Abstract
Understanding the mechanical properties of alginate-based microcapsules according to size and chemical composition allows researchers to zero in on the treatment and methods required to engineer optimized implantable alginate-based artificial cells for chemotherapy. Cross-linked medium viscosity alginate capsules ranging from 1.1% (w/v)-1.8% (w/v) in composition and 200 μm-1200 μm in size, encapsulating ultrasound contrast agents and blue dextran were compressed within a 40 μm high polydimethylsiloxane microfluidic device and subsequently examined using 2D microscopy for strain deformation aimed at the calculation of poisson ratios and volume loss postcompression. Results indicate a decrease in Poisson ratio as a function of alginate concentration, with statistically significant increases in Poisson ratios and percent volume loss as a function of size and composition. For an average of 120 s observation time post compression, in light of the volume loss correlated to the number of cross-links as a function of capsule size and alginate concentration, a strong case for the dominance of poroelasticity vs. viscoelasticity can be made. While there was a decrease in mean Poisson ratio as a function of concentration, at 1.8% (w/v) the mean strain value converged to 0.5, the theoretical ideal isotropic value associated with soft biological tissue.
Highlights
For the past 30 years, biocompatible hydrogels namely agarose, alginate, chitosan, collagen, fibrin and hyaluronic acid have been extensively used in drug delivery, tissue engineering and regenerative medicine due to their biocompatibility, viscoelastic characteristics, and ease of fabrication into specific shapes and sizes namely microcapsules, microfibers and patches [1,2,3,4]
In light of the volume loss correlated to the number of cross-links as a function of capsule size and alginate concentration, and, noisy strain measurements a strong case for the dominance of poroelasticity over viscoelasticity can be made [34,35,36]
Using the approach to classify the time-dependent processes [11], which should occur under either the condition t~Tv for viscoelastic relaxation or the condition t~L2/D for poroelastic relaxation, where t is the average recorded observation time of 120s and L is the radius of contact (~200 μm), the interpretation hinges upon the order of magnitude of the solvent diffusivity
Summary
For the past 30 years, biocompatible hydrogels namely agarose, alginate, chitosan, collagen, fibrin and hyaluronic acid have been extensively used in drug delivery, tissue engineering and regenerative medicine due to their biocompatibility, viscoelastic characteristics, and ease of fabrication into specific shapes and sizes namely microcapsules, microfibers and patches [1,2,3,4]. Biocompatible hydrogels can be modeled as viscoelastic materials that exhibit rubber like characteristics [12,13,14,15] Because of their highly water-swollen nature, hydrogels might lose water if they are deformed, causing changes in the mechanical properties of the gel. The deformation of these elastomeric gels is time-dependent, resulting from concurrent molecular processes. In living tissues and cells, viscoelasticity results from the conformational change of macromolecules and poroelasticity results from the migration small molecules [20,21] Values of the viscoelastic relaxation time τv and the material-specific length (Dτv)1/2 where D is the solvent diffusivity vary greatly among different materials. For a given polymer-solvent pair, the values of τv and (Dτv)1/2 depend on the crosslink density of the polymer network, the concentration of the solvent; multiple times are possible for different molecular processes of relaxation [14]
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