Moving toward more lifelike artificial limbs

The promise of piezoelectrics
A technology more familiar to cell phone users may help with at least one other problem: how to build prosthetic limbs that are both strong and flexible. Pradeep Sharma, a professor of mechanical engineering at the University of Houston, said the solution lies in piezoelectrics, or materials that produce an electric charge in response to mechanical stress.

Piezoelectric materials, mostly ceramics and crystals such as quartz, lack a certain symmetry at the atomic level. Applying a strain across that asymmetric structure by pushing, stretching or otherwise deforming it can produce an electrical charge. Likewise, applying a voltage causes the object to change its shape — the same principle that underlies vibrating cell phones and deployed airbags. “Because of this effect, it has a natural application for prosthetic limbs,” Sharma said. “Our human body works in the same way: you send a signal to the hand and the hand will move.”

But there’s a problem. Ceramic piezoelectric materials offer significant force, but are too brittle to provide a reasonable range of motion. Others made from polymers move fine but have so little oomph, “you would not have enough force to pick up an egg,” he said. “For prosthetic limbs, we must be able to optimize both.” Easier said than done, especially when mixing the two materials has yielded disappointing results.

At the nanoscale level, however, Sharma is exploring how to mimic piezoelectric properties in materials that work better for bioengineering but wouldn’t normally generate electricity. By applying a non-uniform strain, he and his colleagues found they could disrupt the materials’ symmetry ever so slightly and allow them to behave like their electricity-generating counterparts.

The research on what Sharma dubs “piezoelectrics on steroids,” funded by the National Science Foundation and published in March in the journal Physical Review B, could lay the groundwork for scientists to take two materials, join them into a strong and flexible composite and then tinker with the composite’s nanoparticles to coax them into generating electricity.

“Essentially, we’ve opened up a new dimension of engineering by saying you could also tweak the size of particles to tailor a response,” Sharma said. For prosthetics, that could mean changing the size and symmetrical arrangement of the particles to alter the strength of prosthetic limbs while generating for the electrical signals needed to govern motion.

Sharma said he sees his eventual goal as a good fit with nature-inspired ways of strengthening prosthetic materials. Considering that the final goal is to replicate natural movement, he said, “it would be smart if you mimicked nature as much as possible.”

Mimicking nature
In March, researchers at the University of California at Santa Barbara took another step in that direction with a report in the journal Science detailing the unusual features of the Humboldt squid’s parrot-like beak. The squid's beak, one of the hardest organic materials known, has been compared to a knife attached to a bowl of Jell-O. But how can the animal wield a weapon critical to its survival without damaging itself in the process?

The answer lies within a unique network of proteins, water and chitin that changes the beak’s stiffness by 100-fold over a three- to four-inch distance, transitioning from a hard tip to a soft and flexible attachment at its base. Protein content maxes out at the tip, whereas water and chitin concentrations increase toward the base.

From an engineering prospective, the squid’s beak could inspire efforts to use a comparable gradient for bonding dissimilar materials more effectively than existing adhesives, said Frank Zok, a study co-author and professor in the university’s materials department. For a prosthetic arm or leg, that could mean devising a sturdy attachment that doesn’t damage soft body tissue. Nevertheless, how a squid-derived strategy might apply to the human body remains a question.

Chitin, a polysaccharide also found in insect exoskeletons that can form a strong material when pulled into fibers, has already gained notice due to its non-antigenic properties, meaning that the human body is less likely to regard it as foreign and reject it. A modified protein involved in the beak’s stiffness gradient, known as Dopa, also has been tapped for potential engineering applications due to its unusual chemical and mechanical properties.

But study co-author Herb Waite, a marine biochemist,  said the most significant aspect of the new research has less to do with the identity of the molecules than with the role of water in defining the beak’s hard-to-soft gradient.

“I would suggest that the gradient of water is a new design paradigm,” he said. “I really don’t think that many biomedical engineers have considered that option in making better designs for how materials come together.”

Could understanding how the squid beak changes its water content lead to better prosthetics?  Waite isn’t discounting the possibility. But first, he said, scientists must consider how the hydration so essential for bones, ligaments and other biological tissues could alter the stiffness and bonding of polymers, metals and ceramics used in implants.

“The problem with a lot of implant science is that for the anti-interface between hard implants and softer tissue, there’s a great risk for contact damage,” he said. “That’s going to be experienced largely by the softer tissue.” Finding ways of marrying an implant surface with a biological material in a way that doesn’t lead to sores or inflammation for someone wearing a prosthetic will be an ongoing challenge, he said. Nor is it a trivial matter to design a material with a graded distribution from one end to another.

On a more global scale, Waite said the thought-provoking squid beak also highlights the need to conserve the original source of inspiration. For the Humboldt squid, “a fearsome creature to have in the water next to you,” he said the unique gradient of its razor-sharp beak is remarkably pH-dependent and thus susceptible to ocean acidification due to global warming. The ability to mimic nature, after all, depends on retaining a natural world.

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