While organic thin-film transistors built on flexible plastic have been around long enough for people to start discussing a Moore’s Law for bendable ICs, memory devices for these flexible electronics have been a bit more elusive. Now researchers from Tsinghua University, in Beijing, have developed a fully flexible resistive random-access-memory device, dubbed FlexRAM, that offers another approach: a liquid one.

In research described in the journal Advanced Materials, the researchers have used a gallium-based liquid metal to achieve FlexRAM’s data writing-and-reading process. In an example of biomimicry, the gallium-based liquid metal (GLM) droplets undergo oxidation and reduction mechanisms while in a solution environment that mimics the hyperpolarization and depolarization of neurons.

“This breakthrough fundamentally changes traditional notions of flexible memory, offering a theoretical foundation and technical path for future soft intelligent robots, brain-machine interface systems, and wearable/implantable electronic devices.”—Jing Liu, Tsinghua University

These positive and negative bias voltages define the writing of information “1” and “0,” respectively. When a low voltage is applied, the liquid metal is oxidized, corresponding to the high-resistance state of “1.” By reversing the voltage polarity, it returns the metal to its initial low-resistance state of “0.” This reversible switching process allows for the storage and erasure of data.

To showcase the reading and writing capabilities of FlexRAM, the researchers integrated it into a software-and-hardware setup. Through computer commands, they encoded a string of letters and numbers, represented in the form of 0s and 1s, onto an array of eight FlexRAM storage units, equivalent to 1 byte of data information. The digital signal from the computer underwent conversion into an analog signal using pulse-width modulation to precisely control the oxidation and reduction of the liquid metal.

These photographs show the oxidation and reduction state of the gallium-based liquid metal at the heart of FlexRAM.Jing Liu/Tsinghua University

The present prototype is a volatile memory, according to Jing Liu, a professor at the department of biomedical engineering at Tsinghua University. But Liu contends that the memory principle allows for the development of the device into different forms of memory.

This contention is supported by the unusual phenomenon that the data stored in FlexRAM persists even when the power is switched off. In a low- or no-oxygen environment, FlexRAM can retain its data for up to 43,200 seconds (12 hours). It also exhibits repeatable use, maintaining stable performance for over 3,500 cycles of operation.

“This breakthrough fundamentally changes traditional notions of flexible memory, offering a theoretical foundation and technical path for future soft intelligent robots, brain-machine interface systems, and wearable/implantable electronic devices,” said Liu.

The GLM droplets are encapsulated in Ecoflex, a stretchable biopolymer. Using a 3D printer, the researchers printed Ecoflex molds and injected gallium-based liquid-metal droplets and a solution of polyvinyl acetate hydrogel separately into the cavities in the mold. The hydrogel not only prevents solution leakage but also enhances the mechanical properties of the device, increasing its resistance ratio.

“FlexRAM could be incorporated into entire liquid-based computing systems, functioning as a logic device.”—Jing Liu, Tsinghua University

In the present prototype, an array of eight FlexRAM units can store 1 byte of information.

At this conceptual demonstration stage, millimeter-scale resolution molding is sufficient for the demonstration of its working principle, Liu notes.

“The conceivable size scale for these FlexRAM devices can range widely,” said Liu. “For example, the size for each of the droplet memory elements can be from millimeter to nanoscale droplets. Interestingly, as revealed by the present study, the smaller the droplet size, the more sensitive the memory response.”

This groundbreaking work paves the way for the realization of brainlike circuits, aligning with concepts proposed by researchers such as Stuart Parkin at IBM over a decade ago. “FlexRAM could be incorporated into entire liquid-based computing systems, functioning as a logic device,” Liu envisions.

As researchers and engineers continue to address challenges and refine the technology, the potential applications of FlexRAM in soft robotics, brain-machine interface systems, and wearable/implantable electronic could be significant.