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A team of materials scientists has grown two layers of polymer brushes, one on top of another. They have developed a simple way to make a mixture of polymers directly on a surface by printing. These duos of polymers could be used for many purposes, including as biocompatible surfaces.
A polymer brush is made up of a backbone of polymer molecule, with other sections of polymer hanging off it like the hairs on a brush. Many ways to grow these brushes on surfaces have been developed because they have applications in a multitude of areas including as substrates for the control of cell growth, switchable sensors, and for separation of biological molecules. Polymer brushes are often grown by surface-initiated polymerization (SIP), which requires the presence of an initiator molecule and is normally carried out on inorganic surfaces like silicon and glass, rather than on polymeric surfaces. Some brushes have also been produced by microcontact printing. In the latter method, a chemical “ink” is printed directly onto a reactive substrate with a miniature stamp. The ink reacts with the surface of the substrate and makes a pattern that reflects the stamp used. This method produces highly controllable surface patterns.
Stefan Zauscher and co-workers at Duke University, USA, took the concept of reactive microcontact printing one step further. They took an initiator molecule, as used in SIP, and used this as the ink for printing onto a thin layer of polymer brush molecules with carboxylic acid groups on them [poly(acrylic acid)]. The ink reacts with the carboxylic acid groups only where it is printed, creating patterns on the surface of the first brush layer. The team then used the initiator to cause further polymer brushes of a different type [poly(N-isopropylacrylamide)] to grow. The lengths of both polymer brushes can be tuned by varying the familiar parameters of the well-known atom-transfer radical polymerization (ATRP) reaction.
This new combined method is better than existing approaches for growing polymer brushes which print the initiator directly onto a surface, as it allows the user to control the printed feature sizes by changing the printing pressure. The scientists believe that their method could easily be expanded to many other types of polymer, and to make either single or double layers of brushes (the initiator can also be printed directly onto a surface that has carboxylic acid groups in it, as in SIP, which results in a single layer of polymer brush). Such materials would have many uses particularly as they are likely to be biocompatible. This method opens up the possibilities for building polymers, which are much in demand for current and future technologies.