Influence of a Solid Surface on PNIPAM Microgel Films

Abstract

Stimuli-responsive microgels have attracted great interest in recent years as building blocks for fabricating smart surfaces with many technological applications. In particular, PNIPAM microgels are promising candidates for creating thermo-responsive scaffolds to control cell growth and detachment via temperature stimuli. In this framework, understanding the influence of the solid substrate is critical for tailoring microgel coatings to specific applications. The surface modification of the substrate is a winning strategy used to manage microgel-substrate interactions. To control the spreading of microgel particles on a solid surface, glass substrates are coated with a PEI or an APTES layer to improve surface hydrophobicity and add positive charges on the interface. A systematic investigation of PNIPAM microgels spin-coated through a double-step deposition protocol on pristine glass and on functionalised glasses was performed by combining wettability measurements and Atomic Force Microscopy. The greater flattening of microgel particles on less hydrophilic substrates can be explained as a consequence of the reduced shielding of the water-substrate interactions that favors electrostatic interactions between microgels and the substrate. This approach allows the yielding of effective control on microgel coatings that will help to unlock new possibilities for their application in biomedical devices, sensors, or responsive surfaces.

Keywords: PNIPAM; microgels; surface modification; thin films.

Figure 1

AFM images over a 10 × 10 $\mathsf{\mu }$m² area of PNIPAM micrrogels spin-coated on pristine glass substrates at concentrations of (a) C_w = 0.1%, (b) C_w = 0.5%, (c) C_w = 1.0%, (d) C_w = 3.0%, and (e) C_w = 5.0%. Scale bar: 4 $\mathsf{\mu }$m.

Figure 2

Water contact angle (WCA) measurements on (a) pristine glass, (b) glass modified with PEI, and (c) glass modified with APTES.

Figure 3

AFM images over a 10 × 10 $\mathsf{\mu }$m² area of PNIPAM microgels at C_w = 0.1% spin-coated on (a) pristine glass, (b) glass functionalized with PEI, and (c) glass functionalized with APTES. Three-dimensional images of mostly isolated particles on (d) pristine glass, (e) glass functionalized with PEI, and (f) glass functionalized with APTES. Scale bar: 4 $\mathsf{\mu }$m.

Figure 4

AFM images of PNIPAM microgels at C_w = 3.0% spin-coated on (a) pristine glass, (b) glass functionalized with PEI, and (c) glass functionalized with APTES. Scale bar: 4 $\mathsf{\mu }$m.

Figure 5

Water contact angles measured on PNIPAM microgel films at C_w = 3%, at temperatures below the microgel VPTT, spin-coated on (a) pristine glass, (b) glass modified with PEI, and (c) glass modified with APTES and at temperatures above the microgel VPTT measured on the same films on (d) pristine glass, (e) glass modified with PEI, and (f) glass modified with APTES.

Scheme 1

Scheme of surface modifications of (a) pristine glass to obtain (b) glass–PEI and (c) glass–APTES substrates.