Highlight
Biopolymer Gel Microbeads Containing Trapped Emulsion Droplets Show Dramatic Imaging Contrast Effects
Achievement/Results
NSF-funded researchers at Columbia University have developed novel biopolymer gel tracer particles for magnetic resonance imaging (MRI) studies. Prof. Nina Shapley and graduate student Holly Hester-Reilly, supported by a NSF IGERT fellowship, examined MRI contrast effects among alginate gel microbeads containing trapped emulsion droplets, rigid polystyrene particles, and the surrounding suspending liquid. The study was motivated by the need to follow two sizes or two types of particles in a concentrated suspension flow, so that particle size separation or mixing in response to flow may be quantified. Standard MRI techniques are incapable of distinguishing two types of rigid polymer particles, and so the need arises to vary the material of one of the particle types. Since gels are known to exhibit MRI properties in between those of solids and liquids, they were utilized for tracer particles in this study. Gel microbeads were formed at mild, room-temperature conditions by ionic cross-linking of alginate, a natural polysaccharide biopolymer.
Alginate emulsion microbeads were clearly distinguished from rigid polymer particles and suspending fluid by two mechanisms: spin-spin (T2) relaxation time, which indicates the mobility of water molecules in the material, and diffusion, which indicates the presence of small trapped pockets of liquid. The diffusion contrast is particularly useful because it follows a different mechanism from the effect of commercial gadolinium-based MRI contrast agents, which mainly shorten local relaxation times. Here, the imaging contrast was successfully demonstrated in a multiphase flow when a suspension containing both 20.5% by volume rigid polystyrene particles and 2.4 wt% alginate emulsion microbeads passed through a 1:4 expansion flow system and the gel tracer particles were clearly identifiable. While microbead diameters on the order of 1 mm were selected for initial testing, subsequent efforts have resulted in the production and characterization of plain alginate gel beads in the 10-90 micron diameter range, and the formation of emulsion microbeads in this useful size range appears to be feasible.
The results of the investigation were recently published in the Journal of Magnetic Resonance. The alginate emulsion beads likely have greatest potential for use as one particle type among many in a multimodal suspension where detailed concentration profiles or particle size separation must be quantified during flow. This work can also guide developments in the design of graded ceramic and composite materials as well as particulate contrast agents for medical imaging.
Address Goals
Discovery: The development of biopolymer gel tracer particles is a uniquely interdisciplinary project that involves multiple aspects of soft material systems, including cross-linking processes in polymers, emulsification, imaging properties resulting from the particle material microstructure, and the behavior of particles in a suspension flowing through a complex geometry. Hence, the results of the study can transfer new concepts among several research areas, and are relevant to a host of applications. One great advantage of the tracer particles examined here is the ease of production, where detailed chemical synthesis is not required, and therefore the barrier to adoption is quite low. The results have been disseminated in a recent publication (Journal of Magnetic Resonance) and through conference presentations (APS).
Learning: This research investigation and the process of preparing the journal article provided excellent training for graduate student Holly Hester-Reilly in gel microbead synthesis and characterization, complex fluid rheology, experimentation with MRI, image analysis using Matlab and ImageJ software, literature review, and data interpretation.
