Easy Manipulation of Architectures in Protein-based Hydrogels for Cell Culture Applications

The overall goal of this methodology is to create macroporous hydrogels with varying pore sizes for cell culture applications. This method can help answer key questions in the field of 3D cell culture such as the principle feasibility of a novel hydrogel material for 3D cell culture applications. The main advantage of this methodology is that it offers a range of techniques to optimize and adjust a novel material to the requirements of different types of cells.

To begin, dilute one milligram of the RGD peptide or an equivalent cell adhesive peptide in 100 microliters of sterile water to obtain the 10 milligram per milliliter solution. Next, remove the bottom of a 96 well plate and replace it with removal plastic wrap. Mix 100 microliters of BSA stock solution and 100 microliters of THPC stock solution in the 96 well plate to obtain 200 microliters of the hydrogel.

Mix the components by pipetting up and down at least five times to guarantee a uniform hydrogel after polymerization. Place the 96 well plate at room temperature for about 10 minutes until all hydrogels are properly polymerized. Following polymerization, carefully and slowly remove the plastic wrap from the bottom of the plate.

Press the hydrogels out of the 96 well plate using a small stab and transfer them to 1.5 milliliter tubes with sterile PBS pH 7.4. Store the hydrogels in phosphate buffered saline at four degrees Celsius for up to several months. To freeze-dry the hydrogels, first fill 1.5 milliliter tubes with 500 microliters of sterile deionized water and remove the caps.

Transfer the hydrogels to the 1.5 milliliter reaction tubes using forceps. Wrap the 1.5 milliliter tubes tightly with paraffin film using at least three layers for each vial. Use a needle to pierce small holes in the film to enable gas release from the tubes.

To guarantee that the water and hydrogel completely freeze, transfer the vial to liquid nitrogen solution for five minutes. Immediately after removal from the liquid nitrogen, transfer the vials to the freeze-dryer to prevent the material from thawing. As an alternative freezing step, keep the vials at minus 20 degrees Celsius overnight to slowly freeze the hydrogels.

Immediately after removal from minus 20 degrees Celsius, transfer the vials to the freeze-dryer to prevent the material from thawing. After 24 hours and the complete evaporation of the water in and around the hydrogel, thaw the material by removing it from the freeze-dryer. For particle leaching, mix the components of the hydrogel as before.

Following mixing, immediately add sodium chloride until saturation occurs. Add salt until salt crystals can be seen in the solution as a white precipitate. Pore sizes can be altered by grinding the salt crystals prior to use.

Transfer the 96 well plate to a shaker and shake until polymerization takes place. After about 10 minutes, remove the hydrogels from the plate as before. Incubate the hydrogels for at least 24 hours at room temperature in sterile water on a rotator to elute all salt from the hydrogel template.

Store the hydrogels at four degrees Celsius in PBS for up to several months. For gentle formation, prepare hydrogels as described. Then remove a hydrogel from solution using a pincer.

Remove the excess water with highly absorbent paper and place it on top of a block of dry ice. Freeze the hydrogel for 30 seconds and carefully remove it from the block. Do not damage the hydrogel.

Carefully use a spatula to scrape it off the block. Transfer the hydrogel to a 1.5 milliliter tube and dry it overnight at 37 degrees Celsius. Prepare a Rhodamine B stock solution as well as serial dilutions as described in the text protocol.

Remove the hydrogel from the stored solution and transfer it to a 1.5 milliliter tube with one milliliter of 0.01 milligram per milliliter Rhodamine B solution. Stain the hydrogel overnight in Rhodamine B solution at room temperature. The next day, transfer the hydrogel that is to be visualized to 10 milliliters of PBS and wash for at least three hours.

Cut small slices out of the hydrogel using a blade. Then transfer the hydrogel onto an eight well microscopic slide and cover it with PBS. Using a confocal laser scanning microscope, visualize the hydrogel at a wavelength of 514 nanometers.

Sterile filter all stock solutions with a 0.45 micron filter. Begin preparation of the hydrogel and include the cell adhesive peptide prior to hydrogel polymerization. After mixing the components, immediately pipette the mixture into an eight well microscopic slide until the bottom is completely covered.

Transfer the cells into pre-warmed sterile DMEM cell culture medium supplemented with fetal bovine serum, penicillin streptomycin, and nonessential amino acid solution. After counting the cells with a Neubauer counting chamber, carefully pipette 200 microliters of the desired number of cells onto the hydrogel surface. Cover the eight well microscopic slide with a lid and transfer it to an incubator.

After at least four hours of cellular attachment, wash the cells twice with 200 microliters of sterile cell culture PBS. Next, fix the cells with 200 microliters of 3.7%formaldehyde for 10 minutes at room temperature and wash twice with PBS. Permeabilize the cells with 200 microliters of 0.1%Triton X for five minutes before washing twice more with PBS.

Stain the cells with Toluidine Rhodamine by mixing five microliters of methanolic stock with 195 microliters of PBS and adding it to the cells at room temperature. Stain the cells for 20 minutes in the dark before washing twice with PBS. Investigate the cell adhesion properties using a confocal microscope at 514 nanometers.

Analyze the hydrogel properties as described in the text protocol. As shown here, the polymerized and stained hydrogels have no distinctive architectures. After freeze-drying the hydrogels at different temperatures, the average size of the pores within the hydrogel can be extended up to 60 microns.

Salt leaching results in uniform networks with pore sizes of about 10 microns. The size can be further adjusted by grinding salt crystals prior to use. The use of a gradient freezing approach on a block of dry ice leads to the formation of channels with a diameter of about 20 microns and a length of several hundreds of microns.

Prior to functionalization with the cell adhesive moiety, cells cannot adhere to the surface of the hydrogel and show a round morphology. Only after providing the material with the cell adhesive peptide, the cells find focal adhesion points which enable a proper adhesion of the cell onto a material. This is crucial for use of novel materials in 3D cell culture applications.

After watching this video, you should have a good understanding of how to manipulate a hydrogel's architecture with simple methods. This method can be applied for most novel hydrogel systems to learn about the use in cell culture and to optimize the hydrogel's architecture depending on the intended use.