Mechanical characterization of alginate microgel beads

Date
2022-12
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Abstract

Micrometer-sized spherical hydrogel beads, also known as microgels, are used in a wide range of applications, including drug and nutrient delivery, tissue culture, soft actuators, sensors, artificial muscles, and soft robotics. In applications with microgels, mechanical properties play a crucial role. For example, for drug delivery, mechanical strength is crucial as microgels must survive the stress experienced in the needle while injection into the tissues or blood capillaries. Besides, in tissue engineering studies, stiffness is vital as cell growth patterns can depend on the extracellular matrix mechanical strength. Moreover, the elastic modulus is linked to solute permeability. Although measurements of mechanical properties of bulk hydrogels are available, limited data is available about microgel beads. Therefore, a systematic investigation of the mechanical properties of microgel beads is needed. On the other hand, the available techniques are unsuitable for mechanical characterization in a scalable fashion. This thesis will study the mechanical properties of alginate microgel beads produced by a flow-focusing microfluidic device. First, we generated alginate microgel beads of different formulations to see the effect of polymer and crosslinker concentration methodically. We varied the polymer and crosslinker concentration from 1.0 to 2.0 wt% and 100 mM to 200 mM, respectively. From the compression test of the beads, we observed that Young’s modulus of the bead increases with polymer and crosslinker concentration. In addition, we produced beads of diameters ranging from 50 μm to 1000 μm to investigate if the size influences the mechanical strength. Interestingly, we observed that the smallest beads have higher modulus values than the larger beads. Next, we studied the time-dependent poroelastic behavior of the beads by performing load-relaxation tests on 2.0 wt% 200 mM alginate beads at different depths ranging from 10-40 μm. Analyzing the relaxation plots, we observed that the beads showed a time-dependent poroelastic response. We also extracted the poroelastic parameters such as characteristic relaxation time, shear modulus, and diffusivity by fitting the experimental data to a published model. Interestingly, we observed the poroelastic parameters change with compression depth. In addition, we also performed a load relaxation test on 1.0 wt% 200 mM beads to compare the poroelastic parameters of beads of two different concentrations. We noticed that lower concentration yields a smaller modulus and higher diffusivity. Finally, we developed a microfluidic-based technique with four parallel tapered microfluidic channels for scalable mechanical properties determination of microgel beads called tapered microaspiration. We studied the influence of friction on the measurement of elastic modulus and found an insignificant contribution. To validate the technique, we measured Young’s modulus of microgel beads of different polymer concentrations (1.0-3.0 wt%) and found a positive correlation as expected. Finally, we showed that this technique could be used for external agent-driven time-dependent mechanical characterization. In summary, in this thesis, we present a systematic approach to understanding the mechanical behavior of individual microgel beads. We showed that other than polymer and crosslinker concentration, the size of beads can have a significant effect on mechanical strength. Then we showed that the beads are poroelastic, and the poroelastic properties can change with deformation and formulation. Finally, we have also developed a simple microfluidic-based technique for studying the mechanical properties of the beads in a scalable fashion.


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Keywords
Microgel, alginate, mechanical characterization, microfluidics, Young's modulus
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