Implanted sensors are crucial for monitoring patient health, but they come with their own secondary problems such as risk of infection and damage to tissue. But imagine a sensor that did not need to be removed because it degrades by itself over time. Meet the biodegradable sensor!

Sensors are understandably popular, adding various features to consumer devices and functionalities to increasingly ambitious subsystems. Temperature sensors can monitor CPU temperatures, pressure sensors can measure the atmospheric pressure for weather monitoring, gas sensors can detect explosive materials, and IR sensors can detect intruders.

One field in particular that could hugely benefit from the sensor boom is the medical field where up-to-date readings from patients could give valuable insight to doctors. For example, patients who have had brain surgery may need to have their cranial pressure monitored as pressure build up in the brain can be fatal. External sensors (such as heart rate monitors) can be attached to the outside of the body while internal sensors need to be implanted inside the body to take their measurements. The procedures involved in implanting sensors can create opportunities for secondary problems including infections.

In many cases, additional procedures are required to either perform maintenance on an implanted device or remove it once its purpose has been fulfilled. This is where biodegradable sensors come in.

A Sensor that’s Food Safe

A team of engineers from the University of Connecticut, led by senior biomedical engineer Thanh Duc Nguyen, has developed an implantable sensor that degrades inside living tissues. The sensor designed by the engineers is a pressure sensor that utilizes the piezoelectric effect to generate a voltage when a deflection is applied (where the deflection is the result of pressure build up).

The sensor itself is made of molybdenum electrodes encased in a specially treated medical grade polymer, poly (L-lactide) (PLLA). The polymer does not inherently have piezoelectric properties but an engineering student (Eli Curry) from Nguyen’s laboratory discovered that PLLA can attain piezoelectric properties through heat treatment, stretching, and cutting of the material.

The pressure sensor has been tested in mice with positive results (on par with current commercial sensors for medical use) but is yet to be used in human trials.

The biodegradable pressure sensor. Image courtesy of Thanh Duc Nguyen via the University of Connecticut.

While the PLLA material is biodegradable, the sensor's molybdenum electrodes are not. However, this should not be an issue as the electrodes are already used in medical implants such as cardiovascular stents and bone implants.

If such sensors could be made available for general use, then the impact on the medical field could be massive. Patients could be fitted with sensors during their required operation but there would be no need to extract them as they would degrade by themselves. The degrading property stops secondary infections from implant removal surgery and biodegradable wires could be used to connect the sensor to external monitoring devices which do not require sterilization and biological compatibility.

A Stress/Strain Sensor

Another research team from Stanford University has also created a biodegradable sensor for medical monitoring but theirs measures strain and pressure. The sensor is comprised of layers of magnesium electrodes that can be used to measure strain and pressure. Unlike the pressure sensor from the University of Connecticut, this device can be attached to tendons and ligaments and can provide critical data for those recovering from sports injuries and trauma.

This sensor is part of an ongoing Stanford push for biodegradable electronics. Stanford's Professor Zhenan Bao's research group has been exploring the realms of stretchable, flexible, self-healing, wearable circuits and electronic skin for nearly 15 years (and Professor Bao, herself, has been doing research for nearly 30 years).

Last year, Bao's group created what postdoctoral fellow Ting Lei described as a circuit made of "the first example of a semiconductive polymer that can decompose."

2017's biodegradable sensor. Image courtesy of Bao lab via Stanford News Service.

This newest biodegradable strain and pressure sensor is actually composed of two sensors stacked atop one another, each making measurements. Where the University of Connecticut used PLLA, the Stanford group utilized poly-glycerol sebacate (PGS) and poly-octamethylene maleate citrate (POMaC).

You can read more about this new sensor in Nature Electronics.

Beyond Medical Applications

Biodegradable sensors could be extremely valuable for medical applications but there are plenty of other fields where they could be key, as well. If electronics could be made to be fully biodegradable, then e-waste could be made significantly safer and reduce the amount of waste in landfills.

Biodegradable sensors could also find significant use in agricultural uses where electronic sensors could be installed in mass on farmland and be replaced yearly with older sensors degrading into the soil.

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