Switchable and pH responsive porous surfaces based on polypeptide-based block copolymers
Graphical abstract
Introduction
The development of surfaces with switchable properties in response to external stimuli, such as pH, temperature, light and electrical potential, has been actively pursued in the last years for their wide number of practical applications [1]. Particular emphasis has been placed on designing surfaces with tuneable and reversible wettability that are desired in many areas such as antifogging, self-cleaning, separation processes as well as biomedical uses including drug delivery or tissue engineering [2], [3]. Among various kinds of smart polymeric surfaces, pH responsive surfaces with switchable wettability have drawn especial interest because they can be also used to control surface attachment and release of charged biomolecules and species such as peptides, DNA, proteins, cells, and drugs through reversible electrostatic interactions [4], [5], [6], [7], [8], [9]. This reversible immobilization of biomolecules onto surfaces is desired for many biorelated applications including the fabrication of biosensors, microarrays or for tissue engineering purposes [10].
The design of such surfaces with switchable wettability generally required controlling surface roughness and surface chemical composition and/or conformation in order to amplify the effect. A number of methods have been proposed to develop surfaces with such switchable wettability, many of them involving stimuli responsive polymers, with the ability to change its hydrophobicity under the influence of a particular external stimulus [11]. At the same time, the roughness, micro- to nanoscale structures, can enhance both the hydrophilicity and hydrophobicity of these surfaces, thereby broadening the wettability range [12], [13]. In this context, the breath figure (BF) method is one of the most extended approaches for preparing polymeric surfaces with controlled micro and nanostructuration simultaneously with a control of the chemical composition [14], [15], [16]. The term breath figure in its origin refers to the fog that appears on a cold surface such as window when we breathe on it. Later on, in 1994 Francois et al. [17] reported the preparation of porous polymeric surfaces by using this phenomenon. In brief, self-organized porous films are obtained by this method in one single step in a very rapid process consisting in the evaporation of a polymeric solution under moist atmosphere. The evaporation of the solvent decreases the temperature of the solution/air interface inducing the condensation of water droplets on the top. The water droplets self-organize into an ordered hexagonal array and after solvent and water evaporation, honeycomb-patterned films are typically formed [17], [18]. Besides, the use of stimuli responsive polymers conducts to smart porous films and several microporous films functionalized with pH responsive polymers have been prepared by this approach [19], [20], [21], [22]. In these surfaces the wettability character dramatically and reversible changes from highly hydrophilic to hydrophobic in response to variations of the pH. In addition, pH responsive surfaces obtained by breath figure method have been evaluated for biomolecules adsorption and release as a function of pH [9], [23], [24]. Films with amine functional groups inside the pores allowed the immobilization of negatively charged BSA or DNA sequences at basic pH [9], [24]. On the other hand, when the cavities of the porous films are functionalized with, for instance, poly(acrylic acid) (PAA), complexes with polycations such as poly(l-lysine) can be formed at neutral pH wherein the carboxylic groups are negatively charged.
Herein, we designed and prepared smart porous films by the method of breath figure using polypeptides as stimuli responsive polymers. In particular, porous films are obtained from polymeric blends composed of high molecular weight polystyrene as the major constituent and a block copolymer based on poly(l-glutamic acid) (PGA) as the minor component. The PGA segments are biocompatible and pH responsive, at neutral and basic pH the carboxylates are deprotonated, thus the copolymer is negatively charged. This variation of the pH also provokes a structural transition in the secondary structure of the polypeptide between rod-like helical and random coil conformation [25]. Therefore, surface functionalization with PGA would be potentially suitable for a variety of uses, particularly in implant coatings and tissue engineering as this polyelectrolyte can be employed to immobilize biomolecules, such as growth factors. Moreover, it has been studied that the proliferation of cells seems to be influenced by the local rigidity of the cellular microenvironment [26]. In this context, porous surfaces decorated with PGA chains can be an excellent model system to study the mechanical properties of the cellular microenvironment due to its easy synthesis and its homogeneity in comparison with other systems such as proteins inspired materials. In spite of this attractive potential uses, a thorough study of the adsorption kinetics onto this type of porous materials is still required. In this work, the reversible response to the pH of this system is also studied by the adsorption of cationic probes at different pH. In addition, the kinetic of the adsorption process is analyzed in detail.
Section snippets
Materials
Chloroform (CHCl3) was employed without further purification as solvent for the preparation of the porous films. Round glass coverslips of 12 mm diameter were obtained from Ted Pella Inc. Rhodamine 6G (> 99%) for adsorption experiments was purchased from Aldrich. High molecular weight polystyrene (HPS, Aldrich, Mw = 2.50 × 105 g mol− 1) was employed as polymeric matrix and used as received. The block copolymer polystyrene-b-poly(l-glutamic acid) (PS49-b-PGA84) was synthesized following a previously
Results and discussion
Experimental parameters such as relative humidity, polymer concentration, type of polymer, and nature of the support play important roles in the formation of the breath figure microstructures, affecting among other the regularity of the porous films and the final pore size [14], [15], [16]. Whilst typical breath figure approach employs polymer solutions based on one single polymeric component, polymer blends can be successfully used offering interesting advantages. For instance, in this case,
Conclusions
In summary, we reported the preparation of stimuli responsive porous films by the breath figure method using copolymers based on pH sensitive poly(γ-glutamic acid) (PGA) polypeptide. Briefly, honeycomb films were fabricated from polymer blends consisting of high molecular weight PS and the block copolymer PS-b-PGA. The enrichment of the pores in the hydrophilic PGA segments as a result of the breath figure mechanism of formation was demonstrated by fluorescence microscopy. The obtained
Acknowledgements
This work was financially supported by the MINECO (Project MAT2013-47902-C2-1-R and MAT2016-78437-R).
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