Tailored Smart Hydrogels as a Potential Sensing Material for Exhaled Breath Analysis

Smart hydrogels are a very promising sensing material, especially for biomedical applications, due to their stimulus responsiveness, easily achievable biocompatibility and wide range of material compositions. Many different physical or chemical stimuli (e.g. temperature, pH, ionic strength, biomolecules) can cause the volume-phase transition of the polymer which is usually based on the uptake or release of liquid. Hence, in most cases smart hydrogels are employed in aqueous environments [1].<br/>The development of a smart hydrogel material able to maintain the swelling response in gaseous environments would allow to extend the potential for very high selectivity and sensitivity towards analytes in gases such as volatile organic compounds (VOCs).<br/><br/>We have investigated different synthetic smart hydrogel materials with regard to their ability of exhibiting a measurable swelling response in varying relative humidity for the target VOC analytes acetone and isopropanol. The materials include the homopolymers polyacrylamide (PAM), polyacrylic acid (PAA) and poly(<i>N</i>-isopropylacrylamide) (PNIPAAm), and their co-polymer combinations. The swelling response for varying relative humidity from 2% to 100% and different VOC concentrations (up to 100 ppm) was studied by weighing and through piezoresistive pressure sensing. In the latter case, the hydrogels were directly polymerized on a silicon membrane by mask-based UV-polymerization.<br/>We found that PNIPAAm exhibits the most significant swelling response for both target analytes. It furthermore allows to distinguish between the analytes as the weight change for isopropanol is much higher than for acetone.<br/><br/>In a second study we focused on enhancing the stimulus-responsiveness of the material further by increasing porosity through adding of polyethylene-glycol (PEG) into the PNIPAAm pregel solution. After polymerization, PEG is washed out by keeping the sample in deionized water for thirty days, resulting in a porous structure.<br/>However, the pores are not stable and irreversibly collapse when the hydrogel is air-dried for use in a gaseous environment. In order to stabilize the porous matrix and maintain the enhanced swelling response, we determined that the following treatment is necessary after polymerization and washing: (i) freeze-drying of the sample (liquid nitrogen or freezer), and (ii) conditioning in high relative humidity (90 – 100 %). The first step is crucial to preserve the porous network after fabrication. In that state, the polymer chains are stretched and not in equilibrium due to the swelling during washing. In order to allow for relaxation and reordering of the polymer chains, the subsequent conditioning is required where the material shrinks.<br/>Scanning electron microscopy studies confirmed that these steps are essential in preserving the fabricated porous structure. It is furthermore indicated that the addition of PEG is crucial in stabilizing the pores as well. Without PEG, freeze-dried and conditioned samples exhibit the same structural collapse as PEG-modified air-dried and pure air-dried samples [2].<br/><br/>Based on these investigations, we can draw the following conclusions: (i) smart hydrogels can maintain their swelling-response in gaseous environments for a wide range of relative humidity, (ii) PNIPAAm is a promising candidate for acetone and isopropanol detection in gases and (iii) the swelling response can be strongly enhanced by adding of PEG and subsequent freeze-drying and conditioning to stabilize the porous structure.<br/>This study provides a basis for extending the application range of smart hydrogel materials to VOC sensing. Due to the potential for further tunability of the hydrogel material and its favorable mechanical properties, miniaturizable wearable sensor concepts can be envisioned for the future.<br/><br/>References<br/>[1] M. Koetting et al., <i>Mater. </i><i>Sci. Eng. R Rep.</i>, 93:1-49, 2015<br/>[2] S. Wang et al., <i>Tagungsband 16. Dresdner Sensor-Symposium</i>, pp.109-114, 2022