Synthesis of Poly(N-isopropylacrylamide) Janus Microhydrogels for Anisotropic Thermo-responsiveness and Organophilic/Hydrophilic Loading Capability

Summary

We present a protocol to synthesize Janus microhydrogels composed entirely of the same base material, poly(N-isopropylacrylamide) (PNIPAAm), with a clearly compartmentalized structure base on the phase separation of a supersaturated NIPAAm monomer solution. The synthesized Janus microhydrogels show unique properties such as anisotropic thermo-responsiveness and organophilic/hydrophilic loading capability.

Abstract

Janus microparticles are compartmentalized particles with differing molecular structures and/or functionality on each of their two sides. Because of this unique property, Janus microparticles have been recognized as a new class of materials, thereby attracting a great deal of attention from various research fields. The versatility of these microparticles has been exemplified through their uses as building blocks for self-assembly, electrically responsive actuators, emulsifiers for painting and cosmetics, and carriers for drug delivery. This study introduces a detailed protocol that explicitly describes a synthetic method for designing novel Janus microhydrogels composed of a single base material, poly(N-isopropylacrylamide) (PNIPAAm). Janus microdroplets are firstly generated via a hydrodynamic focusing microfluidic device (HFMD) based on the separation of a supersaturated aqueous NIPAAm monomer solution and subsequently polymerized through exposure to UV irradiation. The resulting Janus microhydrogels were found to be entirely composed of the same base material, featured an easily identifiable compartmentalized morphology, and exhibited anisotropic thermo-responsiveness and organophilic/hydrophilic loading capability. We believe that the proposed method introduces a novel hydrogel platform with the potential for advanced synthesis of multi-functional Janus microhydrogels.

Introduction

Hydrogels are a network of hydrophilic polymer chains.¹ An increasing amount of research in the field of hydrogels has promoted significant advances and revealed their similarity to biological tissues; the properties of hydrogels allow the uptake of large amounts of water while maintaining their structure. Environmentally responsive hydrogels have also been studied extensively because of their ability to swell or shrink reversibly in response to external stimuli.² Several triggers, including temperature,^3-5 pH,^6,7 light,^8,9 electric fields,^10,11 and specific molecules, such as glucose,^12,13 h....

Protocol

1. Fabrication of a Master Mold for the Hydrodynamic Focusing Microfluidic Device (HFMD) through Photolithography

1. Design a photomask for the HFMD (Figure 1a) using computer-assisted design (CAD) software according to the manufacturer's protocol.
2. Rinse a 4' silicon wafer with acetone, isopropyl alcohol (IPA), and deionized (DI) water to remove organic and inorganic dust from the wafer.
3. Clean the silicon wafer with O₂ plasma at 100 W of power for 5 min to increase the bonding strength between the wafer and SU-8.
4. Spin-coat 4 ml of the negative photoresist, SU-8 2150, onto the wafer at 3,000 rpm for 30 ....

Results

Figure 3a presents a schematic of the experimental setup used to synthesize Janus microhydrogels via the HFMD. The N-rich and N-poor phases were precisely injected into the HFMD as core fluids 1 and 2 and then merged and broken up into Janus microdroplets at the orifice by the sheath fluid of mineral oil because of the Rayleigh capillary instability. Consequently, Janus microdroplets composed of N-rich and N-poor phases were successfully generated as shown in

Discussion

Two immiscible base materials are generally used to synthesize the Janus microhydrogels. Until recently, Janus microhydrogels consisting of the same base material were rarely reported and the reported Janus microhydrogels did not have a clear internal morphology due to the disturbance caused by the miscibility of the component materials.^{35, 36} In this protocol, we demonstrate a method to synthesize Janus microhydrogels composed entirely of the single base material, PNIPAAm, with a clearly compartmentalized str.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Silicon wafer	LG Siltron	4", Test grade	Wafer for master mold fabrication
Acetone	Samchun Pure Chemical	A0097	Cleaning silicon wafer
Isopropyl alcohol (IPA)	Daejung Chemicals & Metals	5035-4404	Cleaning silicon wafer
Water purification system	Merck Millipore	EMD Millipore RIOs Essential 5	Prepering  deionized water
O2 plasma machine	Femto Science	VITA-A	Cleaning silicon wafer
SU-8 2150 negative photoresist	MicroChem	Y111077 0500L1GL	Photoresist for master mold fabrication
Hot plate	Misung Scientific	HP330D, HP150D	Baking SU-8
SU-8 developer	Microchem	Y020100 4000L1PE	Developing SU-8
Mask aligner system for photolithograpy	Shinu Mst Co.	CA-6M	Photolithography
Sylgard 184 silicone elastomer kit	Dow Corning	1064891	PDMS casting
Laboratory Corona Treater	Electro-technic Products Inc.	Model BD-20AC	PDMS air plasma treatment
N-isopropylacrylamide (NIPAAm)	Sigma-Aldrich	415324-50G	Monomer
N,N'-methylenebisacrylamide (MBAAm)	Sigma-Aldrich	146072-100G	Crosslinker of NIPAAm
4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone, Irgacure 2959	BASF	55047962	Photoinitiator of NIPAAm
ABIL EM 90	Evonik Industries	201109	Sufactant for oil
Vortex mixer	Scientific Industries Inc.	Vortex-Genie 2	Mixing
Tygon tubing	Saint-Gobain	I.D. 1/32", O.D. 3/32", Wall 1/32"	Connecting tube between syringes and HFMD
UV light source	Hamamatsu	Spot light source LC8	Polymerization from NIPAAm to PNIPAAm
Syringes, NORM-JECT (3 ml)	Henke-Sass Wolf GmbH	22767	Loading of materials
Syringe pump	KD Scientific	KDS model 200	Perfusion of materials
Tegitol Type NP-10	Sigma-Aldrich	NP10-500ML	Surfactant for water
Oil red O	Sigma-Aldrich	O0625-25G	Dye for N-rich phase
Oil Blue N	Sigma-Aldrich	391557-5G	Dye for N-rich phase
Yellow food dye	Edentown F&B	NA	Dye for N-poor phase
Green food dye	Edentown F&B	NA	Dye for N-poor phase
Power supply	Agilent	E3649A	Power source for thermoelectric module
Thermoelectric module	Peltier	FALC1-12710T125	Temparature control
Centrifuge machine	Labogene	1248R	Settling down microhydrogels
24-well plate	SPL Life Sciences	32024	Reservoir for observation
Optical microscope	Nikon	ECLIPSE 80i	Optical observation
Image analysis software	IMT i-Solution Inc.	iSolutions DT	Measurement of radius