Hydrogel Learns to Play Pong Over Time

A small, squishy blob of transparent gel can do more than play the video game Pong: it can improve its performance with time.

Connected to a modified version of the game through an electrode array, the straightforward polymer hydrogel showed a quantifiable improvement in accuracy, allowing it to sustain longer rallies. The result reveals a capacity for memory in an exceptionally simple material.

Although the gel is plainly a long way from being an artificial brain, this newly identified capability points towards intriguing directions for future research and development.

"Our research shows that even very simple materials can exhibit complex, adaptive behaviors typically associated with living systems or sophisticated AI," explains biomedical engineer Yoshikatsu Hayashi of the University of Reading in the UK.

"This opens up exciting possibilities for developing new types of 'smart' materials that can learn and adapt to their environment."

Electro-active polymer hydrogel and its memory

The hydrogel is made from an electro-active polymer, or EAP. These polymers alter their size or form when subjected to an electric current, and are widely employed in actuators and sensors as a type of artificial muscle.

In 2022, researchers showed that a clump of human brain cells in a dish could be trained to play Pong. They gave the clump feedback indicating whether it had successfully struck a pixelated "ball" using a pixelated "paddle".

Biomedical engineers Vincent Strong, William Holderbaum and Hayashi, all from the University of Reading, set out to investigate whether an ability resembling learning could also be found in something vastly simpler than human brain tissue.

EAP hydrogel was an obvious candidate for the experiment. Ions - charged particles - move through the hydrogel's network of crosslinked polymer chains when an electric current is applied, causing the gel to change shape.

In earlier work, Hayashi and a separate team showed that this effect could be used to make hydrogel pulse in time with a pacemaker, expanding and contracting in the manner of a beating heart.

During that research, they found that their polyacrylamide hydrogel preserved a "memory" of the pulsation even once the pacemaker had been switched off.

"The rate at which the hydrogel de-swells takes much longer than the time it takes for it to swell in the first place, meaning that the ions' next motion is influenced by its previous motion, which is sort of like memory occurring," Strong explains.

"The continued rearrangement of ions within the hydrogel is based off of previous rearrangements within the hydrogel, continuing back to when it was first made and had a homogeneous distribution of ions."

How the hydrogel played Pong

To take the hydrogel a stage further, the team created a dedicated interface and an adapted Pong game. It used a single paddle that rebounded the ball from the far wall of a digital court, similar to playing table tennis against a wall.

Electrical stimulation conveyed the ball's randomised location to the gel, while the researchers tracked ion flow to determine the paddle's position. They also recorded the duration of every rally - the uninterrupted exchanges between the gel-operated paddle and the wall - and discovered that the rallies lengthened as time passed.

The gel needed roughly 20 minutes to attain its highest Pong skill level.

"Over time, as the ball moves, the gel gathers a memory of all motion. And then the paddle moves to accommodate that ball within the simulated environment," says Strong. "The ions move in a way that maps a memory of all motion over time, and this "memory" results in improved performance."

What the hydrogel's improved performance means

According to the researchers, this memory indicates an emergent capability: one for which the material was neither deliberately designed nor trained. That does not imply that the material is sentient or acting with intent. Rather, it has retained the trace of a physical effect, much as the skin on a cheek can hold the impression left after resting on a creased pillow.

Nevertheless, the result remains compelling and creates several promising paths for investigation. These include identifying the mechanisms responsible for the memory and establishing whether the hydrogel can be trained for other jobs.

"We've shown that memory is emergent within the hydrogels, but the next step is to see whether we can also show specifically that learning is occurring," Strong says.

The research was published in Cell Reports Physical Science.