Ligand accessibility as means to control cell response to bioactive bilayer membranes Yoav Dori, 1 Havazelet Bianco–Peled, 1 Sushil K. Satija, 2 Gregg B. Fields, 3 James B. McCarthy, 4 Matthew Tirrell 1 * 1 Department of Chemical Engineering and Materials, University of Minnesota, Minneapolis, Minnesota 55455 2 Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 3 Department of Chemistry & Biochemistry, Florida Atlantic University, Boca Raton, Florida 33431 4 Department of Laboratory Medicine and Pathology, University of Minnesota, Minneapolis, Minnesota 55455 Received 4 August 1999; accepted 7 September 1999 Abstract: We report a new method to create a biofunc- tional surface in which the accessibility of a ligand is used as a means to influence the cell behavior. Supported bioactive bilayer membranes were created by Langmuir–Blodgett (LB) deposition of either a pure poly(ethylene glycol) (PEG) lipid, having PEG head groups of various lengths, or 50 mol % binary mixtures of a PEG lipid and a novel collagen-like peptide amphiphile on a hydrophobic surface. The peptide amphiphile contains a peptide synthetically lipidated by co- valent linkage to hydrophobic dialkyl tails. The amphiphile head group lengths were determined using neutron reflec- tivity. Cell adhesion and spreading assays showed that the cell response to the membranes depends on the length dif- ference between head groups of the membrane components. Cells adhere and spread on mixtures of the peptide amphi- phile with the PEG lipids having PEG chains of 120 and 750 molecular weight (MW). In contrast, cells adhered but did not spread on the mixture containing the 2000 MW PEG. Cells did not adhere to any of the pure PEG lipid mem- branes or to the mixture containing the 5000 MW PEG. Se- lective masking of a ligand on a surface is one method of controlling the surface bioactivity. © 2000 John Wiley & Sons, Inc. J Biomed Mater Res, 50, 75–81, 2000. Key words: Langmuir–Blodgett; cell adhesion; poly(ethyl- ene glycol); neutron reflectivity; peptide amphiphile; atomic force microscopy INTRODUCTION Controlling the response of cells exposed to a bio- logically active surface, which contains specific func- tional units, is of increasing practical and scientific interest. Many studies have shown that the physical characteristics of a surface, such as surface topography and chemical composition, can greatly affect how cells respond to a surface. 1–8 For example, Chen et al. 9 have shown that the spreading shape and viability of hu- man and bovine capillary endothelial cells can be con- trolled by changing the size and distribution of is- lands, adsorbed with extracellular matrix proteins, on micropatterned surfaces. Another example is the use of “intelligent” polymers to change the surface char- acteristics in response to an environmental stimulus. 10 Such polymers have been used in tissue culture to detach cells from the surface without the need for trypsin. In this article, we present a way to design a biologi- cally active membrane-like surface in which ligand ac- cessibility is used as a means to control the interaction with cells. We focus on membranes containing a bi- nary mixture of a peptide amphiphile, which has a peptide head group covalently linked to lipid tails, 11 and a poly(ethylene glycol) (PEG) lipid. We postu- lated that the relative height difference between the membrane components determines the accessibility of the peptide ligand to cell surface receptors. When the PEG chains are much shorter than the peptide, the peptide ligand is fully exposed and can be recognized by cells. On the other hand, PEG chains that are much longer than the peptide will cover the ligand com- pletely, resulting in a surface that is inert to cells. By *Present address: College of Engineering, University of California-Santa Barbara, Santa Barbara, CA 93106 Correspondence to: Prof. M. Tirrell; e-mail: tirrell@ engineering.ucsb.edu Contract grant sponsors: University of Minnesota; Ful- bright Fellowship; Hebrew Technical Institue Contract grant sponsor: National Institute of Health; Con- tract grant number: R01-HL62427-01 Contract grant sponsor: National Science Foundation; Contract grant number: NSF-BIR-9413241 © 2000 John Wiley & Sons, Inc. CCC 0021-9304/00/010075-07