Biocompatible Lactulose based Resistive Random Access Memory for Bio-Implantable Electronics

The growing interest in digital healthcare has spurred research into the development of bio-implantable devices capable of detecting various biosignals for convenient daily use. Non-volatile memory is an important component of these bio-implantable devices, especially the resistive random-access memory (RRAM) device, which is attracting attention due to its low power consumption, fast switching, and low fabrication cost. In general, RRAM for processing biosignals is often made of inorganic materials, which are not suitable for bio-implantable devices that require biocompatibility. Therefore, numerous studies on organic-based RRAM, which offers several advantages over inorganic-based RRAM, including biocompatibility, low power consumption, and high flexibility, are in progress.<br/>To fabricate bio-implantable organic-based RRAM, we propose Lactulose as a suitable material for the switching layer of RRAM among biocompatible organic materials that are not harmful to the human body. Lactulose has proven its efficacy and safety, as it is registered on the World Health Organization (WHO) list of essential drugs and has the unique characteristic of not being digested or decomposed in the stomach or small intestine. Based on these properties, we determined that Lactulose could be used as a switching layer of bio-implantable RRAM that has a characteristic of biocompatibility.<br/>In order to make a Lactulose -based RRAM device, a series of processes are required. The first step is to prepare a Lactulose solution, which is made by dissolving Lactulose powder (Sigma-Aldrich, USA) in deionized (DI) water as a solvent with a concentration of 3 wt%. Lactulose is completely dissolved by stirring the solution for 24 hours. Simultaneously, prepare a p⁺-Si wafer to deposit the solution. After cleaning process, the wafer undergoes hydrophilic treatment through O2 plasma to help the Lactulose solution adhere well to the wafer’s surface during the spin-coating process. The next step is to deposit the Lactulose solution on the prepared wafer through spin-coating at 3,000 rpm for 30 s. After deposition, the sample is annealed at 80 °C for an hour. Finally, the top electrode, Magnesium (Mg), is deposited on the device after annealing is complete. Since the switching layer is an organic material, an E-beam evaporator is used to minimize damage to the switching layer, and an Mg thin film of about 120 nm is deposited. This results in the fabrication of a two-terminal RRAM having a p⁺-Si /Lactulose/Mg structure.<br/>Fabricated Lactulose-based RRAM had stable bipolar resistive switching properties with a set/reset behavior between the LRS and HRS. When a voltage sweep is applied to a fabricated Lactulose-based RRAM to check the current change, a set behavior was observed at approximately 3.1 V under positive bias, and a reset behavior was observed at approximately -2.8 V under negative bias. The conversion between the LRS and HRS was also verified through the log-scale I-V curve, which indicated that the switching mechanism of the device is likely due to the formation and rupture of conductive filaments in the switching layer. The device also exhibited a stable resistive switching window over 250 cycles without degradation when resistive switching operations in the LRS and HRS regions were performed. The device also shows a retention time of over 8,000 s. Through these endurance and retention results, it was confirmed that Lactulose-based RRAM exhibits high reliability as a memory device. Therefore, the results suggest that Lactulose can be a strong candidate for a bio-implantable RRAM with its performance. Building upon these characteristics and stability, Lactulose-based RRAM demonstrates its potential for use as bio-implantable devices.