Transplant rejection sensor paves way for body-integrated electronics

The first planned use of the technology is a sensor that will detect the very early stages of organ transplant rejection.

Paul Berger, professor of electrical and computer engineering and physics at Ohio State, explained that one barrier to the development of implantable sensors is that most existing electronics are based on silicon, and electrolytes in the body interfere with the electrical signals in silicon circuits. Other semiconductors might work in the body, but they are more expensive and harder to manufacture.

‘Silicon is relatively cheap… it’s non-toxic,’ Berger said in a statement. ‘The challenge is to bridge the gap between the affordable, silicon-based electronics we already know how to build, and the electrochemical systems of the human body.’

In a paper in the journal Electronics Letters, Berger and his colleagues describe a new, patent-pending coating that that they believe will bridge that gap.

In tests, silicon circuits that had been coated with the technology continued to function, even after 24 hours of immersion in a solution that mimicked typical body chemistry.

The project began when Berger talked to researchers in Ohio State’s Department of Biomedical Engineering, who wanted to build an insertable sensor to detect the presence of proteins that mark the first signs of organ rejection in the body. They were struggling to make a working protein sensor from gallium nitride.

‘We already have sensors that would do a great job at detecting these proteins, but they’re made out of silicon. So I wondered if we could come up with a coating that would protect silicon and allow it to function while it directly touched blood, bodily fluids or living tissue,’ Berger said.

In the body, electrolytes such as sodium and potassium control nerves and muscles and maintain hydration. They do this by carrying a positive or negative electric charge that spurs chemical reactions. Those same charges make the electrolytes attractive to silicon, which will readily absorb them. Once inside, the charges alter the electronic behaviour of the silicon so that the readings of a sensor can’t be trusted.

In the study, Berger’s team tested whether electrolytes could be blocked from entering silicon with a layer of aluminium oxide.

The researchers submerged the coated test sensors in fluid for up to 24 hours, removed them from the solution, and then ran a voltage across them to see if they were working properly. The tests showed that the oxide coating effectively blocked electrolytes from the solution so the sensors remained fully functional.

Once developed, a device using this technology could detect certain proteins that the body produces when it’s just beginning to reject a transplanted organ. Doctors would insert a needle into the patient’s body near the site of the implanted organ. Silicon sensors on the needle would detect the protein, and doctors would know how to tailor the patient’s dosage of anti-rejection drugs based on the sensor readings.

The work represents a first step toward fabricating devices that could be implanted in the body long-term, Berger said.