room temperature before bending, after 20 cycles, and after 100 cycles of cyclic bending, with a bending radius of 5.5 mm.The ON/OFF ratio for the PG device before bending is approximately 10 2 , while for ACG01, it reaches 10 4 -10 5 .No significant resistance change is observed for either device after bending.Figures 7(b)  and (d) show the I-V curves of the two devices after 40 min of healing.It is evident from the graphs that the PG film does not exhibit healing characteristics, whereas ACG01, even after 20 and 100 cycles of bending, demonstrates multiple instances of healing.The ON/OFF ratio after healing exceeds 10 4 , indicating a high degree of healing, with the number of healing cycles not affecting the electrical performance.The self-healing behavior of the ACG01 thin film after undergoing severe bending at room temperature, as shown in figures 8(a)-(c), illustrates that the surface cracks of the ACG01 film could achieve virtually complete healing after approximately 40 min.
Figure 9(a) illustrates the I-V curve of the In/ACG01/Ag/GP device after healing, obtained  through a bidirectional voltage sweep from 3 to −3 V, revealing an impressive ON/OFF ratio of 10 5 .The device initially resides in a HRS, transitioning to a low-resistance state (LRS) at −0.8 V during voltage scanning, a process known as 'set.'The LRS persists until the voltage scan reaches 1.45 V, at which point the device returns to HRS, termed 'reset.' figure 9(b) demonstrates that even after 300 cycles of bidirectional voltage scanning, the device maintains an ON/OFF ratio of 10 4 -10 5 and stable set and reset voltages.Figure 9(c) illustrates the cumulative plots of HRS and LRS currents, as well as set and reset voltages, measured at 0.1 V after 40 min of healing for the device.The coefficient of variation (CV), defined as the ratio of the standard deviation to the mean, serves as a quantitative indicator for evaluating the uniformity of device performance.Consequently, it is evident that ACG01 device exhibits more stable current values in both HRS and LRS, with a more concentrated distribution of set and reset voltage.In figure 9(d), it is further demonstrated that ACG01 device maintains memory retention for over 3000 s in both LRS and HRS, underscoring the superior stability of the device.
Figures 10(a) and (b) show SEM images of ACG01 samples that have not undergone bending tests and those that have been subjected to bending, revealing no apparent cracks at the interface junction.Figure 10(c) illustrates that the thickness of the gelatin insulating layer is approximately 34 nm.In figure 10(d), the ON/OFF ratio measured at different bending radii is depicted.Even at a bending radius of 13 mm, the ON/OFF ratio remains within the range of 10 3 -10 4 , while at a bending radius of 10 mm, the ON/OFF ratio slightly decreases to the range of 10 1 -10 2 .The selection of a homogeneous gelatin substrate enhances the structural strength of the device, providing robust interface adhesion and demonstrating improved performance for the RRAM device.
To investigate the current conductive mechanism of device in more detail, the experimental data underwent fitting analysis, as shown in figure 11(a).The LRS current exhibits a slope of approximately 1, indicative of Ohmic contact behavior, suggesting the formation of filament paths during the 'set' process.The fitting results of the HRS current can be categorized into three distinct stages.In the first stage, corresponding to the low voltage region, the current shows a linear relationship with the voltage, exhibiting a slope of about 1.1, indicative of Ohmic contact behavior.Moving on to the second stage, known as the transition region, the current exhibits a quadratic dependence on the voltage, with a slope of around 1.9, consistent with Child's law.Finally, in the third stage, a rapid increase in current is observed, characterized by a slope of approximately 10.3.The threestage current variation of the device is consistent with the space charge limited conduction (SCLC) model [14,15], further corroborating the underlying current conduction mechanism related to the formation of localized filament paths.To further explore the conduction mechanism of ACG01 device, we have analyzed the trend of the LRS current with temperature at a voltage of 0.1 V.As shown in figure 11(b), the relationship between the resistance of the LRS and temperature in the range of 300-340 K is depicted, measured by applying a voltage of 0.1 V.It is observed that  the resistance decreases with increasing temperature, attributed to typical semiconductor behavior [16,17].Furthermore, during the operation of the resistive switch, in the LRS, the dehydrogenation reaction assists in the formation of conjugated double bonds in the carbon layer, yielding the conductive sp 2 carbon filaments [18].In performing the reset process, which is also known as the HRS, the conductive sp 2 carbon filaments are transformed into insulated sp 3 carbon filaments through the hydrogenation reaction.It is confirmed that the ACG01 device has realized the resistive switching characteristic through the conversion between sp 2 and sp 3 carbon filaments.
The significant impact of adding aluminum ions on the formation of carbon filaments is evident in figure 12(a).Carbon atoms form a conjugated structure by sharing π electrons, enabling the movement of electrons throughout the entire molecule.The original form of conjugated polymers lacks inherent charge-carrying carriers, thereby restricting the formation of conductive filaments.Low carrier mobility is a critical factor limiting the performance and applicability of organic semiconductor devices.Consequently, external carriers must be provided, and this can be achieved through the charge transfer process [19].The application of a bias voltage could induce aluminum ions to accept or supply electrons, which leads to the redox reaction in which the aluminum ions are oxidized and electrons are released, promoting the transfer of electrons at the interface between the aluminum and gelatin film layers.However, increasing the concentration of charge carriers contributes to the more stable formation of conductive filaments.Figures 12(b) and (c) demonstrate that the introduction of metal ions resulted in a significant improvement in the CV value, which enhanced the uniformity.

Conclusions
By modifying gelatin films through aluminum ion implantation, the modified gelatin films retained their inherent advantages and successfully exhibited self-healing properties at room temperature.This phenomenon can be attributed to the dynamic metal coordination bonds between aluminum ions and carboxylate groups in the gelatin.In this study, the self-healing gelatin films were applied to RRAM devices, fabricated on biodegradable substrates, thereby creating highly biodegradable green electronic components.
Comprehensive analyses of the devices were conducted to evaluate the impact of different ion concentrations on the self-healing time and electrical performance of the gelatin films.The composition and functional groups of films were characterized using FTIR and XPS analyses.Among the devices, the structure denoted as In/ACG01/Ag/GP exhibited outstanding self-healing properties and electrical performance, demonstrating an impressive self-healing capability, with an average recovery of ON/OFF ratios reaching 10 4 -10 5 after only 40 min of self-healing.Even under a bending radius of 13 mm, the device maintained ON/OFF ratios between 10 3 and 10 4 , with retention times exceeding 3000 s.Notably, the device maintained stable ON/OFF ratio, set voltages, and reset voltages even after 300 operation cycles, showcasing exceptional switching performance and reliability.
The advantages of using a low-temperature and solution-based fabrication method include environmental friendliness, convenience, and costeffectiveness.Additionally, the self-healing properties extend the device's lifespan, offering new opportunities for the development of green electronic components.
)-(c) illustrate the structure diagrams based on PG, ACG01 and ACG1 resistive device, respectively.The resistive switching properties were observed by applying voltage sweep from 3 → −3 → 3 V.The statistical evaluation of the cyclic measurement data indicates that the PG-based device exhibits insignificant resistive variations.In

Figure 6 .
Figure 6.The graph depicting the variation of the ON/OFF ratio over time.

Figure 7 .
Figure 7.The I-V curves of (a) In/PG/Ag/GP and (c) In/ACG01/Ag/GP devices as unbending, 20 times bending and 100 times bending.The I-V curves of (b) In/PG/Ag/GP and (d) In/ACG01/Ag/GP devices as 20 times healing and 100 times healing.

Figure 9 .
Figure 9. (a) I-V switching characteristics of the In/ACG01/Ag/GP device.(b) I-V curves of the In/ACG01/Ag/GP device for 1 st , 50 th , 100 th , 150 th , 200 th , 300 th cycle measurement after healing.(c) Cumulative plots and (d) retention test for the In/ACG01/Ag/GP device.

Figure 10 .
Figure 10.SEM images of (a) ACG without bending and (b) ACG after bending.(c) The cross-sectional image of thickness of the gelatin film.(d) The ON/OFF ratio of the In/ACG01/Ag/GP device at various bending radii.

Figure 11 .
Figure 11.(a) I-V switching characteristic of the In/ACG01/Ag/GP device.(b) The variation of LRS current between 300 and 340 K.

Figure 12 .
Figure 12.(a) The schematic diagram for mechanism of resistive switching.Cumulative plots of the (b) In/PG/Ag/GP and (c) In/ACG01/Ag/GP devices.