TABLE 1.

Physical and structural characteristics of PEG, collagen, and fibrin gels

Material	Main cross-linking mechanism	Solid content (mg/ml)	Average pore diameter (μm)	G′ storage modulus (Pa)	G″ loss modulus (Pa)	δ phase angle (°)	Q swelling ratio	Volumetric swelling
PEG	covalent	21.7	0.025	290 ± 18	2.6	2.3	42 ± 4	4.6
Fibrin	covalent	2	0.6 ± 0.1	26.8 ± 6.4	4.8	5.0	—	1
Collagen	covalent and physical	2	7.4 ± 1.1	6.8 ± 2.7	2.2	17.7	—	1

The average pore size and the swelling ratio Q for PEG was calculated as described in Materials and Methods. Pore size of fibrin and collagen gels is based on perfusion measurements (mean ± SD, n = 4) and calculated by Darcy's law (40). Both values are in good agreement with estimates from image analysis of CLSM projections. The values for storage and loss modulus, G′ and G″, as well as the phase angle, δ, were derived as means from rheological measurements between 0.5 and 1 Hz. For G′, SD for n = 4 is given. Volumetric swelling was measured on a buoyancy balance.