SunLightBots: Turning sunlight into self-sustained, high-power motion in soft robots
TL;DR Subgram, untethered soft robots run on continuous simulated sunlight by letting a light-heated liquid crystal elastomer slowly load a snap-through beam that releases the energy at once and then resets itself by shading. The mechanism reaches a peak of 89 W/kg near 1 Sun and drives a swimmer that advances 3.1 body lengths per cycle under 1.5 Suns.
What they do
Light-driven soft robots harvest energy at low power, but jumping or fast propulsion needs high power. SunLightBots are subgram, untethered robots without electronics that run under continuous simulated sunlight near 1 Sun (100 mW/cm²). A self-resetting snapping mechanism stores the slowly harvested energy and releases it at once, then resets itself, so the robots keep moving as long as the light is on.
How it works
- Self-resetting snap. A light-heated liquid crystal elastomer (LCE) strip contracts and loads a curved silicone beam until it snaps through. The snap closes shutters that shade the LCE; it cools, relaxes and the beam snaps back, reopening the shutters.
- Material tuning. Formulations across two chemistries were screened for low actuation temperature and high stress; the chosen one starts actuating near 60 °C. 1.5 wt% silver and 1.5 wt% copper microparticles gave the fastest photothermal heating.
- Energy barrier. Passive-layer stiffness trades off trigger threshold against stored energy, measured by the height a 16.5 mg steel ball is launched by the snap.
Results
- Peak mass-specific mechanical power of 89 W/kg near 1 Sun, with 0.1 MPa actuation stress at about 60 °C.
- Swimmer with an asymmetric paddle: 3.1 body lengths per cycle under 1.5 Suns.
- Crawler with two tilted legs: one body length under 1.5 Suns.
- Jumper with a stiffer Dragon Skin 30 layer: jumps at about 1.9 Suns (their earlier design needed 6 Suns), more than 20 consecutive jumps with no measurable drop in height.
Limitations
Average speed is limited by slow heating and cooling: the snap is fast, but the cycle rate is set by harvesting and heat transfer. The robots have no steering, sensing or navigation, and were tested under a solar simulator. The authors note that long-term durability and robustness to changing irradiance, angle, temperature and weather still need work.
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Abstract
Soft robots built from active materials can directly convert environmental stimuli into motion, offering a route to energy autonomy and embodied control. However, environmental energy is generally harvested at low power, whereas rapid, mechanically demanding actions require high mechanical output power. Reconciling this mismatch while sustaining self-regulated cyclic actuation is challenging, particularly at the subgram scale, where limited payload capacity constrains the integration of conventional power and control hardware. Here, we present SunLightBots, a class of subgram, sunlight-powered, untethered soft robots that achieve autonomous locomotion under continuous illumination near 1 Sun (100 mW/cm2). Their operation is enabled by a self-resetting snapping (SRS) mechanism that integrates liquid crystal elastomers (LCEs) with snap-through instability to accumulate solar energy supplied at low power, release it as rapid, high-power motion, and reset through photothermal-mechanical feedback. Through coordinated optimization of the LCE actuation temperature, photothermal conversion, actuation stress, and structural energy barrier, the system operates near 1 Sun and achieves a peak mass-specific mechanical power of 89 W/kg. At this irradiance, the optimized LCE generates an actuation stress of 0.1 MPa at approximately 60 deg C. SunLightBots demonstrate sustained swimming, crawling, and repeated impulsive jumping. Under 1.5 Suns, the swimmer advances more than three body lengths per cycle. This work provides a materials-mechanics route toward autonomous soft robots powered entirely by the environment.
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