Design and Development of In Vivo Sensor Systems: The Long and Tortured Road to a Self-Contained, Implantable Glucose Sensor for Diabetes Management - Designing Receptors for the Next Generation of Biosensors - page 215

3.1 Competition/Signaling Component (DBA)
Macromolecular DBA constructs have been used for the first time by our group as
the glucose recognition and signaling agent in a competitive binding assay that will
ultimately be incorporated as a mass-sensitive detection method for the in vivo
determination of glucose concentration. A DBA construct has been described for
use in an in vitro saccharide sensor by James et al. In this example, anthracene units
are used as the dye indicator that correlates fluorescence intensity changes with
saccharide binding [ 59 ]. Although useful for detection of saccharides in an in vitro
environment, this type of detection technique is not applicable to an implantable
device for multiple reasons. The dendrimer constructs have limited aqueous solu-
bility due to the highly insoluble anthracene moiety [ 59 ]. More generally, the use of
anthracene as a candidate for in vivo applications is unfavorable due to sensitivity
issues, toxicity concerns, and lack of metabolic stability [ 60 , 61 ]. The viability of
this type of sensor in a physiological matrix would be compromised, as the material
would continue to lose sensitivity over time due to diminishing fluorescence
resulting from denaturation, photodegradation, and/or indicator poisoning [ 28 ].
This would, in turn, require that the device be continually calibrated and frequently
recharged with fresh reagents [ 7 ]. In addition, there is not a well-established method
for exciting the fluorophore and taking measurements from an implanted
fluorescence-based device without inserting an invasive probe into the subcutaneous
tissue [ 7 ]. The design of our DBA constructs remedies these obstacles to functional
implantation.
The first critical step required for demonstration of the glucose sensing system is
the construction of the DBA competitive agent/signaling component. The selection
of materials for the DBA component was dictated by the need to build synthetic
receptor moieties that would respond with optimal binding sensitivity and selectiv-
ity for glucose in a complex aqueous matrix of potentially competing analytes. In
addition, as deemed essential for extended function in a closed-cycle, long-term
implantable device that is continuously exposed to the lytic nature of physiological
fluid, the synthetic materials used to synthesize the DBAs must be stable and able to
perform without diminished capacity over the lifetime of the sensor. Separately, but
equally important, the materials must not be consumed during the detection process
or require external reagents. For these reasons, our work focused on the develop-
ment of a synthetic saccharide sensor that has the capacity to selectively detect
glucose with long-term integrity in a physiological system.
3.1.1 Boronic Acids as Recognition Elements for Glucose Detection
Boronic acid analogues as synthetic receptor ligands have been extensively
evaluated for their use as the molecular recognition component in the construction
of saccharide sensors [ 62 - 71 ]. It is known that boronic acids rapidly and reversibly