Skin deep

A major obstacle to the development of bioimplantable sensors to constantly monitor deep inside the body is how to communicate with them.

Researchers at Imperial College London hope to solve this by developing a network that would wirelessly power the deeply-implanted sensors, and receive the signals transmitted from the bioimplants.

‘This is something that is going to affect us more and more in the future,’ said the project’s lead investigator Dr Antonio Vilches.

‘As people introduce new implantable technologies, rather than have everyone reinvent the wheel, I propose to come up with a reliable bioimplantable electronics network for the remote monitoring of these sensors, so the sensor designers can just concentrate on their own particular technology’ he added.

Implanted sensors designed to monitor such things as glucose levels and blood pressure, typically resemble transistors with a membrane designed to detect a particular feature and transmit a time-variant analogue signal.

‘That is as far as a sensor need go with this approach, because my platform will then take that signal, digitise it and push it back out into the real world where it can be monitored,’ said Vilches.

In his system a deep sensor and ultrasonic transducer implant would communicate with a subcutaneously implanted transponder (usually around a centimetre below the skin) at an ultrasound frequency of one MHz.

The subcutaneous transponder would then make contact with an external one via inductive coupling, and a patch antenna forming part of the external transponder would create microwaves at a frequency of 2.4GHz to transmit information to a receiver, such as a PC, within a range of three to four metres.

The inductive coupling technology is what allows the sensors to be powered without an internal battery — which is an advantage as a patient’s life could be under threat if he or she has to undergo surgery every time the battery in the sensor runs out. It also has many other existing applications; for example, inductive coupling is used where heart rate controller pumps are fitted directly into the heart.

‘Bioimplants use a lot of power when used for things such as motor control,’ said Vilches. ‘I have seen a heart pump fed, literally, by a car battery. Fortunately, I am only tackling the communication aspect, so mine will be a lot smaller.’

As the researchers aim to address sensors that are deeply implanted, such as those that monitor kidney action, Vilches aims to have an external transponder that could be worn like a belt buckle roughly above the area where the subcutaneous implant would be located (somewhere around the hip).

‘As long as it remains there it does not have to be applied with gel or anything fancy — it will inductively couple the power in and the signals out,’ he said.

Vilches also hopes to make implantable devices that are no larger than a 1p for the deeply-implanted sensor, while the sub-cutaneous transponder would be slightly bigger, the size of a 2p, to accommodate the coupling coil.

Through investigations so far, the scientists have established that the maximum safety limit of ultrasonic power signals is 200 milliwatts. So they will start from that level and work their way down to find the optimum power. According to Vilches, however, signalling is also an ongoing challenge.

‘Our main concern there is that the signal levels coming back from the sensors are likely to be down to -10dBm, and these may well be masked by other internal bodily functions — I really don’t know,’ he said.

‘Apart from the ability to work at very low signal levels in a noisy environment, bodies are extremely good at blocking electromagnetic radiation, so we have to work acoustically. What happens is acoustic signals tend to be reflected like crazy in there, so we will have to use something to mask off the signals we want, and that becomes a signal processing effort,’ he said.

This communication problem does not affect the power being sent in, which Vilches describes as being ‘easy’ because the battery in the external transponder powers the inductive coupling.

The Imperial team is now preparing for the next stage of the project, which will include looking at how ultrasonic signals propagate using metaphysics simulator software and real liquids that emulate body parts, known as biophantoms.