Researchers at Delft University of Technology in the Netherlands have created a soft robotic gripper that moves without a battery, motor, electronic controls or an external power source. The actuator uses the metabolism of living microalgae to convert light into mechanical bending, according to a study published in Nature Communications.
The device is a two-centimetre-long strip made from two bonded hydrogel layers. Each layer responds differently to changes in acidity, or pH. Microalgae grow on the surface of one of the layers, creating a biofilm after the strip is placed in a culture of the freshwater species Chlamydomonas reinhardtii for seven days.
When exposed to red and blue light, the algae carry out photosynthesis and consume nitrate from the surrounding nutrient solution. These processes remove hydrogen ions near the hydrogel, increasing the local pH from around 8 to as high as 11. The change causes chemical groups in the active layer to become negatively charged, absorb water and swell to more than four times their original weight. Because the two layers expand differently, the strip bends.
When the light is switched off, photosynthesis stops. The algae then metabolise stored sugars and release carbon dioxide, making the hydrogel more acidic again. The active layer contracts and the strip bends in the opposite direction, while the pH of the surrounding water changes only marginally.
To demonstrate the concept, the researchers produced an artificial flower, a four-fingered gripper and a hook. Under illumination, the gripper closed around a hollow metal ball weighing 18.4 milligrams. The hook held a 52.5-milligram rubber ring and released it after the light was turned off.
The biological drive is highly energy-efficient but slow. A measurable response begins 10 to 20 seconds after a change in lighting, yet a full 300-degree bend can take up to 18 hours. The university’s demonstration videos therefore use time-lapse speeds between 1,800 and 3,600 times faster than real time.
After 15 hours of illumination, the actuator reached a maximum force of 0.8 millinewtons, roughly equivalent to the weight of an 80-milligram mass. In the experiment, an external holder performed the upward movement needed to lift the objects. The actuator also has to remain in a liquid environment containing dissolved nutrients and would dry out in air.
The algae-based system remained operational for at least 60 days, longer than some earlier biohybrid designs based on animal muscle cells or bacteria. The researchers did not use genetically modified organisms. They suggest that the technology could eventually be useful in applications requiring slow movement and low forces, including light-controlled shading elements, agricultural systems and the handling of delicate samples underwater.
Sources: t3n.