Quantitative Evaluation of Homogeneous Hydrogel Surface Wettability
Authors: Mizuno Hayato Laurence , Tan Eiki , Anraku Yasutaka , Sakai Takamasa , Sakuma Ichiro , Akagi Yuki 
Published: February 2020

A hydrogel is constructed of polymers that are crosslinked to form a three-dimensional network structure with water absorbed within. When using hydrogel as a biomaterial, wettability is one of the important physical properties as it largely affects how protein and cells can adhere on its surface. In previous studies aiming to evaluate the wettability of hydrogel, the affect was evaluated by changing the composition and side chain of the polymer chain constituting the network structure for existing hydrogels such as agarose and gelatin. However, the existing network structure of hydrogel has spatial heterogeneity and topological heterogeneity, and therefore it has been said to be difficult to independently determine which parameter of the network structure was truly affecting wettability. In this research, in order to solve this problem, we aimed to independently and quantitatively evaluate the effect of each parameter on wettability by using the Tetra-PEG gel.Tetra-PEG gel is a type of gel that is composed of two symmetrical tetra-branched structure of Tetra-amine-terminated PEG (Tetra-PEG-NH2) and Tetra-OSu-terminated PEG (Tetra-PEG-OSu). Gelation is done simply by mixing equal amounts of these polymer solutions. As our first attempt, we controlled the polymer volume fraction of the hydrogel and the molecular weight between the crosslinking points by adjusting the concentration of the polymer solution and the molecular weight of the polymer, respectively. To evaluate the effect of charged groups, p-tuned Tetra-PEG gel having ion pairs of positive and negative charged groups was prepared. On the other hand, r-tuned Tetra - PEG gel was prepared to evaluate the effect of the presence of ionic pairs on wettability. Positive or negative charged groups were introduced into the system by changing the mixing ratio of Tetra-PEG-NH2 and Tetra-PEG-Osu, and by so causing either polymer to excessively exist within the hydrogel.Wettability was evaluated through static contact angle measurement. 2 uL of water droplets were dropped onto the surface of the hydrogel, and the contact angle immediately after the dropping was measured. The contact angle measurement results for 5k, 10k, 20k, and 40k Tetra-PEG gel are shown in Fig. 1. For 5k Tetra - PEG gel, contact angle showed values less than 30 degrees, and contact angle showed a decrease with increase in volume fraction. However, contact angle for 5k polymer suddenlyd increase at volume fraction of 0.11.To give a logical and consistent explanation to this result, we introduced the idea of mobility of the polymer chains into the discussion of how the wettability of the hydrogel was determined. When the polymer molecular weight is large, the polymer chain is largely movable, so the PEG chain is bound in the bulk due to the intermolecular force with the water molecules in the gel . As a result, the surface exhibited a hydrophobic property. On the other hand, when the polymer molecular weight is small like 5k Tetra-PEG gel, it is considered that the surface was kept relatively hydrophilic because the polymer chain was bound near the surface and pulled the droplet inwards. Furthermore, in the region of low polymer molecular weight such as 5k Tetra-PEG gel, it is considered that if the volume fraction increases, the polymer chain becomes difficult to move and the hydrophilicity also increases.  Fig. 2 shows the results of contact angle measurement when the charged group concentration was changed. At polymer volume fractions 0.066, 0.081, and 0.096, the contact angle decreased with an increase in the charged group concentration, but decrease in contact angle could not be confirmed at polymer volume fraction of 0.50. In addition, it was confirmed from Fig. 3 that the contact angle of r-tuned Tetra-PEG gel was lower than that of p-tuned Tetra-PEG gel.These results are thought to be because the charged group acted as a hydrophilic group for water molecules which are polar molecules. Wettability did not decrease with p-tuned Tetra-PEG gel with a volume fraction of 0.050, because there were too few charged groups (5 mol/m3) per volume and was not sufficient to visibly affect the surface characteristics. In addition, since the p-tuned Tetra-PEG gel in which the ion pair exists exhibits lower wettability than the r-tuned Tetra-PEG gel, the affinity between the polymer and water decreases due to the presence of the ion pair. As a conclusion, it was revealed that polymer molecular weight greatly affects the mobility of the polymer chain and the wettability, and the wettability is increased by the introduction of the charged group, and when the ion pair is present, wettability decreases.

Figure 1