Gradient Strain Guidance for Cellular Micro-Alignment (Lab Chip 2014)
Biomimetic topography and biomimetic strain stimulation were two of the most common approaches for cellular alignment. In the artificial tissue construct, these two models need to be considered together to coherent the biomimetic cellular alignment under the stimulations of hydrogel shape and the mechanical force for regenerative medicine. Therefore, in this paper, we introduce an active static gradient strain microfluidic chip with flexible PDMS membrane to simultaneously generate a variety of static gradient strain without external mechanical control for cell stimulations in 3D ECM microenvironments. In Fig. 1(a), we develop a flow chip by integrating a TMSPMA-coated flat glass slide (bottom) with a molded PDMS membrane (top) to form a gradient strain chip. When the hydrogel prepolymer solution with cell suspension was overloaded into the flow channel, the PDMS membrane generated a convex curve (as shown in Fig. 1(b)). At this moment, the circular cell-laden hydrogels were formed by UV crosslinking via a photomask. The height of cell-laden hydrogels gradually decreased from the center (circular number 1) to the boundary (circular number 12). After releasing the liquid pressure on the PDMS membrane and removing cell prepolymer residues, in Fig. 1(c), the convex PDMS retracts back to a flat surface and applies force onto the concentric circular hydrogels. Therefore, it generates a gradient static strain on the cell-laden hydrogels. The gradient elongation of concentric hydrogels are observed from day 0 to day 5 at 0, 20, and 40 µL volume injection. Basically, the gradient strain/elongation maintains at least 5 days. Because the circular hydrogel pattern was employed to confine the hydrogel elongation direction, the compressed strain on the hydrogel will convert to the elongated stretch in radius direction. The cell alignment was normalized to a new coordinate system that 0° or 180° represents cell nuclei aligns along the tangent line of the circle, and 90° means cells align along radius direction. NIH3T3 cells were cultured in the gradient strain chip for 3 days. In Fig. 2, cells were in a radius alignment with the radius-to-circle alignment ratio of ~4.2 in hydrogel number 1 (near the center of the circle) and in a circle alignment with the alignment ratio of ~0.22 at hydrogel number 12 (near the boundary). Besides, the alignment ratio from the center (circular number 1) to the boundary (circular number 12) is gradually decreasing, and the cell viability in the gradient strain chip can still be larger than 80% for 5 days.