(Invited) Semi-Interpenetrating Polymer Networks Based on N-Isopropylacrylamide and 2-Acrylamido-2-Methylpropane Sulfonic Acid for Hydrogel-Based Chemical Sensors
Authors: Binder Simon , Gerlach Gerald 
Published: July 2021

Introduction

Stimulus-responsive hydrogels represent sensor elements with a high application potential for the detection of the concentration of a solved species. In combination with a piezoresistive pressure sensor, the hydrogel’s analyte-dependent swelling pressure is transformed into an electrical output signal. In the past, numerous sensors with sensitivities to various analytes, like e.g. ethanol, glucose and pH, have been researched using this sensor principle [1]. However, the time span until a final steady-state value of the sensor’s output signal is reached is typically in the range of minutes to hours due to the excessive diffusion processes associated with the gel's volume phase transition and visco-elastic behaviour.

State of the art

Previous strategies for shortening the response time mostly addressed reducing the hydrogel layer’s dimensions, which concurrently results in a reduction of sensitivity. The measurement method of force compensation using a bisensitive hydrogel represents a sensitivity-preserving approach. The concept of force compensation with a bisensitive hydrogel is to counteract the swelling of the hydrogel with a second stimulus, in this case the temperature. A closed-loop configuration is used to control the temperature with a Peltier element in a way that the hydrogel is continuously kept at a fixed swelling state (Figure 1a). The applied temperature is therefore directly proportional to the analyte concentration to be measured and thus represents the sensor output signal containing the measurement information [2]. By using this configuration, the time-consuming diffusion processes can be reduced significantly. In a previous work, a force-compensated sensor utilizing a bisensitive hydrogel enabled a response time reduction of up to 70% [3].

Method

In [3] an interpenetrating polymer network (IPN) is used as a bisensitive hydrogel, which is sensitive both to the ion concentration c
Na
+ of a saline solution (measurand) and to the temperature ϑ (compensation parameter). This IPN showed sufficient compressive strength for the operation under the influence of the pressure sensor membrane’s restoring force. However, its sequential two-step synthesis complicates the gel structuring process. With regard to enable gel synthesis, gel structuring and gel bonding within one single process step, a single-step synthesis that yields a bisensitive gel with similar compensatory and mechanical properties is desirable. In this work a semi-interpenetrating polymer network (semi-IPN) is presented, which fulfils these requirements. According to IUPAC it is called [net-P(AMPS-co-NiPAAm)]-sipn-PAMPS. It consists of a single statistical copolymer network based on N-isopropylacrylamide (NiPAAm) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) in which long polymer strands of the sulfonic acid (PAMPS) are incorporated, thus contributing to an improvement of the mechanical properties. A second cross-linking reaction is omitted in this way.

Materials

NiPAAm (Acros Organics) was purified by recrystallization from n-hexane while AMPS (Sigma Aldrich), PAMPS (average molecular weight: 2,000,000 g/mol, mass fraction of the solution: 15 wt% in water, Sigma Aldrich), N,N’-methylenebisacrylamide (BIS, Merck) and N,N,N’,N’-tetramethylethylenediamine (TMEDA, Sigma Aldrich) were used without further purification. The initiator sodium peroxodisulphate (NaPS, Riedel-de Haën) was used as a 0.84 molar aqueous solution (1.00 g in 5.0 ml of water).

Synthesis

The semi-IPN was synthesized by redox-initiated free radical polymerization in water in an argon atmosphere. NiPAAm (1057.3 mg, 9.343 mmol), AMPS (59.9 mg, 0.289 mmol), BIS (59.4 mg, 0.385 mmol), PAMPS (363.3 mg of the solution) and NaPS (57.3 µl of the stock solution, 0.048 mmol) were dissolved in 7.9 ml deionized water. Sodium hydroxide solution NaOH (600 µl of a stock solution with concentration 1 mol/l) was added to have predominantly basic conditions. The solution was degassed and cooled in iced water for 10 min. After initiating the polymerization with TMEDA (7.3 µl, 0.048 mmol) the pregel solution with a total volume of 10 ml was filled into polymerization vials and cooled at 15 °C for 3 h. Finally, the semi-IPN hydrogel was cleaned for three days with deionized water to remove polymerization residues. The bisensitive free-swelling characteristics were measured by investigating the mass-based swelling degree Q
m as a function of c
Na
+ and ϑ according to [2]. Flat cylindric samples (diameter: 15 mm, thickness: 3 mm) were fabricated to compare the ultimate compressive strength with the state-of-the-art IPN hydrogel using a rheometer (Anton Paar Physica MCR 301).

Results and Conclusions

Figure 1b shows, that the semi-IPN hydrogel exhibits the desired bisensitive swelling characteristic. For a defined operating point at Q
m ≈ 8, there is an almost linear relation between the measured variable ‘ion concentration c
Na
+’ (scaled logarithmically) and the compensation parameter ‘temperature ϑ’ for the indicated operating range of the sensor (Figure 1c).
Excited at the same stimulus parameters, the semi-IPN samples showed a significant higher compressive strength (> 40 N) than the IPN samples (22.3 ± 4.5 N), possibly due to a denser copolymer network as well as the additional polymer strands present in the network.
Exemplary sensor measurements in the force compensation mode using the bisensitive semi-IPN hydrogel are illustrated in Figure 1d. A significant response time reduction of more than 50% compared to the usual deflection method is also achieved with this hydrogel. Hence, the results prove the suitability of the novel hydrogel within the sensor and lay the foundation for a simplified and reproducible fabrication of the hydrogel layer directly on the sensor’s socket.

References

[1] A. Richter, G. Paschew, S. Klatt, J. Lienig, K.-F. Arndt, H.-J. Adler, Sensors 8(1):561-581 (2008), doi: 10.3390/s8010561.
[2] S. Binder, A. T. Krause, B. Voit, G. Gerlach, IEEE Sensors Letters 1(6) (2017); doi: 10.1109/LSENS.2017.2774922.
[3] S. Binder, G. Gerlach, Technisches Messen 86(4):227-236 (2019); doi: 10.1515/teme-2019-0004.

Figure 1