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Potential Power Source for Implants and Health Concern

In a similar matter to how the mechanical piezoelectric energy generators already used in a few cutting-edge smart prosthetics work, a handful of polymers have been found to produce electrical energy in minute but usable amounts when squeezed and stretched. This is fantastic news for smart implants seeking to use the body's own movements to power their connection to the nervous system. But, the news is tempered with the possibility of serious health concerns.

The piezoelectric capabilities were discovered by researchers at Northwestern University in the US, whilst searching specifically for potential power sources for smart prosthetics and deep brain implants – devices which connect to the electrical impulses of either the central nervous system (brain) or the peripheral nervous system (everything else).

Because these devices send and receive the body's electrical control data, they need to have sufficient power to decrypt the signals and send the appropriate responses back. If they can use the body they are attached to for their power, it avoids the messy problem of relying on batteries which have to be surgically replaced every few years.

The polymers are essentially a mesh of chains that break down over time from stretching and squashing – the very same actions that produce electrical energy. Every time the polymer is squeezed, the pressure has a chance of breaking chemical bonds inside the chain. When this occurs, free radicals – ions with unpaired electrons – are released. Released deep inside the body, free radicals are a nightmare They severely damage organic tissue, triggering premature ageing and many cancers.

So, they are not really suitable as an energy source for smart prosthetics. Unfortunately, this research comes too late in a way, as polymer chains are already used in ordinary dumb prosthetics in several applications. Not suitable for joints, silicon polymers are used in cosmetic surgery for things such as breast implants and face lifts.

The researchers demonstrated that radicals from compressed polymers generate significant amounts of energy that can be used to power chemical reactions in water. This energy has typically been unused but now can be harnessed when polymers are under stress in ordinary circumstances - as in shoe soles, car tires or when compacting plastic bags.

However, at the same time, they discovered the silicone polymer releases a large quantity of harmful free radicals when the polymer is under only a moderate amount of pressure. It doesn't necessarily mean that the polymer is unsuitable for a power source, but it does mean a way of containing the free radicals emitted needs to be found. Worse, it does mean that those already implanted – typically in the soft flesh of the body – are potentially a very real health risk to those with them.

A group of standard silicone implants for breast prosthetics. Regardless of what they are filled with, the outside of the forms are made of silicon polymer - which releases free radicals when it is deformed. Silicon polymer is also used in many other ares where the body will deform naturally.

“We have established that polymers under stress create free radicals with overall efficiencies of up to 30 percent and shoot the radicals out into the surrounding medium where they can drive chemical reactions,” said Bartosz A. Grzybowski, an author of the paper and the Kenneth Burgess Professor of Physical Chemistry and Chemical Systems Engineering. “These radicals can be useful or they can be harmful, depending on the situation.”

Grzybowski and his team are the first to use this energy to drive chemical reactions by simply surrounding the compressed polymer with water containing desired reagents. The radicals created in the polymer migrate toward the polymer/water interface where they produce hydrogen peroxide, which then can drive chemical processes.

“You can get a surprisingly large amount of chemical energy from a polymer under compression,” Grzybowski said. “This energy is, in a sense, free for the taking. Under normal circumstances, the energy is virtually never retrieved from deformed polymers, which then age unproductively. But you could recharge a battery from the energy produced by walking or driving a car. “

However, the free radical problem in generatinbg this much energy is significant. The researchers confirmed that mechanical deformation -- moderate squeezing -- created free radicals in the polymers. They also determined the number of radicals produced in a polymer under pressure is approximately 1016 (10 to the 16th) radicals per cubic centimeter of polymer.

They next filled polymer tubes with water, squeezed the tubes and measured the total number of radicals that migrated into the surrounding solution. They found that nearly 80 percent of the radicals made the trip.

To illustrate the process, they converted a Nike Air LeBron shoe into a “lightning shoe,” where the air pockets in the polymeric sole are filled with a solution of a compound that lights up in the presence of radicals. After a person walked in the shoe for 30 minutes or more, enough radicals were created to generate a blue glow visible to the naked eye.

The researchers studied seven different polymers, including a number of particular public interest. Poly(dimethylsiloxane), a silicon-based material commonly used in medical implants, was but one of them. In the lab experiments, the medium surrounding the polymer and the amount of pressure exerted on the material were similar to what would be found in the human body, Grzybowski pointed out.

“Our findings are somewhat worrisome since every polymeric implant in the human body experiences mechanical stresses and, as we now know, can produce harmful free radicals and liberate them into surrounding tissues, which may contribute to diseases such as cancer, stroke, myocardial infarction, diabetes and other major disorders,” Grzybowski said. “With this knowledge, I am quite happy to have a metal implant in my knee, rather than a polymer implant.


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