Polyelectrolyte multilayer microcapsules: Self-assembly and toward biomedical applications

Molecular bionics - engineering biomaterials at the molecular level using biological principles

Life and biological units are the result of the supramolecular arrangement of many different types of molecules, all of them combined with exquisite precision to achieve specific functions. Taking inspiration from the design principles of nature allows engineering more efficient and compatible biomaterials. Indeed, bionic (from bion-, unit of life and -ic, like) materials have gained increasing attention in the last decades due to their ability to mimic some of the characteristics of nature systems, such as dynamism, selectivity, or signalling. However, there are still many challenges when it comes to their interaction with the human body, which hinder their further clinical development. Here we review some of the recent progress in the field of molecular bionics with the final aim of providing with design rules to ensure their stability in biological media as well as to engineer novel functionalities which enable navigating the human body.

Drug Release Properties of Diflunisal from Layer-By-Layer Self-Assembled κ-Carrageenan/Chitosan Nanocapsules: Effect of Deposited Layers

Engineering of multifunctional drug nanocarriers combining stability and good release properties remains a great challenge. In this work, natural polymers κ-carrageenan (κ-CAR) and chitosan (CS) were deposited onto olive oil nanoemulsion droplets (NE) via layer-by-layer (LbL) self-assembly to study the release mechanisms of the anti-inflammatory diflunisal (DF) as a lipophilic drug model. The nano-systems were characterized by dynamic light scattering (DLS), zeta potential (ζ-potential) measurements, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (XEDS) and Fourier transform infrared spectroscopy (FTIR) to confirm the NE-coating with polymer layers. In addition, kinetic release studies of DF were developed by the dialysis diffusion bag technique. Mathematical models were applied to investigate the release mechanisms. The results showed that stable and suitably sized nanocapsules (~300 nm) were formed. Also, the consecutive adsorption of polyelectrolytes by charge reversal was evidenced. More interestingly, the drug release mechanism varied depending on the number of layers deposited. The nanosized systems containing up to two layers showed anomalous transport and first order kinetics. Formulations with three and four layers exhibited Case II transport releasing diflunisal with zero order kinetics. Hence, our results suggest that these polyelectrolyte nanocapsules have great potential as a multifunctional nanocarrier for drug delivery applications.

A Study of Calcium-Silicate-Hydrate/Polymer Nanocomposites Fabricated Using the Layer-By-Layer Method

Calcium-silicate-hydrate (CSH)/polymer nanocomposites were synthesized with the layer-by-layer (LBL) method, and their morphology and mechanical properties were investigated using atomic force microscopy (AFM) imaging and AFM nanoindentation. Different sets of polymers were used to produce CSH/polymer nanocomposites. The effect of different factors including dipping time, calcium to silicate ratios (C/S ratios) and pH on morphology was investigated. CSH/polymer nanocomposites made with different sets of polymers showed variation in morphologies. However, the Young’s modulus did not seem to reveal significant differences between the nanocomposites studied here. In nanocomposites containing graphene oxide (GO) nanosheet, an increase in the density of CSH particles was observed on the GO nanosheet compared to areas away from the GO nanosheet, providing evidence for improved nucleation of CSH in the presence of GO nanosheets. An increase in roughness and a reduction in the packing density in nanocomposites containing GO nanosheets was observed.

Bioinspired dynamic microcapsules

There is an increasing interest in bioinspired dynamic materials. Abundant illustrations of protein domains exist in nature, with remarkable ligand binding characteristics and structures that undergo conformational changes. For example, calmodulin (CaM) can have three conformational states, which are the unstructured Apo-state, Ca²⁺-bound ligand-exposed binding state, and compact ligand-bound state. CaM's mechanical response to biological cues is highly suitable for engineering dynamic materials. The distance between CaM globular terminals in the Ca²⁺-bound state is 5 nm and in the ligand-bound state is 1.5 nm. CaM's nanoscale conformational changes have been used to develop dynamic hydrogel microspheres that undergo reversible volume changes. The current work presents the fabrication and preliminary results of layer-by-layer (LbL) self-assembled Dynamic MicroCapsules (DynaMicCaps) whose multilayered shell walls are composed of polyelectrolytes and CaM. Quasi-dynamic perfusion results show that the DynaMicCaps undergo drastic volume changes, with up to ∼1500% increase, when exposed to a biochemical ligand trifluoperazine (TFP) at pH 6.3. Under similar test conditions, microcapsules without CaM also underwent volume changes, with only up to ∼290% increase, indicating that CaM's bio-responsiveness was retained within the shell walls of the DynaMicCaps. Furthermore, DynaMicCaps exposed to 0.1 M NaOH underwent volume changes, with only up to ∼580% volume increase. Therefore, DynaMicCaps represent a new class of polyelectrolyte multilayer (PEM) capsules that can potentially be used to release their payload at near physiological pH. With over 200 proteins that undergo marked, well-characterized conformational changes in response to specific biochemical triggers, several other versions of DynaMicCaps can potentially be developed.

Polyelectrolyte Multilayer Nanocoating Dramatically Reduces Bacterial Adhesion to Polyester Fabric

Bacterial adhesion to textiles is thought to contribute to odor and infection. Alternately exposing polyester fabric to aqueous solutions of poly(diallyldimethylammonium chloride) (PDDA) and poly(acrylic acid) (PAA) is shown here to create a nanocoating that dramatically reduces bacterial adhesion. Ten PDDA/PAA bilayers (BL) are 180 nm thick and only increase the weight of the polyester by 2.5%. The increased surface roughness and high degree of PAA ionization leads to a surface with a negative charge that causes a reduction in adhesion of Staphylococcus aureus by 50% when compared to uncoated fabric, after rinsing with sterilized water, because of electrostatic repulsion. S. aureus bacterial adhesion was quantified using bioluminescent radiance measured before and after rinsing, revealing 99% of applied bacteria were removed with a ten bilayer PDDA/PAA nanocoating. The ease of processing, and benign nature of the polymers used, should make this technology useful for rendering textiles antifouling on an industrial scale.

Mechanoresponsive materials for drug delivery: Harnessing forces for controlled release

Mechanically-activated delivery systems harness existing physiological and/or externally-applied forces to provide spatiotemporal control over the release of active agents. Current strategies to deliver therapeutic proteins and drugs use three types of mechanical stimuli: compression, tension, and shear. Based on the intended application, each stimulus requires specific material selection, in terms of substrate composition and size (e.g., macrostructured materials and nanomaterials), for optimal in vitro and in vivo performance. For example, compressive systems typically utilize hydrogels or elastomeric substrates that respond to and withstand cyclic compressive loading, whereas, tension-responsive systems use composites to compartmentalize payloads. Finally, shear-activated systems are based on nanoassemblies or microaggregates that respond to physiological or externally-applied shear stresses. In order to provide a comprehensive assessment of current research on mechanoresponsive drug delivery, the mechanical stimuli intrinsically present in the human body are first discussed, along with the mechanical forces typically applied during medical device interventions, followed by in-depth descriptions of compression, tension, and shear-mediated drug delivery devices. We conclude by summarizing the progress of current research aimed at integrating mechanoresponsive elements within these devices, identifying additional clinical opportunities for mechanically-activated systems, and discussing future prospects.

Encapsulation of photoactive porphyrinoids in polyelectrolyte hollow microcapsules viewed by fluorescence lifetime imaging microscopy (FLIM)

Fluorescence Lifetime Imaging Microscopy (FLIM) was used to investigate the encapsulation of porphyrinoids in multilayer hollow microcapsules assembled layer by layer with poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH).

Branched polymer models and the mechanism of multilayer film buildup

The "in and out diffusion" hypothesis does not provide a conclusive explanation of the buildup displayed by some polyelectrolyte multilayer film systems. Here, we report initial tests of an alternative hypothesis, on which the completion of each adsorption cycle results in an increase in the number of polymer binding sites on the film surface. Polycationic dendrimeric peptides, which can potentially bind several oppositely-charged peptides each, have been designed, synthesized and utilized in comparative film buildup experiments. Material deposited, internal film structure and film surface morphology have been studied by ultraviolet spectroscopy (UVS), circular dichroism spectroscopy (CD), quartz crystal microbalance (QCM) and atomic force microscopy (AFM). Polycations tended to contribute more to film buildup than did polyanions on quartz but not on gold. Increasing the number of branches in the dendrimeric peptides from 4 to 8 reproducibly resulted in an increase in the film growth rate on quartz but not on gold. Peptide backbones tended to adopt a β-strand conformation on incorporation into a film. Thicker films had a greater surface roughness than thin films. The data are consistent with film buildup models in which the average number of polymer binding sites will increase with each successive adsorption cycle in the range where exponential growth is displayed.

Biological applications of LbL multilayer capsules: from drug delivery to sensing

Polyelectrolyte multilayer (PEM) capsules engineered with active elements for targeting, labeling, sensing and delivery hold great promise for the controlled delivery of drugs and the development of new sensing platforms. PEM capsules composed of biodegradable polyelectrolytes are fabricated for intracellular delivery of encapsulated cargo (for example peptides, enzymes, DNA, and drugs) through gradual biodegradation of the shell components. PEM capsules with shells responsive to environmental or physical stimuli are exploited to control drug release. In the presence of appropriate triggers (e.g., pH variation or light irradiation) the pores of the multilayer shell are unlocked, leading to the controlled release of encapsulated cargos. By loading sensing elements in the capsules interior, PEM capsules sensitive to biological analytes, such as ions and metabolites, are assembled and used to detect analyte concentration changes in the surrounding environment. This Review aims to evaluate the current state of PEM capsules for drug delivery and sensing applications.

Polyelectrolyte multilayers with intrinsic antimicrobial functionality: the importance of mobile polycations

Cationic contact-killing is an important strategy for creating antimicrobial surfaces that prevent viable bacteria attachment. Recent studies have shown that highly swollen, compliant surfaces resist bacterial attachment and a sufficient density of mobile cationic charge can effectively disrupt bacterial cell membranes. Polyelectrolyte multilayers (PEMs), a popular coating system for surface modification, have been used to kill bacteria through the incorporation of contact-killing or leaching biocides. In this work, we show that manipulation of multilayer assembly and postassembly conditions (e.g., pH) to expose mobile cationic charge can create antimicrobial PEMs without the addition of specific biocidal species. As a model system, we explored PEMs comprising poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrene sulfonate) (SPS) assembled at high pH and subsequently immersed in low pH solutions. This system undergoes a reversible pH-dependent swelling transition, and we demonstrate that antimicrobial functionality at physiological pH conditions can be turned on and off with suitable pH treatment. In both airborne and waterborne bacteria assays, the viability of two strains of Gram positive Staphylococcus epidermidis (S. epidermidis), one biofilm forming and one nonbiofilm forming, and two strains of Gram negative Escherichia coli (E. coli) was effectively reduced on SPS/PAH multilayers displaying accessible cationic charge. To generalize our results, the pH assembly conditions of PEMs comprising poly(acrylic acid) (PAA) and PAH were also modified to introduce antibacterial capabilities.

Multilayer assembly of hyaluronic acid/poly(allylamine): control of the buildup for the production of hollow capsules

The objective of this work was to investigate the formation of hollow microcapsules composed of hyaluronic acid (HA) and poly(allylamine) (PAH) by layer-by-layer adsorption on CaCO 3 microparticles and subsequent core removal by addition of chelating agents for calcium ions. We found that the molecular weight of HA as well as the HA solution concentration used during deposition are crucial parameters influencing the multilayer structure. Whereas the effect of molecular weight of HA was mainly attributed to the porous structure of the template which allows penetration of polyelectrolytes when their size is below the maximum pore size of the template ( approximately 60 nm), that of the concentration of the HA solution was related to the intrinsic properties of the polysaccharide. Indeed, as shown by quartz crystal microbalance with dissipation monitoring as well as electron microscopy techniques, the latter leads to dense structures for concentrations from five to ten times the critical overlap concentration during adsorption. Such conditions were found to be favorable for the formation of hollow shells. Regarding conditions for core dissolution, we demonstrated the possibility to use either ethylenediaminetetraacetic acid (EDTA) or citric acid as chelating agents. However, in some cases, it was necessary to chemically cross-link the shell to maintain its integrity.

Tuning swelling pH and permeability of hydrogel multilayer capsules

We report on tuning swelling pH transitions of hydrogel hollow capsules that were derived from hydrogen-bonded multilayers via chemical cross-linking. The capsules were either of a single component - a weak poly(carboxylic acid), such as poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), or poly(ethacrylic acid) (PEAA) - or contained two hydrogen-bonding polymers, such as in poly(carboxylic acid)/poly(N-vinylpyrrolidone-co-NH2) (PVPON-co-NH2) or in poly(carboxylic acid)/poly(N-vinylcaprolactam-co-NH2) (PVCL-co-NH2) systems. By varying the acidity of weak polyelectrolytes, the capsule swelling can be tuned over a wide pH range from 5 to 10. We show that differently from one-component capsules, the swelling amplitude of two-component capsules is limited by the number of cross-links provided by amino-containing units of a PVCL-co-NH2 copolymer. For two-component capsules with the same degree of cross-linking, permeability at the minimum swelling pH was decreased for PMAA-neutral copolymer capsules as compared to those of PAA-neutral copolymer. We also demonstrate that swelling pH of one-component capsules in the acidic region can be modulated via reversible association with a polycation. The fine control over the swelling pH transitions and permeability of hydrogel capsules enables their use in biomedical applications.

Nanotechnology in regenerative medicine: the materials side

Regenerative medicine is an emerging multidisciplinary field that aims to restore, maintain or enhance tissues and hence organ functions. Regeneration of tissues can be achieved by the combination of living cells, which will provide biological functionality, and materials, which act as scaffolds to support cell proliferation. Mammalian cells behave in vivo in response to the biological signals they receive from the surrounding environment, which is structured by nanometre-scaled components. Therefore, materials used in repairing the human body have to reproduce the correct signals that guide the cells towards a desirable behaviour. Nanotechnology is not only an excellent tool to produce material structures that mimic the biological ones but also holds the promise of providing efficient delivery systems. The application of nanotechnology to regenerative medicine is a wide issue and this short review will only focus on aspects of nanotechnology relevant to biomaterials science. Specifically, the fabrication of materials, such as nanoparticles and scaffolds for tissue engineering, and the nanopatterning of surfaces aimed at eliciting specific biological responses from the host tissue will be addressed.