Biodegradable implantable sensor monitors healing tendons
They argue that by monitoring in real-time the mechanical forces on tendons after surgical repair, physicians could literally customize rehabilitation protocols for the patient’s needs, finding the exact balance between active exercises and immobilization for speedier recoveries while avoiding rehabilitation-related injuries.
In a paper titled “A stretchable and biodegradable strain and pressure sensor for orthopaedic application” published in Nature Electronics, the team led by Zhenan Bao proposes a laminated structure encompassing a capacitive strain sensor (two thin film comb electrodes sliding relative to each other) and a thin, flexible capacitor featuring a specifically designed micro-structured elastic dielectric layer for the pressure sensor.
For the flexible part, the researchers used two biodegradable elastomers poly(glycerol sebacate) (PGS) and poly(octamethylene maleate (anhydride) citrate) (POMaC) initially developed for tissue engineering applications inside the body and known for their biocompatibility upon degradation, when body fluids eventually permeate through the sensor packaging. The electrodes are made of Mg evaporated on top of a biodegradable polymer substrate (PLLA). Upon hydrolysis, metal magnesium forms highly soluble magnesium oxides that can be evacuated naturally.
Providing independent discrimination of strain and pressure, the laminated sensors were designed to degrade after their useful lifetime, eliminating the need for a second surgery to remove the device. The paper reports a strain sensor designed to operate in the 0-15% strain sensing range, close to the in vivo strain exerted on tendons (under 10%) and capable of discriminating strain as small as 0.4%. The pressure sensor was designed to detect pressures as low as 12 Pa and as high as 430 kPa. The authors also report a response time in the millisecond range as well as an excellent cycling stability for strain and pressure sensing.
The sensor was tested in vitro, with its degradation observed over 8 weeks in a phosphate buffered saline solution at 37°C. It was designed to remain operational in vivo for over two weeks, with in-vivo tests performed subcutaneously on rats. The authors anticipate that such bio-compatible and bio-degradable sensors could find their way beyond orthopaedic rehabilitation monitoring, including as cardiovascular patches and for reconstructive surgery, to monitor the mechanical deformations and pressures in vivo for refined and personalized medicine.
Next, the researchers aim to develop a wireless system made entirely of biodegradable materials, including the circuit used for the wireless transmission of the measured signals though the skin. Last year, Prof. Bao’s team had reported biodegradable organic semiconductor devices.
Stanford University – www.stanford.edu
University College London – www.ucl.ac.uk
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