Miloslav Sailer and Christopher J. Barrett*
Macromolecules 2012, 45, 5704-5711, doi: 10.1021/ma300635n
Highlight by: Dr. Julie Albert, North Carolina State University
The authors describe a fabrication method for assembling gradient layer-by-layer (LbL) films from weak polyelectrolytes in which the assembly pH of the first component [poly(allylamine hydrochloride) (PAH)] is varied in one direction and the assembly pH of the second component [poly(acrylic acide) (PAA)] is varied orthogonally to the first. The resultant 2D combinatorial film provides access to thousands of assembly conditions on a single surface.
The key value of this work lies in its application for the first time of a common gradient fabrication technique to LbL assembly. The authors also complement the method development aspect with an elegant demonstration of the new gradient library as a tool for screening cell viability on LbL surfaces as a function of film assembly conditions. The LbL film libraries were prepared by pumping the polyelectrolyte stock solution into an initially empty reservoir containing the silicon substrate while simultaneously adding HCl or NaOH to the stock solution to vary the pH. Thus, as the liquid level in the reservoir rose, the pH of the deposition solution continually increased or decreased. PAH was deposited first, the sample was rinsed and rotated 90°, and the process was repeated for PAA. This cycle was repeated up to 5 times to fabricate multilayer films. The authors also examined gradients in salt concentration by charging the reservoir with a small volume of highly concentrated solution, which was diluted by the addition of the polyelectrolyte solution. The most noticeable effect of assembly pH on LbL assembly is on the film thickness, though differences in density, surface energy, and modulus properties were also noted. By mapping the viability of human embryonic kidney cells (HEK 293) on the combinatorial surface, the authors clearly identified the assembly pH conditions that provided optimal cell growth (≈pH 4-6 for PAH and ≈pH 4 for PAA).
The ability to easily generate thousands of LbL assembly conditions on a single sample opens doors not only for combinatorial bioscience, as the authors suggest, but also for in-depth characterization of LbL film properties as a function of assembly conditions (e.g., film thickness, interpenetration of polymer chains, wettability, mechanical properties) and for other applications that would benefit from combinatorial screening approaches, such as optical coatings and anti-fouling surfaces.
- On layer-by-layer assembly: (a) “Form and Function in Multilayer Assembly: New Applications at the Nanoscale” Hammond, P. T., Adv. Mater. 2004, 16, 1271-1293. (b) “Biomedical Applications of Layer-by-Layer Assembly: From Biomimetics to Tissue Engineering” Tang, Z.; Wang, P.; Podsiadlo, P.; Kotov, N. A., Adv. Mater. 2006, 18, 3203-3224.
- On polymer surface gradients: (a) “Surface Bound Soft Matter Gradients” Genzer, J. and Bhat, R. R., Langmuir. 2008, 24, 2294-2317. (b) “Solution and Surface Composition Gradients via Microfluidic Confinement: Fabrication of a Statistical-Copolymer-Brush Composition Gradient” Xu, C.; Barnes, S.E.; Wu, T.; Fischer, D. A.; DeLongchamp, D.M.; Batteas, J.D.; Beers, K. L; Adv. Mater. 2006, 18, 1427-1430.