Catechol-Modified Polyions in Layer-by-Layer Assembly to Enhance Stability and Sustain Release of Biomolecules: A Bioinspired Approach Younjin Min and Paula T. Hammond* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States * S Supporting Information ABSTRACT: Although layer−by−layer (LbL) assembly technique has been successfully used in various areas of nanobiotechnology, some LbL-assembled nanostructures have suffered from a lack of stability when they are exposed to certain changes in aqueous environments. In addition, the interlayer diffusion of polyelectrolytes throughout the film during assembly generally limits the control of film architecture and release characteristics. To overcome these limitations, we have utilized a strategy to conjugate catechol groups, largely present in mussel adhesive proteins, to branched poly(ethyleneimine) (BPEI) and poly(acrylic acid) (PAA). Only a fraction of amine or acid groups are modified with catechol groups, thereby preserving their charged nature for use in LbL assembly, while integrating the beneficial adhesive features of catechol groups into LbL films. The structure, physico−chemical properties, and stability of LbL films composing BPEI and PAA without and with catechol modifications were compared. The incorporation of catechol groups led to a doubling of the average film thickness and linear film growth. Upon exposure to PBS pH 7.4, the catechol-containing LbL films underwent far fewer changes in the degree of ionization and film thickness and exhibited stronger mechanical properties, indicative of their enhanced film stability. Finally, when LbL films with catechol modifications were used as physical barrier layers between radiolabeled 14 C−dextran sulfate ( 14 C−DS) and 3 H−heparin sulfate ( 3 H−HS), we observed two different release rates composed of an abrupt release from the surface of 3 H−HS, together with a sustained release from the underlying 14 C−DS. Overall, these films provide a bioinspired multifunctional platform for the systematic incorporation and assembly of biological therapeutics into controlled release films at physiological conditions for biomedical applications. KEYWORDS: polyelectrolytes, self-assembly, mussel adhesive protein, film stability, sustained release, cross-linking ■ INTRODUCTION Layer−by−layer (LbL) assembly technique has been proven to be an ideal method for preparation of multifunctional, nanostructured materials in various aspects of biomedical applications. 1−7 The preparation principles and procedures of the LbL assembly technique are quite simple, mainly relying on electrostatic interactions between oppositely charged polymers. Several reports on the use of LbL films for controlled release via hydrolytic 8 or enzymatic degradation 9 present the potential of these films as delivery systems; however, for certain systems that exhibit interdiffusion or exchange of components during assembly, LbL films can deliver some of their payload in a burst or bolus mode in the presence of external stimuli such as changes in pH or ionic strength. For example, several studies have reported that when polyelectrolyte multilayer films are built at low pH and/or low ionic strength and then transferred to a physiological medium at pH 7.4, the films are disrupted because of the change in charge balance, resulting in film destabilization and loss of materials from the substrate. 10−12 Although a spatially organized LbL film has the potential to produce sequential release of more than one therapeutic component in drug delivery, its development has been challenging because of the phenomenon of interlayer diffusion. The tendency of polyelectrolytes to diffuse throughout LbL systems during the deposition process is believed to be due to a mismatch of charge density between oppositely charged polyelectrolytes, 13,14 and/or the enhanced mobility of polymers with low molecular weight or low charge densities or degree of ionization. 15,16 Thermal, 17 chemical, 18−20 and photo-cross- linking 21 routes have been employed in order to enhance the stability of LbL films in the use of long-term drug delivery applications in physiological media. However, when the incorporation of fragile and sensitive biomolecules such as proteins and plasmid DNA is involved, use of thermal, chemical, and photoreactive routes can be detrimental, as they can denature proteins and cleave DNA. While release of bioactive molecules and polyelectrolyte components can be instantly triggered by a wide variety of stimuli such as pH, 22,23 and ionic strength 12,24 by swelling and destabilizing the films; the fabrication of stable LbL films that can release their payload Received: June 23, 2011 Revised: September 26, 2011 Published: December 1, 2011 Article pubs.acs.org/cm © 2011 American Chemical Society 5349 dx.doi.org/10.1021/cm201801n | Chem.Mater. 2011, 23, 5349−5357