There is much interest in developing sensors made from non-toxic materials that are also biodegradable; current sensors, while innovative and beneficial, are laced with precious metals which are harmful to the environment and human health. This makes them unsuitable for medical uses where they might be in direct contact with the body, or for inclusion in food products.
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Recent work has involved creating electronics and sensors that can naturally disappear into their environment. Such sensors are likely to be based on biodegradable materials like paper, or polylactic acid (PLA), which is already used in bone screws and tissue scaffolds. By combining materials printing with local sintering using a flash photonic process, researchers at École Polytechnique Fédérale de Lausanne have developed printed humidity and temperature sensors and transistors on biodegradable substrates. They envisage these being applied to smart packages and in body-implantable systems.
Medicine
Sensors are already widely used in medicine but often outside the body. Those that are implanted into the soft tissues or organs have the disadvantage of needing to be removed, and can cause damage upon extraction, while also increasing the risk of infection and recovery time.
A team from the University of Connecticut have been working with materials already approved by the US Food and Drug Administration to develop implantable sensors to monitor conditions in the brain, lungs, eyes and heart before dissolving harmlessly. The small, flexible sensors detect changes in pressure within the organs by employing the piezoelectric effect, where applying a force to a material causes a charge to be generated proportional to the force, which can then be detected.
The sensors, which could monitor conditions such as glaucoma, heart disease and bladder cancer, utilize poly (L-lactide) or PLLA, a medically safe biodegradable polymer. It doesn’t normally exhibit piezoelectric properties under normal conditions, but by carefully heating, stretching and cutting the material in a particular way to alter the polymer’s internal molecular structure, the team were able to impart piezoelectric properties.
Although they have been created to replace existing implantable pressure sensors that contain toxic elements, the sensors are not yet fully biodegradable. They consist of two layers of PLLA film sandwiched between molybdenum electrodes encased in layers of PLA. While much of the sensor will breakdown into its individual organic compounds after around four days, the molybdenum will not degrade within the body, though it is at levels the body can tolerate.
Initial tests in which the sensors have been implanted into the abdomen of mice have been successful and have given readings comparable to and as reliable as those obtained from currently commercially available devices.
Another team from the University of Illinois at Urbana-Champaign have developed a small, thin electronic sensor capable of monitoring temperature and pressure within the skull following brain surgery, which could be adapted for other post-operative monitoring. Their sensor is made from extremely thin sheets of dissolvable silicon and are smaller than a grain of rice. They are designed to work for a few weeks before dissolving harmlessly and completely into the body’s own fluids.
Food
When it comes to food, biodegradable sensors could be really useful. In some cases – like with fruit and vegetables – its easy to see when they are getting beyond their best, but with meat and fish, it can be a bit more difficult. Microsensors that are biocompatible and biodegradable could save consumers from the horrible whiff when they open up a packet of meat only to find its going bad.
ETH Zurich researchers have developed an ultra-thin temperature sensor made from biodegradable materials which degrade in just over two months. They encased a superfine, tightly wound electrical filament made of magnesium, silicon dioxide and nitride in a compatible polymer. Magnesium forms part of the diet, while the silicon dioxide and nitride are biocompatible and dissolve in water, and the polymer consists of corn and potato starch - so it is all completely biodegradable and safe. The small, robust and flexible sensor could find uses on fish to ensure that they have been kept cool enough during transportation and pose no threat to human health, for example.
These are just a few examples of how biodegradable sensors might be employed and how they work; there are many more under development that will one day find uses in medicine and food monitoring, maybe even environmental monitoring too.
References and Further Reading
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