@article {18108, title = {Optical Projection Tomography as a Tool for Visualizing Hydrogels Microstructures}, journal = {ESB 2014 - 26th European Conference on Biomaterials}, year = {2014}, month = {2014-08-31 00:00:00}, address = {Liverpool, UK}, abstract = {

INTRODUCTION

Optical Projection Tomography (OPT), is a non-destructive 3D imaging technique, where a suspended specimen, immersed in an index-matching liquid, is rotated and an image is taken at each orientation. The 3D volume of the samples can be reconstructed using back projection algorithm [1].

Hydrogels are transparent biomaterials with a refractive index close to the refractive index of water. This reduces light reflection and scattering in hydrogels in a water solution, enabling OPT imaging without using optical clearing treatments. These optical characteristics enables the use of OPT to image the 3D microstructures of hydrogels and for in vivo OPT imaging of tissue engineered (TE) samples based on hydrogels. Here we present preliminary results of OPT imaging of different Gellan Gum (GG) hydrogels. Projections and 3D hydrogels reconstructions are shown and the statistical properties of the images are analysed.

EXPERIMENTAL METHODS

Four different hydrogels with physical crosslinking [e.g. monovalent cations or spermine (SPM)], different crosslinker quantities and chemical modifications with methacrylate (MA), and one hydrogel with combination of chemical and physical crosslinking, were imaged: (1) GG-MA 2\% UV photocrosslinker, (2) GG-MA 2\% ionic crosslinker, (3) GG 2\% ionic crosslinker [2], (4) GG 1.1\% ionic crosslinker (SPM) and (5) GG 0.6\% ionic crosslinker (SPM). The hydrogels were inserted into fluorinated ethylene propylene tubes and submerged in water. Images were taken with a 5X objective, providing resolution of 3{\textmu}m, around 360o at steps of 0.9 degree. 3D reconstructions were computed with back projection algorithm. Statistical information, as kurtosis (a measure of the shape of the probability distribution of the image histogram) and entropy (a measure of the randomness of the pixels intensities), were calculated from the projections and reconstructed slices in four samples of each hydrogel type.

RESULTS AND DISCUSSION

The projections and 3D reconstruction of hydrogels (1) and (5) are presented in Fig. 1 and 2. The micro-structures of the hydrogels are visually different. These may be related with the properties of the hydrogels as density, viscosity and homogeneity.

Table 1 shows the mean values of the kurtosis and entropy, in the projections and 3D reconstructions, from each type of hydrogel. The hydrogels that present higher visual density of microstructures (hydrogels (1) and (2)) have lower values of kurtosis and higher values\ 

of entropy while the opposite happens for the transparent hydrogels (hydrogels (3), (4) and (5)). High kurtosis values are related with a more homogeneous pixels intensities distribution in transparent hydrogels while high entropy is related with a more random intensity distribution in high density hydrogels.

CONCLUSION

We have shown that OPT is a suitable tool for imaging hydrogel microstructure and that statistical data, such as kurtosis and entropy, could be used to characterize hydrogels according to their optical properties.

REFERENCES

[1] Sharpe, J.\  et al., Science. 296, 2002

[2] Silva-Correia J, et al.,J Tissue Eng Regen Med 5, 2011

ACKNOWLEDGMENTS

We want to thank to Tekes and Finnish Cultural Foundation for supporting this work.

}, keywords = {Gellan Gum, Hydrogels, Optical projection tomography}, author = {Soto, A. M. and Koivisto, J. and Parraga, J. E. and Silva-Correia, J. and Oliveira, J. M. and Reis, R. L. and Kellom{\"a}ki, M. and Hyttinen, J. and Figueiras, E.} }