Bazhang Yu

Use of hydrogel coating to improve the performance of implanted glucose sensors.

In order to protect implanted glucose sensors from biofouling, novel hydrogels (146-217% water by mass) were developed based on a copolymer of hydroxyethyl methacrylate (HEMA) and 2,3-dihydroxypropyl methacrylate (DHPMA). The porosity and mechanical properties of the hydrogels were improved using N-vinyl-2-pyrrolidinone (VP) and ethyleneglycol dimethacrylate (EGDMA). The results of SEM and DSC FT-IT analyses showed that the hydrogel (VP30) produced from a monomeric mixture of 34.5% HEMA, 34.5% DHPMA, 30% VP and 1% EDGMA (mol%) had an excellent pore structure, high water content at swelling equilibrium (W eq=166% by mass) and acceptable mechanical properties. Two kinds of VP30-coated sensors, Pt/GOx/VP30 and Pt/GOx/epoxy-polyurethane (EPU)/VP30 sensors were examined in glucose solutions during a period of 4 weeks. The Pt/GOx/VP30 sensors produced large response currents but the response linearity was poor. Therefore, further studies were focused on the Pt/GOx/EPU/VP30 sensors. With a diffusion-limiting epoxy-polyurethane membrane, the linearity was improved (2-30 mM) and the response time was within 5 min. Eight Pt/GOx/EPU/VP30 sensors were subcutaneously implanted in rats and tested once per week over 4 weeks. All of the implanted sensors kept functioning for at least 21 days and 3 out of 8 sensors still functioned at day 28. Histology revealed that the fibrous capsules surrounding hydrogel-coated sensors were thinner than those surrounding Pt/GOx/EPU sensors after 28 days of implantation.

Synthesis and performance of novel hydrogels coatings for implantable glucose sensors.

Novel hydrogel polymers were prepared, characterized, coated on implantable glucose sensors, and tested in vitro and in vivo. The effects of 2,3-dihydroxypropyl methacrylate (DHPMA) on the swelling, morphology, glass transition (T(g)), and water structure were studied. The results show that the degree of swelling increases with increasing DHPMA content. Scanning electron microscopy (SEM) studies identified uniform, porous structures in samples containing 60-90 mol % DHPMA. Glass-transition temperatures did not change significantly with DHPMA content, but the ratio of freezing to nonfreezing water tended to increase with DHPMA content. Sensors coated with different hydrogels were prepared and in vitro evaluations were performed. The 80% DHPMA hydrogels exhibited optimum sensitivity, response, and stability when coated directly onto the sensor or top of a polyurethane (PU) layer. The histology results show that 80% DHPMA samples exhibit reduced fibrosis and reduced inflammation, resulting in a longer functional life.

Coil-type implantable glucose biosensor with excess enzyme loading.

As part of our overall long-term objective of designing a glucose sensor for long-term subcutaneous implantation, a coil-type implantable glucose sensor loaded with excess glucose oxidase (GOD) inside the coils of a 0.125mm diameter coiled platinum-iridium wire has been developed. The excess GOD was immobilized in a glutaraldehyde/bovine serum albumin (BSA) gel reinforced with cotton and located inside the coils chamber of the sensor. The excess GOD increased the lifetime of the sensor. Based on this coil-type design, various coil-type glucose sensors with cellulose acetate (CA), poly(vinyl chloride)(PVC), polyurethane (PU), poly(bisphenol A carbonate) (PC) and Nafion outer membranes were investigated and compared. Comparatively, Nafion based biosensors provided the best long-term response stability. However, Nafion can still not meet the lifetime requirement of the coil-type sensor with high enzyme loading because the observed function failure of these sensors was indeed caused by outer membrane damage rather than loss of enzyme activity. Additional experiments also revealed that hydrogen peroxide accumulation occurred in the GOD impregnated cotton when the sensors were not polarized which could cause a small false positive measurement. However, this artifact can be easily avoided by using an appropriate measurement technique.

A dexamethasone-loaded PLGA microspheres/collagen scaffold composite for implantable glucose sensors

We have developed a new dexamethasone (Dex)-loaded poly(lactic-co-glycolic acid) microspheres/porous collagen scaffold composite for implantable glucose sensors. The scaffolds were fabricated around the sensing element of the sensors and crosslinked using nordihydroguaiaretic acid (NDGA). The microspheres containing Dex were incorporated into the NDGA-crosslinked collagen scaffold by dipping in microsphere suspension in either water or Pluronic. The loading efficiencies of Dex in the microspheres and the scaffold were determined using high performance liquid chromatography. The microspheres/scaffold composite fabricated using microspheres in the hydrogel solution had a better loading efficiency than when microspheres were in water suspension. The composite fabricated using the hydrogel also showed a slower and more sustained drug release than the standard microspheres in vitro during a 4 week study and did not significantly affect the function of the sensors in vitro. The sensors with the composite that were still functional retained above 50% of their original sensitivity at 2 weeks. Histology showed that the inflammatory response to the Dex-loaded composite was much lower than for the control scaffold at 2 and 4 weeks after implantation. The Dex-loaded composite system might be useful to reduce inflammation to implanted glucose sensors and therefore extend their function and lifetime.

A novel porous collagen scaffold around an implantable biosensor for improving biocompatibility. II. Long-term in vitro/in vivo sensitivity characteristics of sensors with NDGA- or GA-crosslinked collagen scaffolds

We have developed a new 3D porous and biostable collagen scaffold for implantable glucose sensors. The scaffolds were fabricated around the sensors and crosslinked using nordihydroguaiaretic acid (NDGA) or glutaraldehyde (GA) to enhance physical and biological stability. The effect of the scaffolds on sensor function and biocompatibility was examined during long-term (>or=28 days) in vitro and in vivo experiments and compared with control bare sensors. To evaluate the effect of the sensor length on micromotion and sensor function, we also fabricated short and long sensors. 3D porous scaffold application around glucose sensors did not significantly affect the long-term in vitro sensitivity of the sensors. The scaffolds, crosslinked by either NDGA or GA, remained stable around the sensors during the 4 week in vitro study. In the long-term in vivo study, the sensitivity of the short sensors was higher than the sensitivity of long sensors presumably because of less micromotion in the subcutis of the rats. The sensors with NDGA-crosslinked scaffolds had a higher sensitivity than the sensors with GA-crosslinked scaffolds. Histological examination showed that NDGA-crosslinked scaffolds retained their physical structure with reduced inflammation when compared with the GA-crosslinked scaffolds. Therefore, the application of NDGA-crosslinked collagen scaffolds might be a good method for enhancing the function and lifetime of implantable biosensors by minimizing the in vivo foreign body response.