Solvent and solute ingress into hydrogels resolved by a combination of imaging techniques

Abstract

Using simultaneous neutron, fluorescence, and optical brightfield transmission imaging, the diffusion of solvent, fluorescent dyes, and macromolecules into a crosslinked polyacrylamide hydrogel was investigated. This novel combination of different imaging techniques enables us to distinguish the movements of the solvent and fluorescent molecules. Additionally, the swelling or deswelling of the hydrogels can be monitored. From the sequence of images, dye and solvent concentrations were extracted spatially and temporally resolved. Diffusion equations and different boundary conditions, represented by different models, were used to quantitatively analyze the temporal evolution of these concentration profiles and to determine the diffusion coefficients of solvent and solutes. Solute size and network properties were varied and their effect was investigated. Increasing the crosslinking ratio or partially drying the hydrogel was found to hinder solute diffusion due to the reduced pore size. By contrast, solvent diffusion seemed to be slightly faster if the hydrogel was only partially swollen and hence solvent uptake enhanced.

Quantification of the Local Protein Content in Hydrogels Undergoing Swelling and Dissolution at Alkaline pH Using Fluorescence Microscopy

“…Quantitative studies are more challenging. Wagner et al (2016) monitored the swelling and deswelling of polyacrylamide hydrogel using simultaneous neutron, fluorescence and optical brightfield transmission imaging. In a recent study we quantified the swelling of disc-shaped Rhodamine B isothiocyanate (RITC)-labelled whey protein hydrogels at pH ≤ 11 using a conventional wide-field fluorescence microscope (Liu et al 2017).…”

Uptake of Methanol by Poly(methyl methacrylate): An Old Problem Addressed by a Novel Raman Technique

“…The setup used for the wide-field imaging is shown in Figure a. , It uses a monochromatic light-emitting diode (490 nm) as the light source and collimation optics to illuminate the sample uniformly. The light transmitted through the sample is captured using a CCD (charge-coupled device) camera.…”

“…16 In the latter approach, one has to assume that the colorant and solvent uptake proceed with identical rates. In a recent study, the uptake of a solvent by a hydrogel was followed by neutron radiography and compared with the behavior of a fluorescence marker added to the solvent; 17 the uptake rate of the solvent and marker differed, showing that this assumption does not hold as suggested by their different sizes and hence diffusion coefficients. Moreover, both approaches applied by Thomas and Windle do not yield the concentration profiles of the solvent; the colorimetric method could determine the presence of the colorant and hypothetically of solvent but could not quantify the volume fraction with spatial resolution.…”

Neutron, fluorescence, and optical imaging: An in situ combination of complementary techniques

“…The temporal evolutions of these profiles provide detailed and quantitative information on the diffusion processes with very good statistics, which can be quantitatively analyzed to yield, e.g., the diffusion coefficients of the solvent(s) and fluorescein molecules in the solution and hydrogel. A quantitative analysis of the concentration profiles is ongoing 37 and beyond the scope of the present manuscript.…”

“…Polyacrylamide (PAAm) hydrogels with a cross-linker to monomer ratio of 1:60 were synthesized in light water (H 2 O) following a standard protocol. 36,37 The initial water content of the hydrogel can be varied by partially drying of the hydrogel; it shrinks upon drying and re-swells after being contacted with solvent. Here, hydrogel discs with a light water (H 2 O) volume fraction of φ H 2 O = 0.97, radius r ≈ 4.5 mm, and height h ≈ 1.5 mm were cut from the hydrogel and transferred into the sample cell.…”