Study Of Implantable Electrochemical Glucose Sensor

The incidence of diabetes is rising as the improvement of the living standards and the increasing of the elderly population. It has become the third largest disease of the world, after cardiovascular diseases and cancers. The real-time, continuous blood glucose monitoring system can monitor blood glucose levels and control the insulin releasing. This could reduce the complications of hyperglycemia and hypoglycemia.The implantable glucose monitoring system could only work for a few days after implantation. The in vivo failure of the sensor may due to the following reasons. The first is the biofouling caused by inflammation and foreign body response which are related with the biocompatibility of sensors. During the process of biofouling, the immunocytes migrate to the sensor and adhere to the surface. This impedes the diffusion of glucose to the sensor. Moreover, inflammatory cells adhered on the sensors’surface consume the glucose around sensors and release proteolytic enzymes and free radicals which will destroy the sensor. The second reason for the in vivo failure of the sensor is the enzyme degradation. This may due to loss of enzyme activity, the leaching out of the enzyme and the inhibition of enzyme catalysis.We focoused on the structure designing and performance improvement to increase the implantable glucose sensor’s lifetime. The main works and innovations are as follows.1. Gold electrode surface rebuilding by immersing the electrode into NaOH solution and applying a square wave voltage on it. The redox reaction on the surface of the gold electrode generated hydrogen and oxygen bubbles which formed porous nanostructure and rebuilt the gold surface. The porous nanostructure could immobilize more glucose oxidase. We optimize the rebuilding conditions including the rebuilding potential pulse amplitude and the rebuilding time. Finally, we got the uniform and porous nanostructures on the gold electrode. Pt nanoparticles were electrodeposited on the gold surface to increase the catalytic activity of the electrode. We discuss the effect of the electrodeposition time to the sensors’sensitivity and got the optimized electrodeposition time. We immobilized glucose oxidase by copolymerizing it with o-phenylenediamine. The resulted sensor had high sensitivity.2. The biodegradable material poly (lactic-co-glycolic acid) (PLGA) was used as the out coating of the sensor to increase the sensors’lifetime. PLGA was coated on the half area of the sensor surface. The sensitivity of the sensors’area without PLGA coating decreased when soaking in PBS. The initial sensitivity of the sensors’area with PLGA coating is almost zero. When immersed in 37 ℃ PBS, PLGA began to degrade and released the covered glucose oxidase. The latest released enzyme could maintain the sensor’s sensitivity by replacing enzyme of functional inactivation to accomplish the catalytic reaction, so the lifetime of glucose sensors could be increased. At the same time, the permeability of the PLGA increased during degradation which made the permeation of glucose and oxygen to the sensor easier. This also increased the sensor’s sensitivity. The sensors with PLGA out coating remained 75% of the initial sensitivity after immersed in 37 ℃ PBS for 25 days which was much better than the sensors without PLGA coating.3. The conducting polymer polyaniline (PANI) was used to increase the amount of enzyme immobilization due to its porous structure and high specific surface area. At the same time, the excellent conductive character of PANI could improve the electron transfer rate of the enzyme and electrode. This would reduce the response time of the sensor. Glucose sensors prepared with PANI had a sensitivity of 4 uA mM-1 cm-2 and a linearity of 0-14 mM. The sensitivity and the linearity were enough for monitoring the glucose levels of diabetes. The prepared glucose sensors were immersed in 37℃ bovine serum to imitate the in vivo conditions and evaluate its long time stability. When soaked in 37℃ bovine serum for 44 days, the sensor still remained 45% of its initial sensitivity and this lifetime was longer than most of the sensors tested in bovine serum reported by other researchers. PLGA was coated on the surface of the PANI based sensors. The PLGA coated PANI glucose sensor remained 80% of its initial sensitivity when immersed in 37 ℃ bovine serum and this lifetime basically met the long term requirement of implantation.4. Stainless steel electrode substrate was used to replace the gold electrode substrate to meet the requirement of hardness when implant in subcutaneous. We optimized the technological conditions of sensor fabrication including electrodeposition time and voltage of Pt nanoparticles, elcetropolymerizing time and current of PANI nanofibers. We also tested the sensors’sensitivity, linearity and anti-interference property under different operation voltage and the best voltage was chosen. The sensors based on stainless steel substrate had a sensitivity of 1 uA mM-1 cm-2 and a linearity of 0-25 mM with the correlation coefficient of 0.998 under the constant potential at+0.6 V vs. Ag/AgCl. What’s more, the sensor had a good anti-interference to uric acid, ascorbic acid and acetaminophen.5. The working electrode, counter electrode and reference electrode were packaged together and tested the stability of the sensor under 0.6 V voltage for 24 hours. Signal fluction within 24 hours was about 100 nA and it was equal to the current change caused by 1mM glucose concentration change. The packaged sensor was implanted into the subcutaneous of rat back. After implantation for 6 days and 11 days, we tested the sensor in vivo. Results showed that the sensor could follow the blood glucose change in real time. What’s more, sensitivity, response time and baseline of the sensor had almost not change tested at 6 days and 11 days. This reveals that the sensor has good stability in vivo.