NMR measurement of small-molecule diffusionin PVA hydrogels: a comparison of CONVEX and standard PGSE methods

Abstract

Hydrogels are biocompatible polymeric materials that are becoming increasingly
important in biomedical applications, such as drug delivery and tissue engineering.
Understanding of small-molecule diffusion in these systems is important in the contexts
of controlled drug release; transport of nutrients (e.g., O2 and growth factors) into the
gel; and transport of cellular waste out of the gel. In this work, the diffusion coefficient of the aromatic amino acid phenylalanine (Phe) in non-crosslinked and crosslinked poly(vinyl alcohol) (PVA) hydrogels was measured using two NMR diffusion methods, CONVEX and the standard pulsed-gradient spin-echo (PGSE). Pulsed field-gradient
(PFG) NMR measurements provide the advantage of measuring the molecular selfdiffusion
coefficient directly and without having to rely on the physical release of the
solute, but are often difficult to perform in tissues and hydrated polymers due to a large water signal. CONVEX is a recently proposed diffusion method that alleviates this
problem by means of NMR excitation-sculpting water suppression. In the measurements
presented here, CONVEX results were superior to those from PGSE measurements with
respect to every test applied, and enabled a reliable comparison of the diffusion
coefficients of Phe in crosslinked and non-crosslinked hydrogels. The value of D(Phe)
was smaller in the non-crosslinked hydrogel than in the crosslinked gel; this finding is
discussed in the paper.