Smart Sensing Sutures for Real-Time Glucose Monitoring

Real-time monitoring of glucose levels in living organisms is a key indicator of various biological processes. Specifically, monitoring glucose and other biomarkers in avian species (e.g. house sparrows) is a key indicator of the animal’s stress level. Current stress level monitoring methods are invasive and require multiple interactions with the animal (collecting blood samples at different time points). Therefore, a less invasive and continuous monitoring method is preferred.<br/>Flexible bioelectronics have emerged in the fields of precision health and medicine as they offer seamless electronic-tissue interface in terms of physical contact and mechanical properties (comparable elastic moduli). Threads have previously been utilized in biomedical applications to realize thread-based transistors, biosensors, and even drug-delivery platforms. Threads’ one-dimensional, flexible characteristics ensure seamless integration with underlying tissues and organs better than two-dimensional, planar platforms and ensure intimate contact with tissue/organ surfaces, as exemplified by sutures. The fibers they are comprised of can be specifically chosen from biocompatible sources, such as silk and polylactic-co-glycolic acid (PLGA), to tailor their use more towards biomedical applications. Further, thread-based biosensors are small and light enough not to interfere with flight movement during continuous real-time monitoring.<br/>In this study, we realize sensing sutures for glucose monitoring using electrochemical principles. We use a three-electrode system: the working and counter electrodes are made of medical-grade sutures while the reference electrode is a commercial Ag/AgCl (1 M KCl) electrode. The conductive layer on the working and counter electrodes is gold via solution-based electroless deposition and plating. This produces a more uniform and connected coating layer of gold on the thread’s rough, uneven surface. Compared to other conductive coatings, such as carbon which requires multiple coatings for adequate conductivity, electroless deposition of gold produces a more flexible conductive suture. Gold nanoparticles were synthesized, characterized, and deposited on the sutures. In addition to the conductive layer, an electron-mediating layer (Prussian blue) is electrically deposited and enzyme-chitosan bilayers are coated. Glucose oxidase, which is the enzyme immobilized in the enzyme-chitosan matrix, catalyzes the electron-producing reaction that glucose undergoes in the presence of oxygen and, as a result, alters the current in the system. At all steps, FTIR and SEM are utilized to confirm the presence and quality of each coated layers.<br/>We demonstrate functionality of sensors by obtaining calibration curves and test data for real-time glucose monitoring, sensor stability, sensor repeatability, and analyte interference. The sensing sutures had a sensitivity value of 0.933 ± 0.115 nA/mM within a range from 0 mM to 54.9 mM glucose with an LOD value of 0.62 mM. They demonstrated successful real-time monitoring of change in glucose concentration over an operating period of 40 minutes and sensor stability of at least five days. In addition, the sensing sutures showed selectivity towards glucose even with the presence of other molecules, such as lactate. Tests and measurements were performed in 1X phosphate-buffered saline (PBS). Furthermore, future tests will be done using ex-vivo tissue models and in-vivo animal models. While glucose sensing has been reported, sensing sutures can be expanded to other analytes, such as lactate, electrolytes, chemokines, and metabolites.