Princeton Plasma Physics Laboratory (PPPL) collaborator Lenore Rasmussen has the gift of serendipity. Two disparate life experiences sparked the polymer chemist's interest in the development of electro-responsive "smart materials"—electrically driven polymers that are strong and durable enough to act as artificial muscles in prosthetic devices and robotics.
Her early experience identifying DNA proteins and an injury suffered by her cousin in a farm accident triggered her interest in development of the materials. She brings to this work an extensive background in chemistry, biology, and biochemistry.
Rasmussen was using electrophoresis—the movement of suspended particles through a gel under the action of a strong electric field—to separate and identify protein molecules and DNA. "There are little wells in which you put your proteins or DNA samples. You turn on the electricity and watch how they migrate. Different proteins or DNA fragments will go through the gel at different speeds that depend on their molecular weights. The larger, heavier molecules will have a harder time getting through. One of the wells would contain known proteins for comparison. For DNA, the smaller fragments would move further and longer ones would end up closer to the starting point," explained Rasmussen. But, as fate would have it, one day she made a mistake formulating the gel. "I goofed up mixing stuff together and (as a result) the gel responded to the electricity by contracting—a eureka moment," she said. Later, while she was a grad student at Purdue pursuing a degree in biophysics, one of her cousins was spreading hay on a land reclamation project. He slipped and his leg got caught in the hay spreader. His foot was not detached, but much of the muscle and circulation in the calf of his leg were damaged. Initially, doctors were not sure he would keep the leg. If gangrene set in, he would have to have it amputated. "I was the scientist and biologist in the family, so they asked if I could go and look at prosthetics to see what was out there in case he needed one. While I really liked what I saw for legs, I really hated what I saw for arms and hands. As it turns out, my cousin's leg healed. He had a lot of recovery and still has a slight limp. But I kept thinking about my experience with the gels in DNA analysis and the need for better prosthetics. So I went on to Virginia Tech partly to get the background in polymer chemistry that I would need to develop artificial muscles," said Rasmussen.
Currently, prosthetics for the arm and hand are not functional unless they utilize three-pronged metal devices that are controlled mechanically. Rasmussen wondered if a prosthetic limb could respond directly to a neural impulse and whether they could be made more attractive and highly functional. In 2003 she established Ras Labs, LLC, a small, for-profit, innovative research and development laboratory devoted to projects that utilize polymer chemistry, biochemistry, biology and engineering. Rasmussen envisions artificial muscles, or actuators, that are comprised of an electro-responsive polymer gel (the smart material) containing embedded electrodes, all encased in a flexible coating that acts as a kind of skin.
The smart material is cross-linked, meaning that a side bond has been formed between polymer chains to increase strength and toughness. The embedded electrodes serve a dual role: providing the electric stimulus, much like a nerve, and attaching the smart material to a lever, like a tendon attaches muscle tissue to bone.
When the electrodes are energized with direct current, the smart material contracts or expands, depending on the formulation. It then relaxes when the current is turned off, acting much like real muscle tissue responding to a neural impulse from the brain. The goal is for both the electro-responsive smart material and the embedded electrodes to move as a unit, analogous to muscles and nerves moving together. Rasmussen tested a variety of polymers and found that poly (hydroxyethylmethacrylic acid)-poly(methacrylic acid) cross-linked network gels respond quickly to electricity and have all the other needed properties. But one challenge remained: after repeated cycles, the polymer detached often from the electrodes. However, from her former affiliation with Virginia Tech and with Johnson & Johnson's (J&J) Ethicon division, Rasmussen recalled that J&J performed plasma sterilization of its medical needles, and then coated them with polymers that allow them to slide more quickly into the patients, reducing discomfort. Plasma treatment not only sterilizes metal, but also improves the adherence of the polymer.
A potential solution was at hand. A colleague put Rasmussen in touch with Lew Meixler, PPPL's Head of Applications Research and Technology Transfer.
Rasmussen's discussions with Meixler resulted in the establishment of a Cooperative Research and Development Agreement (CRADA) last December between PPPL and Ras Labs. The CRADA, with PPPL participants Lew Meixler and Yevgeny Raitses, revolves around PPPL's plasma sterilization equipment, an excellent apparatus in which to treat metal samples with plasma.
To date, tests conducted at PPPL are encouraging, resulting in improved bond strengths.
Stainless steel and titanium metals are being treated with plasma comprised of ions of nitrogen, helium, or hydrogen. Following treatment, a polymer coating is sandwiched between two pieces of treated foil. Whatever is learned from the PPPL plasma treatments, Rasmussen will continue her quest for electro-responsive smart materials that can have a profound impact on prosthetics and robotics, with excellent control, dexterity, and durability.
If she is successful, a lot of folks may benefit.
Lew Meixler and Lenore Rasmussen prepare a metal wire sample for plasma treatment at PPPL.
