Influence of poly(N-isopropylacrylamide) (PIPAAm) graft density on properties of PIPAAm grafted poly(dimethylsiloxane) surfaces and their stability

Yoshikatsu Akiyama

doi:10.1016/j.heliyon.2021.e06520

Abstract

A previous report shows that poly(N-isopropylacrylamide) (PIPAAm) gel grafted onto poly(dimethylsiloxane) (PDMS) (PI-PDMS) surfaces with large PIPAAm graft density (Lar-PI-PDMS), is prepared by using electron beam irradiation, demonstrating that applied mechanical stretching affects properties of the Lar-PI-PDMS surface. However, the influence of PIPAAm graft density on the properties of PI-PDMS surfaces and their stability are not understood. To provide insight into these points, the properties of PI-PDMS surfaces with low PIPAAm graft density (Low-PI-PDMS) surfaces with stretched (stretch ratio = 20%) and unstretched states were examined as stretchable temperature-responsive cell culture surface using contact angle measurement and cell attachment/detachment assays, compared to those with Lar-PI-PDMS, as previously reported. Long-term contact angle measurements (61 days) for unstretched Low-PI-PDMS and Lar-PI-PDMS surfaces indicated that the cross-linked structure of the grafted PIPAAm gel suppressed hydrophobic recovery of the basal PDMS surface. The cell attachment assay revealed that the stretched Low-PI-PDMS surface was less cell adhesive than that of the unstretched Low-PI-PDMS surface despite of a larger amount of adsorbed fibronectin (FN). The lower cell adhesiveness was possibly explained by denaturation of adsorbed FN, which was induced by the strong hydrophobic property of the stretched Low-PI-PDMS surface. The cell detachment assay revealed that dual stimuli, low temperature treatment and mechanical shrinking stress applied to the stretched Low-PI-PDMS surface promoted cell detachment compared to a single stimulus, low temperature treatment or mechanical shrinking stress. These results suggested that the PIPAAm gelgrafted PDMS surface was chemically stable and did not suffer from hydrophobic recovery. External mechanical stretching stress not only strongly dehydrated grafted PIPAAm chains, but also denatured the adsorbed FN when the grafted PIPAAm layer was extremely thin, as in Low-PI-PDMS surfaces. Thus, PI-PDMS may be utilized as a stretchable temperature-responsive cell culture surface without significant hydrophobic recovery.

Highlights

A temperature-responsive cell culture surface (TRCS), which was first invented by Okano et al, has been a powerful tool for the fabrication of cell-sheets [1, 2]
The grafted PIPAAm layer was deposited as a PIPAAm gel, because polymerization, cross-linking and grafting polymer proceed at the same time during electron beam (EB) irradiation [9]
In the case of Lar-PIPAAm gel-grafted PDMS surface (PI-PDMS) and Low-PI-PDMS, two new signals emerged at 1650 cmÀ1 and 1549 cmÀ1, which were assigned to the C1⁄4O and N–H groups of grafted PIPAAm, respectively

Summary

Introduction

A temperature-responsive cell culture surface (TRCS), which was first invented by Okano et al, has been a powerful tool for the fabrication of cell-sheets [1, 2]. They demonstrated that cell-sheets are transplantable to damaged human tissues and organs to helptheir recovery, while thick tissue with high cell density (e.g., heart muscle) was able to be fabricated by repeatedly stacking cell-sheets [3, 4, 5]. Akiyama et al have demonstrated that precise control of the thickness of the nanoscale grafted PIPAAm layer is a key factor for TRCSs to show temperature-dependent cell attachment and detachment character [3]. The polymer-thickness dependency on cell adhesive and hydrophobic properties of these TRCSs has been previously explained in terms of different molecular mobilities

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Conclusion