Materials and Experimental Approaches
Preparation of Hydrogel-Based Composite Membrane
The monomer HEMA (70 wt%), water (30 wt%), the cross-linker ethylene glycol dimethacrylate (EGDMA, 0.1 mol% of HEMA), and the catalyst tetramethyle- thylenediamine (TEMED) were mixed to form a homogeneous aqueous solution (HEMA solution). Single-walled CNTs (purity > 90 wt%, length = 1-3 pm, from Chengdu Organic Chemicals Co. Ltd.) were added into HEMA solution to form suspension with 1-2 mg/mL of CNTs. This suspension was firstly cooled to 4°C and then mixed quickly with 20 wt% of ammonium persulfate (APS) solution (1 vol% of suspension) under stirring, followed by casting in molds (for both cylindrical sample and thin membrane) and left to stand for 24 hours at 4°C. The reaction of polymerization was completed until the initiator APS was fully consumed. The suspended CNT particles were bonded by both physical and chemical bonds with the base material, PHEMA hydrogel (Kharismadewi et al., 2016; Massoumi et al., 2016). This synthetic process and the content of chemicals used in the process were chosen based on that, it can give the composite the best mechanical and electrical characteristics (Ansari et al., 2016).
Purification Treatment of Hydrogel-Based Composite
After the synthesis of the hydrogel-based composites, swelling and cleansing processes have to be conducted to release the residual stress between polymer chains and remove the water-soluble contaminants (Zhao et ah, 2015). In this work, the samples of hydrogel composites after synthesis were immersed in phosphate-buffered saline (PBS) to absorb ions. PBS was used as an electrolyte for the purpose of mimicking a biocompatible environment for any potential bioelectric applications (Huang et ah, 2017). During this stage, ions and solvent molecules entered into the gaps between molecular chains and pushed them apart. Meanwhile, the solvent molecules attached to the chains and occupied the space between the hydrogel chains, which made the volume of swollen hydrogel composites larger than those samples before the swelling. The PBS was renewed every 4 hours to remove the residuals inside the composites. Such a renewal process was conducted six times before conducting experimental tests on the samples.
Experimental Approaches for Tests
Two cumulative release experiments were conducted, with the purposes of (i) investigating the diffusion characteristics of PHEMA-CNT composite, when it is interacted with glucose molecules; (ii) studying the capacity of absorption of PHEMA-CNT composite hydrogel to glucose.
For the first type of cumulative release experiment (Purpose I), the experimental set-ups, conditions, and descriptions are illustrated in Figure 11.2. Steps 1-3 are the preparation procedures, aiming to fabricate the PHEMA-CNT composite, which is filled with glucose molecules. Three days of stirring ensure that the glucose concentration inside the PHEMA-CNT composite sample saturates completely. Step 4 aims to obtain the speed that glucose molecules diffuse out from the PHEMA-CNT composite sample, by monitoring the glucose concentration in the PBS buffer over time. The constant environment for the diffusion experiment under 37°C and 100 r/min stirring is maintained in a water bath and using a magnetic stirring system.
According to Step 4 in Figure 11.2, 3 mb solution is withdrawn from the PBS buffer solution outside the PHEMA-CNT composite sample and immediately replaced by 3 mL fresh 37°C PBS buffer solution at the designated time interval points (tx). The samples of such withdrawn solution are stored in a -5°C fridge in order to avoid the glucose diffusing out from the PHEMA-CNT composite sample to be consumed by germs, which may affect the accuracy of the experimental results. Thus, an ICB SBA-90 laboratory glucose meter (±0.01 g/L) is employed to measure the glucose concentration in the withdrawn solution samples. However, in Step 4, a small amount of glucose exists in 3 mL solution, which is extracted from the PBS buffer solution every time at tx, but the 3 mL replaced PBS buffer solution does not contain any glucose. This process causes a glucose loss, thus underestimating the amount of glucose that is released from the PHEMA-CNT composite sample. Aiming to eliminate such glucose
FIGURE 11.2 Process of the cumulative release experiment to determine diffusion characteristics of glucose in PHEMA-CNT composite.
loss, the amount of glucose released from PHEMA-CNT composite sample has to be calculated:
where M(tx) and Mm(tx) are the actual amount and measuring amount of glucose at the designated time interval points tx, respectively; x is the order number, which indicates the withdraw time of the 3 mL PBS buffer solution. The equilibrium value of M(tx) equals the value of M„ from Equation (11.3).
The experimental set-ups, conditions, and descriptions for the other type of cumulative release experiment (Purpose II), which aims to study the capacity of absorption of PHEMA-CNT composite to glucose, are summarized in Figure 11.3. Step 1 in Figure 11.3 is the same as Step 3 in Figure 11.2, ensuring the glucose is completely saturated inside the PHEMA-CNT composite sample. The experimental steps of such a cumulative release experiment (Purpose II) are described as follows: (1) Place the PHEMA-CNT composite sample into PBS buffer solution to diffuse the glucose out from the sample. (2) Keep the environment under 37°C, 100 r/min stirring for 24 hours, which allows the release of the glucose to reach the equilibrium. (3) Extract 3 mL of the PBS buffer, mark as a sample number y, and store in fridge to keep temperature of -5°C, to avoid consumption of the glucose by germs. (4) Then place the PHEMA-CNT composite sample into 300 mL fresh PBS buffer solution, and repeat Steps 2 and 3. This repeated process is terminated if the concentration of glucose in the PBS buffer is zero (Step 4). To guarantee the zero concentration of glucose, many repeats of Steps 2 and 3 have to be implemented. In this study, seven
FIGURE 11.3 Process of the cumulative release experiment to determine the capacity of absorption of PHEMA-CNT composite on glucose molecules.
times of repeat were carried out, which means samples marked with у (у = 1, 2, 3,.. .,7) have been obtained. Therefore, the amount of glucose that is absorbed by the PHEMA-CNT composite sample (Me,ucose [mol]) is given by:
where My denotes the mass of glucose (g) that diffused out from the PHEMA- CNT composite sample at the time corresponding to the number y. Cy is the measured concentration of glucose (g/mL) from the withdrawn solution sample, which is marked as number y. As previously, the concentration of glucose was measured utilizing the ICB SBA-90 laboratory glucose meter (+0.01 g/L). Thus, the capacity of absorption (Г) of the PHEMA-CNT composite sample to glucose can be given by: