# Microparticle Hydrogel Material Properties Emerge from Mixing-Induced Homogenization in a Poly(ethylene glycol) and Dextran Aqueous Two-Phase System

## TL;DR

This study explores how mixing poly(ethylene glycol) and dextran in a two-phase system affects the material properties of microgels.

## Contribution

The study reveals that microgel properties depend on the homogenized average concentration of PEG in the two-phase system.

## Key findings

- Microgel polymer concentration correlates with the average PEG concentration in the ATPS.
- Mechanical properties depend on both PEG concentration and ATPS volume ratio.
- Phase separation homogenizes the ATPS, determining microgel material properties.

## Abstract

Polymer–polymer aqueous two-phase systems (ATPSs)
are attractive
for microgel synthesis, but given the complexity of phase separation,
predicting microgel material properties from ATPS formulations is
not trivial. The objective of this study was to determine how the
phase diagram of a poly(ethylene glycol) (PEG) and dextran ATPS is
related to the material properties of PEG microgel products. PEG-dextran
ATPSs were prepared from four-arm 20 kDa PEG-norbornene and 40 kDa
dextran in phosphate buffered saline (PBS), and the phase diagram
was constructed. PEG microgels were synthesized from five ATPS formulations
using an oligopeptide cross-linker and thiol-norbornene photochemistry.
Thermogravimetric analysis (TGA) revealed that the polymer concentration
of microgel pellets linearly correlates with the average concentration
of PEG in the ATPS rather than the separated phase compositions, as
determined from the phase diagram. Atomic force microscopy (AFM) and
bulk rheology studies demonstrated that the mechanical properties
of microgels rely on both the average concentration of PEG in the
ATPS and the ATPS volume ratio as determined from the phase diagram.
These findings suggest that PEG-dextran ATPSs undergo homogenization
upon mixing, which principally determines the material properties
of the microgels upon gelation.