SPECTROSCOPY, IMAGING, OTHER TECHNIQUES |
Noninvasive Assessment of Collagen Gel Microstructure and Mechanics Using Multiphoton Microscopy
Christopher B Raub 1, Vinod Suresh 1, Tatiana Krasieva 1, Julia Lyubovitsky 1, Justin D Mih 1, Andrew J Putnam 1, Bruce J Tromberg 1 and Steven C George 1*
1 University of California, Irvine
* To whom correspondence should be addressed. E-mail: scgeorge{at}uci.edu.
Submitted on September 25, 2006
Revised on October 6, 2006
Accepted on 1 December 2006
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Abstract |
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Multiphoton microscopy (MPM) of collagen hydrogels produces second harmonic generation (SHG) and two-photon fluorescence (TPF) images, which can be used to non-invasively study gel microstructure at depth (~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°C) and glutaraldehyde (GTA) to manipulate collagen hydrogel fiber diameter, space-filling properties, and cross-link density. MPM and scanning electron microscopy (SEM) reveal that as polymerization temperature decreases (37°C to 4°C) fiber diameter and pore size increase, whereas hydrogel storage modulus (G', 23 ± 3 Pa to 0.28 ± 0.16 Pa, respectively, mean ± SE) and mean SHG decrease (minimal change in TPF). In contrast, GTA 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 which are strongly correlated with bulk mechanical properties. Thus, optical imaging may be a useful noninvasive tool to assess tissue mechanics.
Key Words:
collagen, fluorescence, optical imaging, rheology, second harmonic
