Wilhelm T. S. Huck

Polymer brushes via surface-initiated polymerizations

Polymer brushes produced by controlled surface-initiated polymerization provide a route to surfaces coated with well-defined thin polymer films that are covalently bound to the substrate. All of the major controlled polymerization techniques have been applied to the synthesis of polymer brushes and examples of each are presented here. Many examples of brush synthesis in the literature have used the living atom transfer radical polymerization (ATRP) system, and in this tutorial review a particular focus is given to examples of this technique.

Extracellular-matrix tethering regulates stem-cell fate

To investigate how substrate properties influence stem-cell fate, we cultured single human epidermal stem cells on polydimethylsiloxane (PDMS) and polyacrylamide (PAAm) hydrogel surfaces, 0.1 kPa-2.3 MPa in stiffness, with a covalently attached collagen coating. Cell spreading and differentiation were unaffected by polydimethylsiloxane stiffness. However, cells on polyacrylamide of low elastic modulus (0.5 kPa) could not form stable focal adhesions and differentiated as a result of decreased activation of the extracellular-signal-related kinase (ERK)/mitogen-activated protein kinase (MAPK) signalling pathway. The differentiation of human mesenchymal stem cells was also unaffected by PDMS stiffness but regulated by the elastic modulus of PAAm. Dextran penetration measurements indicated that polyacrylamide substrates of low elastic modulus were more porous than stiff substrates, suggesting that the collagen anchoring points would be further apart. We then changed collagen crosslink concentration and used hydrogel-nanoparticle substrates to vary anchoring distance at constant substrate stiffness. Lower collagen anchoring density resulted in increased differentiation. We conclude that stem cells exert a mechanical force on collagen fibres and gauge the feedback to make cell-fate decisions.

Microdroplets in Microfluidics: An Evolving Platform for Discoveries in Chemistry and Biology

Microdroplets in microfluidics offer a great number of opportunities in chemical and biological research. They provide a compartment in which species or reactions can be isolated, they are monodisperse and therefore suitable for quantitative studies, they offer the possibility to work with extremely small volumes, single cells, or single molecules, and are suitable for high-throughput experiments. The aim of this Review is to show the importance of these features in enabling new experiments in biology and chemistry. The recent advances in device fabrication are highlighted as are the remaining technological challenges. Examples are presented to show how compartmentalization, monodispersity, single-molecule sensitivity, and high throughput have been exploited in experiments that would have been extremely difficult outside the microfluidics platform.

Role of the extracellular matrix in regulating stem cell fate

The field of stem cells and regenerative medicine offers considerable promise as a means of delivering new treatments for a wide range of diseases. In order to maximize the effectiveness of cell-based therapies — whether stimulating expansion of endogenous cells or transplanting cells into patients — it is essential to understand the environmental (niche) signals that regulate stem cell behaviour. One of those signals is from the extracellular matrix (ECM). New technologies have offered insights into how stem cells sense signals from the ECM and how they respond to these signals at the molecular level, which ultimately regulate their fate.

Variable Adhesion of Micropatterned Thermoresponsive Polymer Brushes: AFM Investigations of Poly(N‐isopropylacrylamide) Brushes Prepared by Surface‐Initiated Polymerizations

Thermoresponsive polymer brushes have been grown via surface-initiated atom-transfer radical polymerization from micropatterned substrates (see Figure). Atomic force microscopy both in topography and adhesion force mode shows that the brush domains undergo reversible phase changes in water around the polymer’s lower critical solution temperature. These switchable patterned surfaces may form a new class of microdevices.

Surface grafted polymer brushes as ideal building blocks for “smart” surfaces

Polymer brushes are assemblies of macromolecules chemically tethered at one end to a substrate. They provide an alternative to self-assembled monolayers because of the intrinsically large size of the building blocks and the ensuing entropic contribution to the film morphology. In this article, an overview of a number of representative polymer brush systems will be presented and their potential application for surfaces with controlled wettability, smart surfaces and nanoactuators will be explored in some detail.

Antibacterial and Antifouling Polymer Brushes Incorporating Antimicrobial Peptide

Surface-initiated atom transfer radical polymerization (ATRP) has been used to prepare antifouling copolymer brushes based on 2-(2-methoxyethoxy)ethyl methacrylate (MEO(2)MA) and hydroxyl-terminated oligo(ethylene glycol) methacrylate (HOEGMA). The amount of hydroxyl reactive groups incorporated into the brushes was varied by changing the composition of the monomer mixture. These coatings were subsequently functionalized by a natural antibacterial peptide, magainin I, via an oriented chemical grafting on hydroxyl groups, which maintains the activity of the peptide. The antibacterial activity of the functionalized brushes was successfully tested against two different strains of gram-positive bacteria.

Vertically segregated polymer-blend photovoltaic thin-film structures through surface-mediated solution processing

Surface treatment and solvent evaporation control are used to promote vertical segregation in polyfluorene-blend thin films. This surface-mediated control of the compositional structure in the direction normal to the plane of the film has important implications for optimizing charge transport in solution-processed conjugated polymer-blend optoelectronics. Here, the surface energy of the hole-collector electrode of photovoltaic devices is modified by deposition of self-assembled monolayers to favor segregation of the hole-accepting component of the blend to the substrate. Devices fabricated with intentionally vertically segregated blends showed external quantum efficiencies of up to 14%, which is ten times higher than that of devices fabricated without surface modification.

25th anniversary article: Designer hydrogels for cell cultures: A materials selection guide

Cell culturing, whether for tissue engineering or cell biology studies, always involves placing cells in a non-natural environment and no material currently exist that can mimic the entire complexity of natural tissues and variety of cell-matrix interactions that is found in vivo. Here, we review the vast range of hydrogels, composed of natural or synthetic polymers that provide a route to tailored microenvironments.

Coupling Microdroplet Microreactors with Mass Spectrometry: Reading the Contents of Single Droplets Online

Fully integrated: Mass spectrometry has been integrated into a detection scheme for microdroplets that are created within microfluidic channels (see picture, scale bar 200 microm). This technique allows droplets to be identified based on the compounds they contain, and combines fluorescence screening with MS analysis. These experiments indicate how similar approaches can be applied to the ambitious goals of on-chip protein evolution and chemical synthesis.