3D Printable Poly(N-isopropylacrylamide) Microgel Suspensions with Temperature-Dependent Rheological Responses

Abstract

Microgel suspensions have garnered significant interest in fundamental research due to their phase transition between liquid-like to paste-like behaviors stemming from tunable interparticle and particle-solvent interactions. Particularly, stimuli-responsive microgels undergo faster volume changes in response to external stimuli in comparison to their bulk counterparts, while maintaining their structural integrity. Here, concentrated and diluted suspensions of poly(N-isopropylacrylamide) (PNIPAm) microgels are dispersed to different packing fractions in water for the characterizations of temperature-responsive rheological responses. In the intrinsic volume phase transition (VPT), polymer chains collapse, and microgels shrink to smaller sizes. Additionally, the intermicrogel and microgel-solvent interactions vary in VPT, which results in microgel clusters that significantly affect the linear shear moduli of suspensions. The effect of the temperature ramp rate of PNIPAm microgel suspensions on rheological responses is characterized. Moreover, the effect of the mass fraction of microgels on the relative viscosity of dilute microgel suspensions is investigated. These results shed light on understanding the heating and cooling rate-dependent temperature responsiveness of PNIPAm microgel suspensions, establishing pathways to regulate the rheological characteristics in temperature-responsive microgel-based platforms. Therefore, this work envisions technological advancements in different fields such as drug delivery, tissue engineering, and diagnostic tools.

Figure 1

Fabrication of PNIPAm microgel suspensions and their temperature responsiveness. (a) Schematic of the preparation of PNIPAm microgel suspensions using a water-in-oil template. PNIPAm precursor droplets are polymerized and cross-linked upon UV irradiation to form microgels. PNIPAm microgel suspensions are obtained by washing and redispersing microgels to DI water with different mass fractions. (b) Micrographs of MG640 microgels in diluted suspensions at 25 and 45 °C, respectively. Diameters (d) of microgels are measured from optical images using ImageJ. (c) Corresponding schematics of the size change of PNIPAm microgels upon VPTT. (d) Effect of temperature on the size of the single microgel in diluted MG640 and MG320 suspensions. (e) Representative images of the microgel clusters in water recorded with a high-speed camera using DIA at (i) 25 °C, (ii) 32 °C, and (iii) 45 °C. (f) Corresponding schematics of microgel clustering at (i) 25 °C, (ii) 32 °C, and (iii) 45 °C. (g) Effect of temperature on the size of microgel clusters in diluted MG640 and MG320 suspensions. The average hydrodynamic diameters of the microgel cluster are measured from DIA using Anton Paar’s Kalliope Software.

Figure 2

Effect of temperature on rheological behaviors of concentrated PNIPAm microgel suspensions with different cross-link densities. (a) Young’s moduli of PNIPAm layers prepared from MG640 and MG320 recipes at 25 and 45 °C, respectively. (b) Schematic of the evaporation-avoid rheometer setup. (c) Photographs of concentrated MG640 suspensions before (25 °C) and after testing (45 °C). (d) Oscillation amplitude of concentrated MG640 and MG320 suspensions that determine G′ (solid circles) and G′′ (hollow circles) at 25 °C. The blue shade area indicates LVR at 25 °C. (e) Oscillatory frequency sweep of concentrated MG640 and MG320 suspensions at 25 °C. (f) Oscillation amplitude of concentrated MG640 and MG320 suspensions at 45 °C. The red shade area indicates LVR at 45 °C. (g) Oscillatory frequency sweep of concentrated MG640 and MG320 suspensions at 45 °C.

Figure 3

3D printing of concentrated PNIPAm MG320 suspensions. (a) Extrusion morphology of the MG320 ink using a 27G nozzle. (b) Printing results of 1-layer rectilinear structure and 2-layer grid structure using MG320 ink at different syringe temperatures. (b) Photographs of the printing process and the top view of the printed lattice structure of the gummy bear. Scale bar = 1 cm. (c) Side view and top view of the 3D printed octopus structure. Scale bar = 1 cm. (d) Printability diagram of MG320 ink as a function of the syringe and printing bed temperatures. The dashed lines indicate a boundary between printable inks with unprintable inks.

Figure 4

Effect of the temperature ramp rate on concentrated PNIPAm microgel suspensions with different cross-link densities. (a) Contact angles of PNIPAm layers prepared from MG640 and MG320 recipes as a function of temperature. Contact angle images at 20.4, 33, and 45 °C are shown as inserts. (b) DSC thermograms of concentrated MG640 and MG320 suspensions at a ramp rate of 5 °C/min. (c–e) G′ of concentrated MG640 suspensions for four cycles of heating and cooling ranging from 25 to 45 °C with ramp rates at (c) 1 °C/min, (d) 3 °C/min, and (e) 5 °C/min. (f–h) G′ of concentrated MG320 suspensions for four cycles of heating and cooling ranging from 25 to 45 °C with ramp rates at (f) 1 °C/min, (g) 3 °C/min, and (h) 5 °C/min.

Figure 5

Viscoelastic properties of concentrated and diluted PNIPAm microgel suspensions with different cross-link densities. (a, b) The viscosity of concentrated MG640 and MG320 suspensions at (a) 25 °C and (b) 45 °C, respectively. (c, d) Correlation and linear fitting between the apparent shear viscosity and mass fraction of diluted MG640 and MG320 suspensions at (c) 25 °C and (d) 45 °C, respectively.