KEY CONCEPTS
• Polymer gels use a process known as transient de-swelling to engage in self-autonomous propulsion.
• The movement occurs because solvent evaporates at different rates from different areas of the polymer gel. 
• Self-propulsion is created because different curvature geometries are produced as the polymer strip starts moving. 

The demand for developing strategies for generating energy in a sustainable manner is moving researchers to figure out how to configure devices that can utilize their working environments. Productivity and efficiency gains can be made while emissions can be limited.

In a previous TLT article,¹ a device known as a metal-air-scavenger was developed that extracts energy from metal present in its environment. The researchers took advantage of the energy stored in the chemical bonds of metal, which are higher than for natural sugars (such as glucose). When used to power an electric vehicle, the metal-air-scavenger combined the energy density found in lithium-ion batteries with the extraction ability of energy harvesters.

Another approach that can be used is self-propulsion through the use of a power-amplified system. This method is widely used by many organisms in nature, according to Al Crosby, professor of polymer science and engineering in the College of Natural Sciences at the University of Massachusetts Amherst in Amherst, Mass.

Crosby says, “Power amplification is developed through a latch mediated spring actuation system by organisms such as the Venus flytrap. These systems overcome typical tradeoffs of actuators, like muscles, which can either move fast and exert little force or move slowly and exert large forces. Their approach is to use slow actuators to store large forces in springs and to engage a latch to keep the spring energy sorted until needed. Once triggered, the latch releases and the spring recoils at speeds and forces much greater than the actuator can exert.”

Crosby gives an example of how a human being takes advantage of a similar mechanism when using a bow to shoot an arrow. He says, “The person stores energy in the bow, which serves as a spring, and when they release their finger (or latch) the bow recoils to shoot the arrow great distances.”

Many of the strategies used by organisms to store energy involve regulating osmotic pressure through the transport kinetics of water. This means that a likely candidate to emulate this approach might be a polymer gel that can readily absorb a solvent.

Crosby indicates that polymer gels, shaped to undergo snap instabilities, can serve as an effective latch. They have been evaluated for this effect but no one, to date, has developed a self-regenerative strategy that can be done autonomously without the need of an external force such as a motor. He says, “In most cases, the polymer gel will conduct a once through snapping process to move but is limited unless a motor is available for reengagement.”

A new approach has now been undertaken to enable polymer gels to self-propel through their environment by taking advantage of evaporation.

Transient de-swelling
Crosby and his colleagues determined that polymer gels can move in a self-autonomous manner through a process known as transient de-swelling. He says, “Polymer gels de-swell or lose their solvent to evaporation. The typical thinking is that this process is uniform because the polymer gel retains the same shape, after complete de-swelling. In actuality, the solvent evaporates from the polymer gel at different rates depending on location: more solvent evaporates along edges as compared to the faces of a polymer gel for the same amount of time. This means that the polymer gel changes shape during the process.”

As the solvent evaporates, the researchers noticed that thin polymer gel strips would undergo successive snapping motions. The movement was based on how the solvent evaporated at different rates from different areas of the polymer gel through a process known as transient de-swelling.

Based on this discovery, a model system that involved swelling crosslinked polydimethylsiloxane (PDMS) with hexane was studied. Crosby says, “PDMS is a material we have used in our work for a long period of time. It is easy to crosslink and works well with hexane, which is a solvent that readily swells PDMS and evaporates quickly.”

In conducting experiments, the researchers placed a strip of PDMS on a polytetrafluoroethylene substrate. Crosby explains, “We used this material to minimize the influence friction might have in hindering the motion of the strip. Polytetrafluoroethylene also is incompatible with hexane, which prevents the solvent from absorbing into the substrate.”

The researchers found that the motion of the tip of the polymer strip was approximately one meter per second. The different curvature geometries produced when the polymer strip starts snapping lead to repeating self-propulsion.

Crosby says, “During the initial motion, a local asymmetric change in the surface area is created, leading to the formation of convex and concave faces in the polymer gel. The convex face exhibits a higher surface area, leading to greater solvent evaporation (de-swelling) than the concave face. This creates a snapping instability, leading to motion that converts one face to the other. The reversibility of this process leads to the polymer gel self-propelling.”

The researchers then switched to toluene, which is a solvent with a lower diffusion rate in PDMS and a lower evaporation rate than hexane. Crosby says, “We found that the number of transitions from concave to convex increased with toluene due to the longer evaporation times, but otherwise the mechanism was still applicable.”

To show how shape geometry can impact the snapping process, the researchers designed an axisymmetric shell structure that is similar to the Venus fly trap (see Figure 2). The jumping process induced through transient de-swelling led to a calculated peak power density of 312 watts per kilogram that is greater than what is found with jumping insects of a similar mass.


Figure 2. An axisymmetric shell structure prepared with a polymer gel and similar to a Venus fly trap can exhibit self-autonomous propulsion through a process known as transient de-swelling. Figure courtesy of the University of Massachusetts Amherst.

Future work will focus on the use of more environmentally friendly polymers and solvents. Crosby says, “We are now evaluating polymer gels that are hydrophilic and can be used in aqueous solvent systems where self-propulsion can be induced through ionic means.”

Additional information can be found in a recent article² or by contacting Crosby at acrosby@umass.edu.

REFERENCES
1. Canter, N. (2020), “New power sources: Extracting energy from the environment,” TLT, 76 (8), pp 12-13.
2. Kim, Y., Berg, J. and Crosby, A. (2021), “Autonomous snapping and jumping polymer gels,” Nature Materials, click here.