Characteristics of Precipitation-formed Polyethylene Glycol Microgels Are Controlled by Molecular Weight of Reactants

Summary

This work describes the formation of poly(ethylene glycol) (PEG) microgels via a photopolymerized precipitation reaction. Increasing the PEG molecular weight increased microgel diameter and swelling ratio. Simple adaptations to the PEG microgel precipitation reaction are explored for future applications of microgels as drug delivery vehicles and tissue engineering scaffolds.

Abstract

This work describes the formation of poly(ethylene glycol) (PEG) microgels via a photopolymerized precipitation reaction. Precipitation reactions offer several advantages over traditional microsphere fabrication techniques. Contrary to emulsion, suspension, and dispersion techniques, microgels formed by precipitation are of uniform shape and size, i.e. low polydispersity index, without the use of organic solvents or stabilizers. The mild conditions of the precipitation reaction, customizable properties of the microgels, and low viscosity for injections make them applicable for in vivo purposes. Unlike other fabrication techniques, microgel characteristics can be modified by changing the starting polymer molecular weight. Increasing the starting PEG molecular weight increased microgel diameter and swelling ratio. Further modifications are suggested such as encapsulating molecules during microgel crosslinking. Simple adaptations to the PEG microgel building blocks are explored for future applications of microgels as drug delivery vehicles and tissue engineering scaffolds.

Introduction

By definition, microgels are hydrogels of any shape with an equivalent diameter of approximately 0.1-100 μm¹. Because of their size and characteristics, polymeric microgels present a versatile tool for advancing drug delivery and tissue engineering systems. While bulk hydrogels are extensively utilized as tissue engineering scaffolds and drug delivery vehicles with great success^2-4, a recent shift to microscale control of scaffolds provides a unique opportunity for microgels to be used as base materials for building scaffolds. In addition, microgels have a high surface area to volume ratio for cellular interactions and in solution have a low viscosity that makes them ideal for injections. Finally, microgels can be formed using numerous polymers by a variety of methods dependent on the desired microgel characteristics, making them highly customizable for a variety of applications.

Techniques to produce microgels include suspension, emulsion, dispersion, or precipitation polymerizations. Emulsion and suspension polymerizations typically require organic solvents and surfactants or stabilizers to form the microgels. The nature of these methods yield a highly disperse particle size distribution⁵. Dispersion and precipitation reactions render particles with a lower polydispersity⁶; however particles formed by dispersion polymerization still require the use of stabilizing agents⁶. Microgels formed by precipitation reactions are unique in that they form particles of uniform size and shape without the use of stabilizers or surfactants. Microgel formation is achieved when growing polymer chains phase separate from the continuous phase by enthalpic or entropic precipitation⁷. Precipitation polymerization is often at high temperatures that can be lowered by the use of kosmotrophic salts, which decrease the solubility of the polymer in the solvent⁸. This work focuses on microgels formed from poly(ethylene glycol) (PEG) by a photopolymerized precipitation reaction under biologically-compatible conditions, with variations to alter microgel properties and encapsulate molecules for drug delivery applications.

Previous studies with PEG hydrogels show that the polymerization conditions greatly influence the physical and mechanical properties of hydrogels, namely the hydrogel water content and compressive modulus^3,9. These crosslinked materials are of interest because the relationship between structural and physical properties described by Flory¹⁰ can be utilized to tailor the crosslinked hydrogel for specific applications. These principles are similar for microgels. Precipitation-formed PEG microgels have been found to have potential for regenerative medicine¹¹, however further investigation into the microgel properties was necessary to enhance their repertoire for future biomedical applications. This report describes the procedure to fabricate microgels by precipitation reaction and alter characteristics, such as microparticle diameter, polydispersity index (PDI), density, and swelling, that would be important to further develop these materials for drug delivery or regenerative medicine.

Protocol

1. Preparing Solutions for Use in Microgel Fabrication Before beginning the precipitation reaction, make the necessary solutions and warm them to 37 °C. The required solutions include 0.5% photoinitiator, 1.5 M Na2SO4, buffer solution, 200 mg/ml PEG-diacrylate (PEG-DA) solution, and 1x phosphate buffered saline (PBS). Note: Acrylate all PEG precursors according to published methods and store at -20 °C under argon until use12. Weigh out photoinitiator and di…

Representative Results

Microgel size is dependent on polymerization conditions. Figure 2A illustrates how microgel diameter increases with increasing PEG starting molecular weight and crosslinking time. Representative images of microgels used for sizing for various molecular weights and crosslinked for 30 sec are shown in Figures 2C-G. For PEG with a molecular weight of 3,000 Da, the microgel average diameter increased from 1.65±0.26 to 2.20±0.54 μm as UV exposure increased from 30-600 sec (<str…

Discussion

Physical properties of PEG microgels were examined for changes in polymerization conditions. For this precipitation reaction, a buffer solution, 20% (w/v) PEG-diacrylate (MW 3,000, 4,600, or 6,000 Da) solution, and photoinitiator were mixed and warmed to 37 °C. Addition of 1.5 M Na₂SO₄, a kosmotrophic salt that increases the interactions between water molecules, caused PEG to momentarily precipitate upon its addition. This effect is more prominent with higher molecular weight PEG^8</su…

Materials

Phosphate Buffered Saline (PBS)	MP Biomedical	2810305			ThermoStat plus	Eppendorf	5352YL404573	Tube Warmer
Triethanolamine (TEOA)	J.T. Baker	9468-01	Preheat to 37 °C prior to pipetting		LSE Vortex Mixer	Corning	S004164
Hydrochloric acid (HCl)	BDH Aristar	BDH3028			XP205 Analytical Balance	Mettler Toledo	11106024
Sodium Sulfate	J.T. Baker	3891-01			CureSpot 50	ADAC Systems	A121-031
Irgacure 2959	Ciba	029891301PS04			SevenMulti pH Meter	Mettler Toledo	51302813
Ovalbumin (OVA)	Invitrogen	34782			Class II, Type A2 Biological Safety Cabinet	Nuaire	1.39284E+11
PEG 1,500	Alfa Aesar	A16241			Centrifuge 5430 R	Eppendorf	5428AH010419
PEG 3,000	Fluka	03997-1KG			Sonicator 8890	Cole Parmer	8890R-DTH	SN QAC039907293D
PEG 4,000	Alfa Aesar	A16151			Coverslip 22 mm x 30 mm, 1.5 thickness	Fisherbrand	12-544-A
PEG 4,600	Sigma	373001-250G			Inverted Microscope with camera	Zeiss	1022923629
PEG 6,000	Fluka	03394-1KG
PEG 10,000	Alfa Aesar	B21955
Dextran 70	TCI	D1449