Cellulose nanofiber aqueous solution, which remained virtually transparent for more than one week, was prepared by using the clear upper layer of diluted cellulose nanofiber solution produced by wet jet milling. Glucose oxidase (GOx) was easily dissolved in this solution and GOx-immobilized electrode was easily fabricated by simple repetitious drops of GOx-cellulose solution on the surface of a platinum-iridium electrode. Glucose sensor properties of the obtained electrodes were examined in phosphate buffer solution of pH 7.4 at 40°C. The obtained electrode provided a glucose sensor response with significantly high response speed and good linear relationship between glucose concentration and response current. After an initial decrease of response sensitivity for a few days, relatively constant sensitivity was obtained for about 20 days. Nevertheless, the influence of electroactive compounds such as ascorbic acid, uric acid and acetoaminophen were not negletable.

A low invasive glucose sensor, which has sensing region at the tip of a fine tapered electrode, was developed for continuous glucose monitoring. Platinum-Iridium alloy electrode was settled at the middle of tapered PEEK tube and was employed as sensing electrode. Electrodeposition of glucose oxidase in the presence of surfactant, Triton X-100, was performed for high-density enzyme immobilization, and was followed by the electropolymerization of o-phenylenediamine for the formation of functional entrapping and permselective polymer membrane. Amperometric responses of the prepared sensors to glucose were measured at a potential of 0.60 V (vs. Ag/AgCl). The sensor showed the sensitivity of 5.387 mA/cm2 mM, r2=0.992, and a good linear range from up to 21.6 mM glucose.

Good control of blood glucose degree is quite important for the prevention of serious diabetes complications onset and progression. Accurate recognition of the blood glucose degree helps the patient to provide appropriate treatments, such as insulin therapy. Recently, implantable glucose sensors for continuous glucose monitoring (CGM) are of significant importance on diabetic health care, since it not only lower the physical and mental load on glucose measurement, but also present continuous glucose trend, which is useful for treatment evaluation. However, since the length of sensor device inserted in skin is about 1 cm, development of lower invasive CGM system is expected for the improvement of diabetic patients quality of life. In this study, a low invasive patch type glucose sensor, which has sensing region at the tip inside of a tube, was proposed. The schematic illustration of the tip of such glucose sensor is shown in Figure 1. Since the sensing region is at the inside of a fine needle tube, it requires only the sensor tip to be implanted in the tissue for glucose monitoring. In other words, it can be possible to perform as a patch type sensor, which impresses the user as a sticking sensor instead of implanting sensor. Glucose oxidase was immobilized inside the tube using the combination of electrodeposition and electropolymerization technique, which was similar to the procedure proposed by Wilson’s group [1,2]. That is, phosphate buffer solution (pH 7.0) containing GOx and Triton X-100 was first poured inside the tube electrode and applied a potential of 1.3 V (vs Ag/AgCl) for 1 h to form GOx layer on the surface. Phosphate buffer solution containing o-phenylenediamine (o-PD) was next poured inside the tube and a potential of 0.7 V (vs Ag/AgCl) was applied for 15 min to induce the electropolymerization of o-PD. Properties of the obtained sensor were evaluated both by in vitro and in vivo measurements. Acknowledgment This study was supported in part by a Grant-in-Aid for Scientific Research (C) No. 24500510 from Japan Society for the Promotion of Sciences (JSPS). References [1] N. Matsumoto, X. Chen, G. S. Wilson, Anal Chem, 74, 362 (2002). [2] X. Chen, N. Matsumoto, Y. Hu, G. S. Wilson, Anal Chem, 74, 368 (2002).

Nearly transparent cellulose aqueous solution was prepared using the clear layer of diluted cellulose dispersion solution produced by wet jet milling. Glucose oxidase (GOx) was immobilized on platinum-iridium electrode by applying certain amount of cellulose aqueous solution containing GOx and dried for at least two days in room temperature.  The obtained GOx-immobilized electrodes provided good response current up to 22.4 mM.  Good linear relationship between glucose concentration and response current was observed.  Correlation coefficient of 0.994 was obtained ranging from 0 to 11 mM on the electrode prepared using 500 mg/L cellulose and 250 mg/L GOx mixture solution.  The sensor response was significantly fast and steady state current was obtained within one second. The stability of GOx-immobilized electrode response was measured for 30 days. After initial decrease of response current for the first few days, relatively constant responses were obtained for twenty days.

The importance of glucose determinations for the diagnosis and effective treatment of diabetes has been well recognized. Therefore, much effort has been devoted to developing an effective sensor for the continuous estimation of glucose concentration in subcutaneous tissue [1]. Miniaturization of the sensor device is an important subject, since smaller devices are less invasive, both physically and psychologically. A finer device will excite fewer pain receptors in the skin and will cause less tissue damage and less pain. However, fabrication of finer sensor request precise technique to immobilize enzyme in a specific area. Conventional methods of enzyme immobilization are covalent attachment,cross-linking, hydrogel entrapment, electropolymerized polymer entrapment, and the combination of two or more methods. Among the variety of procedure for the immobilization of enzyme, electropolymerized polymer entrapment is interesting, since the enzyme will be immobilized just at the position where the electricity was passed. Similar to this procedure, Matsumoto et al reported that the layer of enzyme can be formed on the electrode by applying a potential of 1.3 V (vs Ag/AgCl) in the enzyme solution containing a nonionic surfactant such as Triton X-100. Since the obtained enzyme layer was not stable, they formed a electropolymerized polyphenol film in order to improve the stability, which also functioned as a permselective film to eliminate the influence of electroactive species excite in the biological fluid such as ascorbic and uric acids. In this study, glucose oxidase was immobilized using electrodeposition procedure and polymer film was formed to stabilize the enzyme film. Glucose sensor properties of the obtained electrode were investigated using in vitro and in vivo measurements. 1. N. Matsumoto, X. Chen and G. S. Wilson, Anal Chem, 74, 368 (2002).