Development of a glucose- and temperature-sensitive hydrogel for force-compensated biomedical sensors

Stimulus-responsive hydrogels are polymers that change their water content as a function of external factors and measurands such as salt and ion concentrations, pH or glucose. In the past, numerous sensor principles with hydro- gels as sensing materials have been reported. A common challenge with these sensors is their slow transient response, as the volume phase transition (VPT) of a hydrogel is associated with extensive diffusion processes and viscoelastic creep. A recently presented technique uses force compensation to counteract these effects and shorten hydrogel- response times [1]. The principle is based on a dual-sensitive hydrogel, i.e., the hydrogel is designed to have an addi- tional sensitivity to temperature . This allows one to counteract measurand-induced VPT by means of a closed-loop temperature control and to keep the hydrogel’s swelling state at a fixed bias point. Instead of the hydrogel’s swelling response, the balancing temperature is being used as a measurement signal. In previous work, we have documented that a response time reduction of up to 70% could be achieved when using a dual-sensitive hydrogel with sen- sitivity to sodium chloride and temperature [1]. In this paper, we present a novel dual- sensitivity hydrogel designed to measure physi- ologically relevant glucose concentrations. The dual-sensitivity to glucose and temperature, respectively, is achieved by copolymerization of N-Isopropylacrylamide (NiPAAm) and phenyl- boronic acid (APB) [2]. However, the hydrogel was specifically designed to meet the require- ments for the application in a force- compensated hydrogel-based sensor, i.e., glu- cose-induced swelling can be quasi-statically compensated by temperature-control. The swelling behavior of this dual-sensitive hydrogel net-P(NiPAAm-co-PBA) is shown in Fig. 1. For each glucose concentration cG in the physiolog- ically relevant range, there is a corresponding temperature with which the degree of swelling Qm can be maintained at a constant bias point. For example, if a bias point of Qm,0 = 9.9 is cho- sen, a swelling caused by a change of cG = 5 mM to cG = 10 mM can be counteracted by controlling the temperature from 25 °C to 30 °C. This shows that the swelling degree of the hydrogel can be compensated quasi- statically and it is suitable for the force- compensated hydrogel sensor. The integration of this hydrogel into the sensor and response time reduction are currently being studied.