Hydrogel Synthesis

The pre-gel solution was created in a 1000µl test tube; materials, role in polymerization and amounts added are listed in Tables 1.

Material	Purpose	Structure	Mole percent
2,2-Dimethoxy-2-phenyl-acetophenone (DMPAP)	Free readical initiator (photoinitiator)		0.0012
2-Hydroxethyl methacrylate (HEMA)	Polymer backbone		21.2121
Tetraethylene glycol dimethacrylate (TEGDMA)	Crosslinker		3.0303
Ethylene glycol (EG)	Solvent		75.7576

Table 1. Hydrogel pre-gel constituents, their roles in hydrogel free radical polymerization, chemical 2D polymeric structure and the amounts added to the pre-gel solution.

Synthesis

1. Prior to beginning hydrogel synthesis, a synthesis mold was assembled from two glass slides and three, 520-micron thick polyolefin-sheet spacers; this configuration was held together by binder clips as shown in Figure 6. The large glass slides were offset by a few millimeters to create a channel for pipetting the pre-gel solution into the mold.
2. Before beginning hydrogel synthesis, obtain a 1000µl test tube, the chemicals described in Table 1, a micropipette with clean tips, and setting mold (Figure 6). All work should be performed in a fume hood with proper Personal Protective Equipment (PPE). PPE includes safety glasses or goggles, a lab coat and protective gloves.

Figure 6: Hydrogel synthesis mold, created from two glass slides, three strips of 520-micron thick polyolefin-sheet as spacers, and large binder clips.

3. Add 0.0012 mole percent 2,2-Dimethoxy-2-phenyl-acetophenone (DMPAP), the solid photoinitator (free radical initiator that is initiated by light), to the test tube first.
4. Add 21.2121 mole percent 2-hydroxethyl methacrylate (HEMA), the backbone molecule, and 3.0303 mole percent tetraethylene glycol dimethacrylate (TEGDMA), the crosslinking molecule to the test tube, using new pipette tips each time. TEGDMA will chemically crosslink the HEMA chains in the presence of free radicals by connecting the polymer chains into a network polymer.
5. Mix the solution using a vortex machine until a homogenous solution is achieved.
6. Measure 0.25 grams of Bromocresol Purple and rinse it into the solution using 75.7576 mole percent ethylene glycol (EG), the solvent. The pigment is only for viewing purposes (the hydrogel is transparent otherwise), and EG serves as a solvent to dissolve the initiating free radical initiator and keeps the hydrogel flexible.
7. Mix the solution using the vortex machine until the pigment completely dissolves and the solution is homogenous.
8. Using a micropipette, deposit the solution into the mold by aligning the tip of the micropipette with the offset edge of the large glass slides and uniformly injecting the pre-gel solution into the center of the mold.
9. Place mold 5 centimeters below a UV emitting flashlight (Warson SK66) and irradiate the mold for one minute. UV light cleaves the bonds in the initiator species, turning them into free radicals which can then attack the polymer and crosslinker molecules. When fully networked, the hydrogel should be a rubber solid with a jello-like consistency.
10. Remove mold from the light and disassemble the mold configuration. Remove hydrogel from the glass slides.
11. Rinse both sides of the hydrogel with deionized water to remove any unreacted chemical species and oligomers from the product.
12. To characterize how various UV-light exposure times affect degree of crosslinking and swelling ability, this procedure can be repeated while varying step nine. For characterization, the solution was exposed to UV light for 1 minute, 1.5 minutes and 5 minutes, producing a total of three hydrogels.

Characterization

The swelling degree of the hydrogel can be calculated by drying, hydrating and then re-drying the polymer.

1. Place the finished hydrogels in a container with an alcohol such as Isopropyl Alcohol so that they are totally submerged. Leave in the alchohol for 4-8 hours, when the alcohol has replaced all the ethylene glycol in the hydrogel.
2. Remove the hydrogels from the alcohol and leave to dry in the open, about 30 minutes. The alcohol evaporates more quickly than water or the solvent, allowing the hydrogel to maintain its structure.
3. Weigh the dried hydogels.
4. Submerge the hydrogels in DI water for at least 30 minutes, until they're completely swollen. Remove the gels from water, gently wipe dry and weigh.
5. Calculate the swelling degree using the equation:  , where  is the weight of the swollen polymer and  is the weight of the dried polymer.