Affinity-based turbidity sensor for glucose monitoring by optical coherence tomography: toward the development of an implantable sensor

We investigated the feasibility of constructing an implantable optical-based sensor for seminoninvasive continuous monitoring of analytes. In this novel sensor, analyte concentration-dependent changes induced in the degree of optical turbidity of the sensing element can be accurately monitored by optical coherence tomography (OCT), an interferometric technique. To demonstrate proof-of-concept, we engineered a sensor for monitoring glucose concentration that enabled us to quantitatively monitor the glucose-specific changes induced in bulk scattering (turbidity) of the sensor. The sensor consists of a glucose-permeable membrane housing that contains a suspension of macroporous hydrogel particles and concanavalin A (ConA), a glucose-specific lectin, that are designed to alter the optical scattering of the sensor as a function of glucose concentration. The mechanism of modulation of bulk turbidity in the sensor is based on glucose-specific affinity binding of ConA to pendant glucose residues of macroporous hydrogel particles. The affinity-based modulation of the scattering coefficient was significantly enhanced by optimizing particle size, particle size distribution, and ConA concentration. Successful operation of the sensor was demonstrated under in vitro condition where excellent reversibility and stability (160 days) of prototype sensors with good overall response over the physiological glucose concentration range (2.5-20 mM) and good accuracy (standard deviation 5%) were observed. Furthermore, to assess the feasibility of using the novel sensor as one that can be implanted under skin, the sensor was covered by a 0.4 mm thick tissue phantom where it was demonstrable that the response of the sensor to 10 mM glucose change could still be measured in the presence of a layer of tissue shielding the sensor aiming to simulate in vivo condition. In summary, we have demonstrated that it is feasible to develop an affinity-based turbidity sensor that can exhibit a highly specific optical response as a function of changes in local glucose concentration and such response can be accurately monitored by OCT suggesting that the novel sensor can potentially be engineered to be used as an implantable sensor for in vivo monitoring of analytes.