Liquid asset

Swiss researchers have developed a sensor with no electronic components that can monitor the progress of orthopaedic implants in a healing bone, and could eventually be made to biodegrade in the body.

The device, embedded in an implant, monitors implant deformations to avoid overload during physiotherapy and provide more information about the healing process of the bone. More of the load shifts from the implant to the bone during rehabilitation, and this is reflected by changes in the deformation.

Felix Gattiker, a research scientist from Swiss materials science research institution EMPA, said the problem with existing methods is they need sensors based on electromagnetic signal transmission.

‘That is not a problem in itself, but you need specialist equipment to read the sensor and to transfer the data from the sensor to an exterior device,’ said Gattiker. ‘And, not only are they expensive, but you cannot fabricate them from biodegradable materials and have to operate to remove them.’

EMPA’s sensor is made entirely of polymethyl methacrylate (PMMA) commonly known as plexiglass, but the team is working on a new version fabricated from biodegradable materials such as polycapro- lactone (PCL).

‘Such a sensor would decompose in the body within a couple of months so there would be no need to remove it when the bone was healed,’ said Gattiker. ‘These sensors are particularly interesting for use alongside the biodegradable implants which are coming on to the market as there is no need for re-operation.’

The sensor is a rectangular box, or reservoir, filled with a liquid such as distilled water, connected to a microchannel situated on top of it. The sensor is attached to an orthopaedic implant. When the implant is bent slightly, the reservoir is squeezed, causing the liquid to flow out of the reservoir and into the microchannel. There is a direct relationship between the fill level of the microchannel and the deformation of the reservoir.

The fill level of the microchannel is measured using standard ultrasound equipment with the simple addition of new image processing algorithms to measure the levels.

‘The idea is to use commercially available scanners rather than having to buy or develop a new special-purpose unit,’ said Gattiker. ‘A doctor or surgeon would not accept the additional cost of a new single-purpose scanner. The commercial version of the system will be very affordable as it consists of the basic plastic or a biodegradable sensor and a software update for the ultrasound scanner.’

The initial application for the sensor will be in facial surgery, but it could also be used in applications such as broken femurs. The criterion is that the sensor can only be used when the surrounding tissues are penetrable by ultrasound. there are some situations where organs are in front of the implant, meaning such a sensor could not be used effectively.

One of the key challenges the researchers are addressing is to make the sensor entirely stable and watertight. With regard to the ultrasound readout, they are working on signal processing issues and the proper evaluation of the data.

So far the team has carried out tests using simulated in-vitro materials, which mimic real tissue. ‘This is only an approximation, you cannot simulate all the aspects of real tissue,’ said Gatticker. ‘The in-vitro experiments performed well and we could get readouts with the sensor, but real tissue is not homogeneous, and differences in it cause deflection of the ultrasound. As a result, there are many artefacts in the image, and we’re working on dealing with these.’

Development of the sensor began in 2003, and work on the ultrasound-based readout started in the summer of 2005. The project to date has been funded by EMPA and NCCR — the Swiss National Science Foundation — and the team is seeking further funding to progress the system into an animal testing phase.

Gattiker has plans to carry out some in-vitro experiments using real tissue. ‘I just go to the butcher and buy some meat — it’s much closer to a real application,’ he said.

By the end of the project the researchers hope to have a working prototype and received industry backing. A medical implant company has already expressed an interest. After production of the prototype, it could be at least another three years until the sensor could be used in patients.

The sensor is also very sensitive to temperature and could have future applications as an in-vivo thermometer and be used to monitor cancer treatment.

Berenice Baker