Harnessing Mechanical Cues to Enhance Cellular Migration in a Novel Tissue Engineered Dermal Substitute

Purpose: Current dermal replacement products perform sub-optimally in complex wound beds, such as those that have been irradiated or those with exposed hardware, mostly as a result of insufficient cell invasion and vascularization. Angiogenesis is the result of multi-step processes which involve complex interactions between endothelial cells and their microenvironment. Cells sense the rigidity of their environment in a process called mechanotransduction, which is effected through integrin-mediated adhesions. Directional cell migration based upon substrate rigidity has previously been observed in a process termed durotaxis. We have fabricated a novel micropatterned microsphere scaffold (MSS) composed of differential densities of type I collagen in order to harness these signaling cues and promote rapid cell invasion and vascularization. Herein we compare the performance of MSS to a widely utilized, commercially available dermal replacement product (Integra®) in vitro and in vivo.

Methods: Microspheres composed of 1% type I collagen 50-150um in diameter were created and encased in a 0.3% type I collagen bulk. For our in vitro study, polydimethylsiloxane (PDMS) wells of 4mm diameter and 2mm height were filled with the microsphere scaffolds. 3mm Integra® disks were placed inside PDMS wells. Non-microsphere containing 1% and 0.3% collagen scaffolds served as controls. A monolayer of human umbilical vein endothelial cells (HUVEC) was seeded onto this three-dimensional platform, stimulated with 1uM sphingosine-1-phosphate, and cultured for 3 days. The collagen hydrogels were then imaged using confocal microscopy and z-stacks obtained to quantify cell invasion. For the in vivo study, 8x2mm MSS disks were created, along with 1% and 0.3% collagen controls. 8mm Integra® disks were created, and the silicone layer was removed to allow invasion from either side (comparable to the other discs). A disk of each type was then implanted subcutaneously in the dorsum of 8-week old wild-type mice. The scaffolds were removed at 7 and 14 days, imaged, and analyzed with ImageJ.

Results: In vitro results demonstrated significantly higher cell counts in both MSS and Integra® scaffolds compared to controls (p<0.001). Invading HUVEC penetrated significantly deeper in MSS compared to Integra® (mean depth of 73.5um vs. 40um, p<0.001), as well as 0.3% and 1% controls (mean depth of 13.4um and 12.2um respectively, p<0.001). In vivo results demonstrated robust cellular invasion throughout depth of the MSS construct, with more cells reaching the equator of the scaffold compared to Integra® and controls at both 7 days (p>0.05) and 14 days (p=0.03). Immunohistochemistry verified the presence of CD31 positive, CD45 negative cells within the MSS constructs.

Conclusions: These studies demonstrate superior cellular invasion of MSS both in vitro and in vivo compared to the current gold standard dermal regenerative template. Our novel hydrogel scaffold composed only of differential densities of type I collagen harnesses mechanical cues to significantly enhance cellular migration into the graft, offering a promising alternative to currently available dermal replacement products.