2.1. Preparation of the hydrogels
Agarose was dissolved in sterile PBS to obtain a 1% (w/v) solution by heating on a hot plate. The aqueous suspension of 0.1% (w/v) type I collagen was obtained by slowly hydrating collagen flakes in distilled water for 3 h, under magnetic stirring at 4 °C.
To prepare the agarose-collagen blended hydrogels (A-C), the warm agarose solution (at around 45 °C, the gel point is at 36 °C) was added to the collagen suspension. The hydrogels were prepared inside the wells of a 24 well plate: each well was loaded with 1 mL of A-C hydrogel. The starting agarose solution was diluted to obtain 3 different working concentrations, 0.5, 0.25, and 0.125%, respectively. The starting collagen solution was diluted to 0.02% in the final blend. The components were mixed gently with a glass rod and allowed to gel at room temperature. The gelation time varied from 30 seconds up to 10 minutes in the case of hydrogels containing 0.5% and 0.125% agarose, respectively. All the glassware was sterilized in an autoclave prior to be used, and the preparation of the hydrogels was carried out under a laminar flow hood. The preparation scheme is presented in Figure 1.