Polymer brushes with properties that can be tuned with temperature have found a wide range of applications for the control of cell adhesion. PNIPAM is a choice polymer for such purpose as it switches from cell repellant to cell adhesive around 32°C, close to the physiological temperature. Still, the way this change occurs was not fully described so far.
Most of the studies used to assess the change in conformation use spatially-averaged measurements such as liquid-phase ellipsometry or neutron reflectivity. Here, spectral reflectance in the visible range was used instead, allowing the sample to be characterized at each temperature within a fraction of second, with a micrometric spatial resolution, and with the same instrument that can further be used for studying cell adhesion or detachment on the brush.
Using this setup, the density profile of the brush as a function of the distance from the grafting surface could be retrieved as a function of temperature, confirming the existence of a phase separation (between a dense, weakly hydrated layer near the grafting surface and a more hydrated outer layer). Data at various temperature also demonstrated that the brush could be fully characterized in terms of chain length, grafting density and polydispersity, something that has so far proven challenging to achieve with non destructive methods. The ease of use and rapidity of this method also permits investigating in details the effect of several parameters such as the chain length and grafting density. This study shows, for example, that the hysteretic behaviour of the brush as a function of temperature is related to the chain entanglement during collapse at high temperature, providing insight into the thermophysics of the brush swelling.
This study thus paves the way for the rational study and design of polymer brushes for cell adhesion control.