Swelling of a hemi-ellipsoidal ionic hydrogel for determination of material properties of deposited thin polymer films: an inverse finite element approach

Abstract

Selective deposition of polymers at the surface of an ionic hydrogel is conventionally used to tailor properties of the composite material for application within for instance drug release and cell encapsulation. Here we describe a method for determination of the mechanical properties of a thin polymer film deposited on an ionic hydrogel core. The ionic strength-dependent hydrogel swelling is affected by the crosslink density and thickness of the deposited polymer layer. A hemi-ellipsoidal geometry of the hydrogel, corresponding to that employed in proof-of-concept experiments, is used to enforce biaxial deformation of the deposited layer when the ionic hydrogel core is equilibrated at various ionic strengths. The ionic strength dependent equilibrium swelling ratio of the hydrogel with deposited polymer film is modeled using a finite element approach. The free energy of the hydrogel core includes contributions accounting for the polymer mixing, the elastic deformation of the network and the Donnan equilibrium. The latter type of contribution is not included in the neutral thin layer in the present study. Adding the polymer multilayer/shell at the surface reveals that the ionic strength-dependent swelling constraint is more pronounced the thicker and stiffer the film is. Combining thickness measurements of the polymer film with high resolution interferometric determination of reduction in swelling capacity of ionic hydrogels, an equivalent elastic property of the polymer layer is obtained using inverse finite element analysis. In the proof-of-concept experiments, analysis of data obtained for chitosan-alginate multilayers composed of four and eight polymer bilayers deposited on anionic acrylamide-based hydrogel core suggest that these bilayers show an elastic stiffness one order of magnitude larger than the one of the hydrogel core.

Buckling initiation in layered hydrogels during transient swelling

“…localized sharp folds) [10,11,12,13,14,15,16,17,18,19,20], as illustrated in Figure 1. While creasing mainly occurs as the first mode of instability when homogeneous gels are exposed to large swelling [12,18], wrinkling patterns are first and foremost found in gels having a gradient stiffness through the thickness, caused by a variation in the crosslinking density within the gel [14,21,22], or by the deposition of a thin and stiff film at the outer surface of the gel, possibly to alter properties like permeability, stability or biocompatibility [23,24,25]. In addition, the impact of the mechanically layered or anisotropic character of natural entities on morphogenesis attains growing interest [26,18] with structure formation in aging skin as an example [27].…”

Cyclodextrin triggered dimensional changes of polysaccharide nanogel integrated hydrogels at nanometer resolution

“…The swelling state2 of the CHPMA4 and CHPMA7 hydrogels with no added cyclodextrins to the aqueous solution were selected as the reference states. 45 Higher substitution ratio leads to the higher crosslink density. Increasing the crosslink density of hydrogels will reduce the free volume within the hydrogel network structure and lead to the 50 reduction of the water holding capacity.…”

“…4) obtained 50 from finite element analysis of swelling of hydrogels constrained at the fiber optical base. 45 The power law coefficient of 2.6 different from 3 arises due to constraining of the hydrogel to the end of the optical fiber resulting in an increase of the length of the hydrogel along the fiber axis and decrease of the overall swelling volume relative to unconstrained swelling. Selecting V/V0 = 1.02 as the basis for obtaining estimates of CD concentrations that represent equal perturbation of the CHP 5 hydrogels [CD]ref (Table 1) (Fig.…”

“…44 This contrasts most of the observed relative changes lopt/lopt, typically 100 times or larger, and indicate that the term n1l , changes in physical length of the hydrogels are the dominating contribution to the experimentally determined lopt . 45 The relative binding constants of the various CDs are estimated assuming the same relative volume changes, e.g., V/V0 = 1.02, represent equal perturbation of the cholesterol domains by the CD. The experimentally determined lopt/lopt is converted to V/V0 using the relation V/V0 [(lopt+lopt)/lopt] 2.6 (Fig.…”

“…17 For instance, cholesterol-bearing pullulan (CHP) form nanogel (20-30 nm) in water by self-assembly. The main driving force of the nanogel formation is hydrophobic interaction via cholesteryl groups as 45 functions on association domains. 17 The cholesterol association domains of the nanogel possess unique properties and specific functions for the nanogel, including the ability to encapsulate hydrophobic drugs or soluble proteins.…”