Noninvasive assessment of collagen gel microstructure and mechanics using multiphoton microscopy

Multiphoton microscopy of collagen hydrogels produces second harmonic generation (SHG) and two-photon fluorescence (TPF) images, which can be used to noninvasively study gel microstructure at depth ( approximately 1 mm). The microstructure is also a primary determinate of the mechanical properties of the gel; thus, we hypothesized that bulk optical properties (i.e., SHG and TPF) could be used to predict bulk mechanical properties of collagen hydrogels. We utilized polymerization temperature (4-37 degrees C) and glutaraldehyde to manipulate collagen hydrogel fiber diameter, space-filling properties, and cross-link density. Multiphoton microscopy and scanning electron microscopy reveal that as polymerization temperature decreases (37-4 degrees C) fiber diameter and pore size increase, whereas hydrogel storage modulus (G', from 23 +/- 3 Pa to 0.28 +/- 0.16 Pa, respectively, mean +/- SE) and mean SHG decrease (minimal change in TPF). In contrast, glutaraldehyde significantly increases the mean TPF signal (without impacting the SHG signal) and the storage modulus (16 +/- 3.5 Pa before to 138 +/- 40 Pa after cross-linking, mean +/- SD). We conclude that SHG and TPF can characterize differential microscopic features of the collagen hydrogel that are strongly correlated with bulk mechanical properties. Thus, optical imaging may be a useful noninvasive tool to assess tissue mechanics.