CHEMISTRY & MATERIALS
Surface engineering The small stuff matters
Understanding the molecular properties of polyelectrolytes gives unprecedented control over responsive polymer microstructures
Polymer films that undergo nanoscale structural transformations in response to external stimuli are key components of devices like biosensors and artificial membranes. One of the best materials for manufacturing such responsive materials is polyelectrolyte multilayers (PEMs) — polymer chains bearing charged units that can assemble layer-by-layer onto solid surfaces. Maxim Kiryukhin at the A*STAR Institute of Materials Research and Engineering and co-workers1
have now gained new insights into PEM growth
which may enable construction of three-dimensional microstruc- tures for on-demand drug delivery. The standard method of producing PEMs involves repeatedly
dipping a flat substrate into positively and negatively charged polyelectrolyte solutions. Although this approach can generate large-scale coatings with precise vertical thicknesses, it is far more challenging to achieve similar control at confined dimensions. Previous studies have shown that PEMs grown onto micro- sized surface templates have non-uniform thicknesses that could severely compromise device performance. Kiryukhin and co-workers systematically investigated a PEM assembled from poly(diallyldimethylammonium chloride)/ poly(sodium 4-styrenesulfonate), or PDADMAC–PSS, to help resolve this problem. Unlike other multilayers, the PDADMAC– PSS system can shift from a linear layer-by-layer growth mode to one with exponentially increasing film thickness, simply by increasing the ionic strength of the polyelectrolyte dipping solution. By exploring these two regimes on templates contain- ing differently sized ‘microwells’, the team hoped to discover the critical factors needed to produce consistent microstructures. First, the researchers imprinted an array of microwells ranging
from 2 to 25 micrometers onto a sacrificial plastic template made of poly(methyl methacrylate), or PMMA, and then coated the surface with a PDADMAC–PSS multilayer film. After trans- ferring the PEM onto a silicon support, they dissolved away the PMMA template. Depending on the microwell pattern, the polyelectrolyte con-
centration, and number of multilayers, this technique generated numerous structures including hollow microchambers and solid microstubs (see image). The team found that in exponential
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Scanning electron microscope images of (PDADMAC-PSS)10 films assembled on PMMA templates with ’microwells’ of 25 (left) and 7 (right) micrometers. Thinner films form inside larger microwells, and vice-versa. All scale bars: 10 µm.
growth modes, aggregation of PDADMAC chains produced variable template coatings; thicker films could form inside the microwells and vice-versa, depending on the particular template shape. Weak bonds between the PDADMAC aggregates gave these films gel-like mechanical properties. In linear growth modes, the solvated polyelectrolyte exists as isolated chains that coat the template evenly and make the microstructures rigid and glass-like. Stable hollow microchambers formed in this mode when the PEM film was thicker than 400 nanometers. According to Kiryukhin, the highly ordered arrays of sealed
microchambers produced with this method could allow pro- grammed release of tiny ‘cargos’ from individual cavities — oppor- tunities that the team are currently pursuing.
1. Kiryukhin, M. V., Man, S. M., Sadovoy, A. V., Low, H. Y. & Sukhorukov, G. B. Peculiarities of polyelectrolyte multilayer assembly on patterned surfaces. Langmuir 27, 8430–8436 (2011).
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A*STAR RESEARCH OCTOBER 2011– MARCH 2012
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