Princeton Plasma Physics Laboratory Assists with the Development of Artificial Muscle

PPPL's Lew Meixler collaborates with Lenore Rasmussen on the plasma treatment of electrodes for intelligent materials project at PPPL.

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 developing the materials.

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 graduate student at Purdue University 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, it would have to be 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," said Rasmussen. "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," Rasmussen stated.

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, comprised of an electro-responsive polymer gel (known as 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. The thin elastomeric coating also serves as a moisture barrier, preventing evaporation and leakage of the electrolyte solution in the polymer, and allowing the actuators to be fully operational anywhere.

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 and poly methacrylic acid served as a cross-link to network gels, which respond quickly to electricity and have all the other needed properties. But one challenge remained: after repeated cycles, the polymer often detached 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 patients, reducing discomfort.

A potential solution was at hand. Rasmussen contacted Lew Meixler, PPPL's Head of Applications Research and Technology Transfer. Their discussions resulted in the establishment of a Cooperative Research and Development Agreement (CRADA) last December between PPPL and Ras Labs. The CRADA revolves around PPPL's plasma sterilization equipment, an excellent apparatus in which to treat metal samples with plasma. Different ions are being studied to find a suitable metal and plasma combination that solves the detachment problem.

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. Titanium, in particular, is suitable for use within the body. Oxygen ions derived from synthetic air (for safety) are also used. Ions are driven onto the surface of a 0.5-inch by 1.5-inch metal foil by a 40-volt electric potential for 12 hours. Following treatment, a polymer coating is sandwiched between two pieces of treated foil. The composite is then sent to the University of Pennsylvania or to the Princeton Textile Research Institute, which performs adhesion tests on the small samples that fit into PPPL's apparatus. A standardized testing apparatus controls the speed and strain with which the composite is peeled apart. Future tests will be conducted with actual wire electrodes treated in the PPPL apparatus.

In addition to identifying a suitable plasma treatment for metals, the tests at PPPL should provide insight into the mechanism responsible for improved adhesion of the polymer. Preliminary studies have shown that the plasma ions rough up the metal surface on a molecular scale and make the surface super clean by removing any oils that might be present. "Right after the peel test we check to see where the break has occurred. If necessary, we use electron microscopy to view the surfaces," Rasmussen said. "If the polymer comes off the metal cleanly, the interface is the problem. If there are patches of the polymer remaining on the metal, then the failure was in the polymer itself—or there could be other things going on."

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 many people may benefit.

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