Image credits Shutterstock.com/Sergey Ryzhov
The US Stanford scientist Paige Fox and her team from the Division of Plastic & Reconstructive Surgery at the Stanford University Medical Center alongside Zhenan Bao and his team from the Department of Chemical Engineering at Stanford University have developed an implantable strain and pressure sensor. The sensor is made from fully biodegradable polymers and could be used to track the healing of tendons and then degrade after its useful lifetime.
Their work presents the implementation of fully stretchable and biodegradable organic materials as biomechanical sensors. They primarily used organic materials because they offer the advantages of versatility in molecular tuning for desirable degradation kinetics, as well as easy processing, and mass production capabilities.
The researchers also selected materials that are well established for their excellent biocompatibility upon degradation, potentially reducing the timeline for clinical translation. In contrast, the cytotoxicity of carbon nanotubes (CNTs) may prevent the use of degradable CNT-PLLA (polylactic acid) composites in biomedical implants.
The key elements of the material design of the sensor are the two biodegradable elastomers poly(glycerol sebacate) (PGS)17 and poly(octamethylene maleate (anhydride) citrate) (POMaC)18. Both materials were initially developed for tissue engineering applications inside the body. PGS is approved by the US Food and Drug Administration (FDA) for biomedical use, while POMaC has been subject to extensive biocompatibility studies, demonstrating its cell and tissue biocompatibility comparable to that of PLLA control.
The sensor consists of Photocrosslinked POMaC networks (PPOMAC) for the Top Layer, and ester bond crosslinked photocrosslinked POMaC (EPPOMAC) for the Bottom Layer. The synthesis of PPOMaC and EPPOMaC was performed using the mixture of maleic anhydride (Fluka, CAS 108-31-6), citric acid (Sigma-Aldrich, CAS 77-92-9) and 1,8-octanediol (Sigma-Aldrich, CAS 629-41-4).
The prepolymer was dissolved in tetrahydrofuran (THF, ~5 g in 20 ml), and purified by dropwise precipitation into 2l of deionized water. Photocrosslinked POMaC networks (PPOMaC) were formed by crosslinking through free-radical polymerization. EPPOMaC was produced by further crosslinking PPOMaC through the available free functional groups of citric acid.
In addition to their established biocompatibility upon degradation, they are excellent candidates for this application in terms of their mechanical properties and degradation characteristics, which can be tuned by varying the polymerization conditions. The electrodes are made of Mg evaporated on top of a biodegradable polymer substrate (PLLA).
The sensor fabrication involved the following steps:
1. Synthesis of POMaC top and bottom encapsulation layers.
2. Preparation of POMaC elastomers.
3. Synthesis of PGS and fabrication of microstructured dielectric layer for the pressure sensor
4. Fabrication of biodegradable metal electrodes.
5. Sensor assembly.
Thus, in future, the patients undergoing any surgery will be provided this fully biodegradable sensor with the ability to monitor, in real time, the mechanical forces on tendons after surgical repair could allow personalized rehabilitation programmes to be developed for recovering patients.
Fig. 1. A stretchable strain and pressure sensor that can track the healing of tendons. a, Strategy for using the mechanical sensor in rehabilitation. The fully biodegradable sensor could be attached to an injured tendon and could continually monitor strain and pressure applied to the tendon in order to avoid overload while it heals. b, Expanded schematic of the structure of the strain and pressure sensor, which is made from biodegradable polymers (PGS, POMaC and PLLA) and magnesium electrodes. c, A picture of the complete sensor. Credit: adapted from ref.doi:10.1038/s41928-018-0071-7, Macmillan Publishers Ltd.
Paige Fox, Zhenan Bao and colleagues thus reported a stretchable and biodegradable strain and pressure sensor that has low hysteresis, excellent cycling stability and a well-controlled dissolution and functional lifetime. The device is designed to offer post-surgical tracking of the mechanics of injured tendons and help prevent the tendons being overloaded, which can hinder rehabilitation.
Once their team accomplishes this wonderful device, Fox said that “the high-sensitivity, fast time response and biodegradability of their sensor means that it could also be of value in biomedical applications beyond orthopaedic rehabilitation monitoring. For instance, it could be relevant to cardiovascular patches and reconstructive surgery, where the monitoring of mechanical deformations and pressures in real time in vivo will allow for refined and personalized medicine. Future research will consist of developing a wireless system made entirely of biodegradable materials, including the circuit used for wireless transmission of measured signals through the skin.”