Abstract: “Actuators are responsible for moving and controlling other systems. Hydrogel actuators have been developed for therapeutic purposes in surgical applications but lose their mechanical strength under too much swelling. Closed-celled, fluid-filled systems, like plants, however, can swell with an increase in mechanical stiffness. Using this as our motivation, we try to synthesize these closed-celled, fluid-filled soft composites that could be used as actuators. This composite is made of emulsified salt water inner phase and PDMS, a two part polymer formed from intersecting polymeric chains. The plant tissue analog (PTA) allows for semi-permeable fluid motion that is biocompatible, flexible, and robust. Since the material is osmotically active, the composite can swell upon water intake due to a difference in chemical potential while maintaining cellular rigidity. This project first investigated the maximum swelling ratios of PTA of different salt concentrations, glycerol concentrations, mixing speeds, wall thickness, and cavity diameters measured via swelling in deionized water or salt water until the sample ceases to swell or ruptures. Furthermore, actuators were created by combining different PTAs in a variety of geometries. The actuators contained parts that are soft and other parts that are stiff to have a bending or twisting motion. Under swelling, the PTAs mimicked a sphere, twist motion, and time delayed wiggle action.”
Text Transciption_Nutrobkina_Slowik
Great talk. I’m curious what applications these soft actuators can be used for.
Hi Siraj,
Thank you for asking. These soft actuators are biocompatible, flexible and have mechanical integrity. Because of these properties, the plant tissue analogs (PTAs) have been developed into soft actuators to be utilized in therapeutic applications specifically for surgical purposes. Furthermore, this research has potential to be used in osmotically active soft machines, non-toxic, active tissue implant material, and water relation inputs for climate models.
Milena N.
Absolutely gorgeous work! I really enjoyed the presentation, your explanations, the fact that the two of you were explaining your shared project. Very well done!
Now for the science: What type of nanoparticles (NP) and what concentration did you use for the PTA synthesis? If possible, it would be nice to check if changing NP type and/or concentration can help with the mechanical properties and avoid rupture.
You showed great results for bending actuators in a variety of designs. However, most of them have positive or zero gaussian curvature. Have you attempted creating actuators that will exhibit negative gaussian curvature as saddle-shapes, helicoids, etc.?
Hi Vianney,
Thanks for asking. With regards to your first question, the nanoparticle we used is made from silica. We chose this specific nanoparticle because in order to be able to add >74% water in PDMS, we needed highly hydrophobic nanoparticles. This was a cheap and easily available commercial nanoparticle that fit the criteria. Not many kinds of nanoparticles are surface modified to this degree of hydrophobicity. Using a metallic nanoparticle, such as a titanium or silver, might enhance the mechanical property. However, they would have a marginal effect because we use such small quantity of nanoparticles (~ 3%). It would also be considerably more expensive. We also don’t expect it to avoid ruptures because nanoparticles stabilize emulsions by adsorbing to the surface of the cavities. From previous analysis, with the silica naoparticles, we already get complete surface coverage so it can’t get any better.
With regards to your second question, plant tissue analogs (PTAs) can only actuate by bending or twisting. Therefore, Gaussian geometries are not feasible. However, it’s not not to say it’s impossible. One way it might be possible is by having spatial variations in the PTA concentration such that some parts will swell and some parts will shrink, creating a negative curvature. Overall, more experiments are necessary in order to find a method to synthesize and actuate Gaussian geometries.
Milena N.
Very nice presentation. I was wondering, once the material is swelled if it is then dried will it go back to its original state, or is this process irreversible?
Hi Steven,
Thank you for asking. Although our main focus was on swelling the plant tissue analog (PTA) to create actuators, we have investigated whether PTAs have reversible swelling properties. We performed a single repeat swelling cycle (i.e.: swell – dry – swell). From this trial, the swelling data was about the same in the second swelling as compared to the first. However, there was less swelling in the second round because over-swelling might have occurred in the first round. Therefore, it’s possible for the sample to rupture during the initial swelling, which would make the PTA have an irreversible swelling property. Overall, the process is for the most part reversible and the PTA goes back to almost its original state after swelling.
Milena N.