Additive Manufacturing May be Key in Commercializing ‘Liquid Metal Ram’

A new approach to storage systems, achieved by researchers from Tsinghua University in China, allows for flexible memory without compromising the performance of electronic devices. Funded by the National Natural Science Foundation of China, the China Postdoctoral Science Foundation, and the Shuimu Tsinghua Scholarship Program, this research introduces “Liquid Metal Memory” in a recent publication in “Advanced Materials.”

Storage systems, being vital components of electronic devices, must become far more flexible as the world sees more advanced wearable electronics, biomedical devices, and soft robotics. These data storage systems must stretch, bend, and twist to extremes without affecting the emerging devices' performance.

Achieving flexible memory has been challenging due to the limitations of conventional storage methodologies. The latest study proposes a novel class of storage principles inspired by the human brain's polarization and depolarization mechanisms.

By introducing oxidation and deoxidation behaviors of liquid metals, the team achieved fully flexible memory. Researchers utilized reversible electrochemical oxidation to modulate the target liquid metals' overall conductivity, creating a significant 11-order resistance difference for binary data storage, as noted.

To obtain the best storage performance, systematic optimizations of multiple parameters were conducted. Conceptual experiments showed memory stability in extreme deformation scenarios, including 360° twisting, 180° bending, and 100% stretching. Further tests demonstrated better performance with smaller unit sizes of the memory.

The team concluded that their storage system achieves a swift storage speed of over 33 Hz and a long data retention capacity of more than 43200 s, with stable, repeatable operation up to 3500 cycles. These remarkable performance metrics indicate that the “groundbreaking method” can overcome the innate rigidness limitations of existing electronic storage units while paving the way for innovative neuromorphic devices.

Thus, liquid metal memory fundamentally alters traditional concepts of flexible memory, offering practical avenues for future applications in bio-inspired artificial intelligence systems, soft robotics, and wearable electronics.

An Unconventional Approach: Using a Liquid Metal

The increasing use of flexible devices means the demand for the deformable characteristics of memory will be growing, said Jing Liu, a professor in the Department of Biomedical Engineering at Tsinghua University in Beijing, in an interview.

The flexible resistive RAM device is dubbed FlexRAM and is developed with an unconventional approach — liquid. This liquid metal RAM stores information in a solution environment, much like how our brain does, which is about 70% water. 

By taking this biomimetic approach, FlexRAM sets itself apart from current memory systems, which are solid. The biomimetic approach, according to Liu, is similar to “the aqueous working environments found within living organisms.” 

So far, the flexibility of existing memory devices has been limited because they are usually created by laying down the inflexible memory components onto soft materials. This makes the devices flexible only partially and leads to peeling and cracking when the device is deformed. 

FlexRAM aims to change this by using an alloy composed of gallium and indium as the memory component to fabricate their storage device. Gallium-based liquid metals are an attractive material due to their excellent features, such as high electrical and thermal conductivity, low toxicity, and low viscosity with a fluidic nature at room temperature.

Inspired by the brain, the material goes through oxidation and reduction in a solution environment, much like our brain neurons. The neuron is polarized when the plasma membrane inside has a negative charge compared to the outside, and when any changes make it less negative, that's depolarization.

Additionally, the material maintains its liquid state at room temperature. This facilitates their oxidation to form a dense gallium oxide layer on the surface of the liquid. This layer of gallium oxide corresponds to a high electrical resistance state of the storage system and a low-resistance state of elemental gallium, the reduced form of the liquid. 

A high resistance ratio, the difference between the resistance of these two states, is essential to memory storage performance.

Achieving High Integration and Scalability

When it comes to performance, memory storage devices need to have a lot of characteristics, including energy efficiency, fast read and write speeds, high storage density, data retention, durability, and reliability. The problem comes in striking a balance among these aspects while maximizing the flexibility of the device.

So, to develop a device that can handle high levels of deformation, the team of researchers used a stretchable polymer called Ecoflex as its encapsulation material.

Then, the team utilized a 3D printer to print Ecoflex molds. 3D printing or additive manufacturing enables the production of complex objects. It allows the production of items that were not possible economically with traditional manufacturing. AM basically means creating three-dimensional objects by putting in layers of material in a computer-created design.

Due to its cost-effectiveness, 3D printing has made manufacturing accessible to the masses for the first time. Meanwhile, its ability to offer design flexibility and rapid prototyping makes this technique popular among scientists and researchers.

So, once the device was created, droplets of the gallium-based liquid metal were put into the mold cavities. To prevent the solution leakage, researchers also used droplets of the polyvinyl acetate hydrogel solution, which was injected separately due to its ability to increase the resistance ratio of the device and enhance its mechanical properties.

The size of the liquid metal droplet was of significance here as it considerably affects the high-resistance state/low-resistance state ratio in the device. A smaller droplet size leads to an increased ratio because of the amplified impact of the surface oxide film. So, the smaller the size of the droplet, “the more sensitive the memory response.”

Liu said:

“Reducing the droplet size benefits the integration and scalability of the FlexRAM, making fully flexible, high-density memory a promising option for diverse engineering developments.” 

Reading, Writing, & Storing Data

Now, when it comes to encoding data, FlexRAM does it through the oxidation and reduction processes of the liquid metal. 

So, the way it works is that the gallium-based liquid metal gets oxidized when a low voltage is applied. This gives it a high-resistance state of “1”. On reversing the voltage polarity, the liquid metal gets back to its initial low-resistance state of “0”. This reversible switching process allows memory to be stored and erased in the device.

To demonstrate FlexRAM's reading and writing capability, the researchers integrated the device into a software and hardware setup. By using computer commands, the team wrote a string of numbers and letters onto an array of eight FlexRAM storage units.

These letters and numbers were encoded in the form of 0s and 1s and corresponded to 1 byte of data information, which is far from consumer-grade memory capacity.

In the next step, the team used a technique called pulse-width modulation, which converted the digital signal from the computer into an analog. The technique allowed them to carefully control the oxidation and reduction of the liquid metal.

Then, the team applied a short 1-volt test voltage during information reading in order to measure the resistance state of the system without making changes to the redox state of the metal. The current is then transmitted to the computer, which converts the signal into 0 or 1 using an algorithm. Finally, the encoded message is displayed on an LED screen.

While the prototype is a volatile memory, the principle allows for the development of the device into different forms of memory.

This can be seen in the observation that the data stored in the device is found to persist even when the power is switched off. This could mean that the device has promise as a form of flexible storage and maybe beyond RAM. Liu noted:

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

The FlexRAM can further retain its data for up to 43,200 seconds or 12 hours in a low or no-oxygen environment. Also, the device can be used again and again while maintaining a stable performance for more than 3500 cycles. While a good start, it is nowhere near what traditional but non-flexible memory is capable of, which is in millions.

Vast Application Potential

While the device has demonstrated promising performance, its response time and level of integration are not up to commercial standards. This means there's still a need for improvements on several fronts, including the manufacturing procedure, which, as of now, involves filling the materials in sequence.

The team aims to use intelligent and automated manufacturing processes along with 3D airborne printing and packaging technology. 

Still, the tech is very young and will take years to be fully realized. Having said that, the proof-of-concept is encouraging, and this new approach has attracted interest from the industry with several liquid-based concepts being explored.

One such research was demonstrated a couple of years ago when two new liquid-based storage concepts were proposed — colloidal and electrolithic memory, which have the potential for extremely high-density nearline storage applications.

Again, by taking inspiration from advances in life sciences, the storage medium for making a dense array of access devices was proposed to be a liquid containing ions, molecules, or (nano-)particles, which can be manipulated in larger volumes to an access device that is part of a dense array.

IMEC, an R&D and innovation hub in nanoelectronics and digital technologies, foresees the introduction of liquid memory from 2030 onwards. It anticipates that with these approaches, the bit storage density can be pushed towards the 1Tbit/mm2 range at a lower process cost per mm2. It also noted that for these storage solutions to be viable for nearline applications, the technology must have adequate response time, energy consumption, bandwidth (e.g., 20Gb/s), cycling endurance (103 read/write cycles), and the ability to retain data over a decade.

In another instance, back in 2020, researchers got a charge out of liquid metal batteries. Here, the salt electrolyte, metal anode, and cathode were all in liquid form. Compared to solid-state batteries, liquid metal batteries benefit from the fast diffusion of ions between electrodes, which means rapid charge-discharge cycles. 

Moreover, mechanical stresses are much less, and it removes the need for membranes and separators while enhancing long-term stability and utility. The research stated that liquid metal batteries, though heavy, are non-flammable and could be more suitable for large-scale electricity storage.

Most recently, scientists discovered a liquid metal-based composite that enables robust electrical and mechanical connections between soft circuitry and rigid electrical components. The researchers hope to have this material, called E-CASE, which is an electrically conductive adhesive with silver and eutectic gallium-indium (EGaIn), play a role in electronics, robotics, and sensors.

So, as researchers address challenges and refine the technology, FlexRAM can also find its use in implantable electronics, soft robotics, and brain-machine interface systems in the future.

Additive Manufacturing Companies 

#1. Materialise

The Belgium-based 3D printing services provider serves a range of industries, including automotive, aerospace, and health care. Over the past couple of months, Materialise entered into several partnerships, including with Ricoh USA to promote the use of 3D-printed anatomic models, with Proponent to 3D print cabin solutions for aircraft, Nikon SLM Solutions to develop advanced Build Processors, and with Ansys to simplify simulation for 3D printing.

With a market cap of $329 million, the share of Materialise (MTLS:NASDAQ) has been trading at $5.57, down 15.16% YTD. The company posted a revenue (TTM) of $272 mln and has an EPS (TTM) of 0.05 and a P/E (TTM) of 116.53. The company reported a 3.2% increase in its total revenue to $63.6 mln from the previous year during its 3Q23 earnings report while its EBITDA saw a 55% jump and net profit increased by 184% to $4.2 million.

#2. EOS GmbH

Germany-based EOS GmbH is a leading industrial 3D printing manufacturer that has launched an FDR technology that enables the production of fine details without sacrificing quality. Meanwhile, the company's Smart Fusion eliminates support structures, lowers cost, minimizes material use, and reduces post-processing requirements. Its new systems further allow a fully automated solution that scales in line with the production needs. 

Besides EOS GmbH and Materialise, 3D printing companies such as Stratasys, GE Additive, Desktop Metal, Formlabs, and Renishaw can help in commercializing Liquid Metal Ram. Meanwhile, the likes of Soft Robotics, Shadow Robot Company, Neuralink, CTRL-labs, BrainGate, Apple, and Samsung can benefit from this new approach to storage systems.

Final Word

Liquid Metal Ram's ability to withstand nearly any deformation promises a great future for electronic devices, further enriching our lives. However, they are still in an early stage, with more research and work needed before they can be commercialized. 

Here, additive manufacturing can play a key role by allowing for customized designs and better integration of different components for enhanced performance and reliability. Moreover, it allows for rapid prototyping, enabling researchers and companies to make quick improvements while reducing waste, offering scalability and on-demand production.

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