Recent years have seen increasing interest in the construction of nanoscopically layered materials involving aqueous‐based sequential assembly of polymers on solid substrates. In the booming research ...area of layer‐by‐layer (LbL) assembly of oppositely charged polymers, self‐assembly driven by hydrogen bond formation emerges as a powerful technique. Hydrogen‐bonded (HB) LbL materials open new opportunities for LbL films, which are more difficult to produce than their electrostatically assembled counterparts. Specifically, the new properties associated with HB assembly include: 1) the ease of producing films responsive to environmental pH at mild pH values, 2) numerous possibilities for converting HB films into single‐ or two‐component ultrathin hydrogel materials, and 3) the inclusion of polymers with low glass transition temperatures (e.g., poly(ethylene oxide)) within ultrathin films. These properties can lead to new applications for HB LbL films, such as pH‐ and/or temperature‐responsive drug delivery systems, materials with tunable mechanical properties, release films dissolvable under physiological conditions, and proton‐exchange membranes for fuel cells. In this report, we discuss the recent developments in the synthesis of LbL materials based on HB assembly, the study of their structure–property relationships, and the prospective applications of HB LbL constructs in biotechnology and biomedicine.
Surface‐templated hydrogen‐bonded layer‐by‐layer assembly results in a variety of functional materials, including films of hydrogen‐bonded polymers (top), polymer–particle ultrathin coatings, two‐component surface hydrogels, and hollow two‐component capsules (bottom). These assemblies have promising applications as pH‐ and/or temperature‐responsive drug delivery systems, materials with tunable mechanical properties, release films dissolvable under physiological conditions, and proton‐exchange membranes for fuel cells.
While antioxidants are widely known as natural components of healthy food and drinks or as additives to commercial polymer materials to prevent their degradation, recent years have seen increasing ...interest in enhancing the antioxidant functionality of newly developed polymer materials and coatings. This paper provides a critical overview and comparative analysis of multiple ways of integrating antioxidants within diverse polymer materials, including bulk films, electrospun fibers, and self-assembled coatings. Polyphenolic antioxidant moieties with varied molecular architecture are in the focus of this Review, because of their abundance, nontoxic nature, and potent antioxidant activity. Polymer materials with integrated polyphenolic functionality offer opportunities and challenges that span from the fundamentals to their applications. In addition to the traditional blending of antioxidants with polymer materials, developments in surface grafting and assembly via noncovalent interaction for controlling localization versus migration of antioxidant molecules are discussed. The versatile chemistry of polyphenolic antioxidants offers numerous possibilities for programmed inclusion of these molecules in polymer materials using not only van der Waals interactions or covalent tethering to polymers, but also via their hydrogen-bonding assembly with neutral molecules. An understanding and rational use of interactions of polyphenol moieties with surrounding molecules can enable precise control of concentration and retention versus delivery rate of antioxidants in polymer materials that are critical in food packaging, biomedical, and environmental applications.
We report on association of tannic acid (TA) with neutral or charged polymers in solution and at surfaces and contrast hydrogen-bonded and electrostatically associated polymer/TA complexes and ...TA/polymer layer-by-layer (LbL) films as per their stability in the pH scale. The neutral polymers used for hydrogen bonding with TA were poly(N-vinylcaprolactam) (PVCL), poly(N-vinylpyrrolidone) (PVPON), poly(ethylene oxide) (PEO), or poly(N-isopropylacrylamide) (PNIPAM), and the polymer used to explore electrostatic binding with TA was 90% quaternized poly(4-vinylpyridine) (Q90). Association of TA with polymers in solution was explored by measuring the turbidity of solutions. At surfaces, LbL film deposition and pH stability were followed by phase-modulated ellipsometry and in-situ Fourier transform infrared spectroscopy in attenuated total reflection mode (ATR-FTIR). While electrostatically stabilized films of TA with Q90 could not be deposited at low pH values (pH = 2), hydrogen-bonded films of TA with PVCL, PVPON, PEO, and PNIPAM could be constructed at pH 2 and did not dissolve until a critical dissolution pH of 9.5, 9, 8.5, and 8 (measured in 0.01 M buffer solutions) for PVCL/TA, PVPON/TA, PEO/TA, and PNIPAM/TA, respectively. In addition, all multilayers could be also constructed at pH 7.5 in solutions with low ionic strength. The high pH stability of these systems as compared to multilayers of the same neutral polymers with polyacrylic (PAA) or polymethacrylic (PMAA) acids is due to higher pK a value of TA of ∼8.5 as estimated in this paper. We also show that multilayers of TA with a copolymer of N-vinylpyrrolidone containing 20 mol % of primary amino groups, PVPON-NH2-20, were highly stable in a wide pH range from 1.3 to 11.7 because of combined stabilization through both electrostatic and hydrogen-bonding interactions. For all systems, pH windows for deposition and stability of LbL films at surfaces correlated with the phase behavior of TA complexes in solution. High pH stability of hydrogen-bonded films of TA as well as the capability of tuning the critical pH value for film dissolution in the range close to physiological pH values makes such multilayer systems promising candidates for biomedical applications.
Soft materials with widely tailorable mechanical properties throughout the material's volume can shape the future of soft robotics and wearable electronics, impacting both consumer and defense ...sectors. Herein, a platform of 3D printable soft polymer networks with unprecedented tunability of stiffness of nearly three orders of magnitude (MPa to GPa) and an inherent capability to interbond is reported. The materials are based on dynamic covalent polymer networks with variable density of crosslinkers attached to prepolymer backbones via a temperature‐reversible Diels–Alder (DA) reaction. Inherent flexibility of the prepolymer chains and controllable crosslinking density enable 3D printed networks with glass transition temperatures ranging from just a few degrees to several tens of degrees Celsius. Materials with an elastomeric network demonstrate a fast and spontaneous self‐healing behavior at room temperature both in air and under water—a behavior difficult to achieve with other crosslinked materials. Reversible dissociation of DA networks at temperatures exceeding ≈120 °C allows for reprintability, while control of the stereochemistry of DA attachments enables reprogrammable shape memory behavior. The introduced platform addresses current major challenges including control of polymer interbonding, enhanced mechanical performance of printed parts, and reprocessability of 3D‐printed crosslinked materials in the absence of solvent.
A family of reversibly crosslinked polymer networks with wide‐ranging mechanical properties is developed. The materials are highly suitable for use with facile 3D printing techniques, seamlessly bond in spite of mismatch in stiffness, can autonomously self‐heal in air and underwater at room temperature, and enable reprogramming their shape through plastic deformation of the network.
Here, we report on upper critical solution temperature (UCST)-type polyamino acid-based block copolymer assembled in a layer-by-layer system with a natural polyphenol. The UCST-type block copolymer, ...polyvinylpyrrolidone-b-polyureido(ornithine-co-lysine) (PVP-b-PUOL), was synthesized via ring opening polymerization followed by postpolymerization functionalization with ureido groups. PVP-b-PUOL exhibited UCST behavior that was controlled by both molecular weight and ornithine-to-lysine ratio. Importantly, PVP-b-PUOL with a UCST of 33 °C assembled into block copolymer micelles below UCST and dissociated above UCST, yet this behavior was not repeatable in solution due to β-sheet formation and large-scale aggregation. To overcome this limitation, UCST micelles (UCSTMs) were deposited within layer-by-layer (LbL) films via hydrogen bonding with tannic acid (TA). Significantly, the assembled TA/UCSTM films stabilized the micelles from desorption while maintaining their morphology and prevented β-sheet formation even after continuous exposure to 40 °C for 7 days. Moreover, these films demonstrated repeatable swelling–deswelling for up to five temperature cycles from 25 to 40 °C. The thermo-switchable hydrophobicity of micellar cores was used for trapping of a model active molecule, pyrene, and its on–off temperature-controlled release, demonstrating the potential of TA/UCSTM films for controlled delivery of active molecules.
We report on hydrogen-bonded layer-by-layer (LbL) films as a robust, reusable platform for temperature-triggered “on-demand” release of drugs. Films with high drug loading capacity, ...temperature-controlled on–off drug release, and stability at physiological conditions were enabled by assembly of tannic acid (TA) with temperature-responsive block copolymer micelles (BCMs), which were pre-formed by heating solutions of a neutral diblock copolymer, poly(N-vinylpyrrolidone)-b-poly(N-isopropylacrylamide) (PVPON-b-PNIPAM), to a temperature above the lower critical solution temperature (LCST) of PNIPAM. The BCM/TA films exhibited temperature-triggered swelling/deswelling transitions at physiological conditions (swelling ratios of 1.75 and 1.2 at 37°C and 20°C, respectively). A model drug, doxorubicin (DOX) was incorporated into the film at a high drug-to-matrix ratio (~9.3wt.% of DOX per film mass), with a total loading capacity controlled by the film thickness. At 37°C, DOX was efficiently retained within the hydrophobic BCM cores of BCM/TA films, whereas exposure to a lower temperature (20°C) triggered fast DOX release. While neither bare BCM-containing films nor films loaded with DOX showed cytotoxicity at 37°C, drug released from films at lower temperature exhibited high potency against breast cancer cells. Repeated on/off drug release was demonstrated with 1.5-μm-thick DOX-loaded films, allowing at least three 30-min cooling cycles with consistent DOX (~12–16% of loaded DOX released for each cycle) released over a 4-day period. Despite significant stress associated with multiple swelling/deswelling cycles, films maintained their structural integrity in PBS, and each film could be repeatedly loaded with drug and used more than 15 times with only ~7% loss in film thickness and no obvious changes in reloading capacity or release profiles. This work presents the first proof-of-concept utility of temperature-responsive BCM-containing films for repeated on-demand release of a drug.
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Chemically cross-linked polymer matrices have demonstrated strong potential for shape stabilization of molten phase change materials (PCM). However, they are not designed to be fillable and removable ...from a heat exchange module for an easy replacement with new PCM matrices and lack self-healing capability. Here, a new category of shapeable, self-healing gels, “salogels”, is introduced. The salogels reversibly disassemble in a high-salinity environment of a fluid inorganic PCM lithium nitrate trihydrate (LNH), at a preprogrammed temperature. LNH was employed as a high latent heat PCM and simultaneously as a solvent, which supported the formation of a network of polyvinyl alcohol (PVA) chains via physical cross-linking through poly(amidoamine) dendrimers of various generations. The existence of hydrogen bonding and the importance of low-hydration state of PVA for the efficient gelation were experimentally confirmed. The thermal behavior of PCM salogels was highly reversible and repeatable during multiple heating/cooling cycles. Importantly, the gel–sol transition temperature could be precisely controlled within a range of temperature above LNH’s melting point by the choice of dendrimer generation and their concentration. Shape stabilization and self-healing properties of the salogels, taken together with tunability of their temperature-induced fluidization make these materials attractive for thermal energy storage applications that require on-demand removal and replacement of used inorganic PCM salt hydrates.
We report on highly efficient, bioresponsive, controlled-release antibacterial coatings constructed by direct assembly of tannic acid (TA) with one of several cationic antibiotics (tobromycin, ...gentamicin, and polymyxin B) using the layer-by-layer (LbL) technique. These films exhibit a distinct “self-defense” behavior triggered by acidification of the immediate environment by pathogenic bacteria, such as Staphylococcus epidermidis (S. epidermidis) or Escherichia coli (E. coli). Films assembled using spin-assisted and dip-assisted techniques show drastically different morphology, thickness and pH-/bacteria-triggered antibiotic release characteristics. While dip-deposited films have rough surfaces with island-like, granular structures regardless of the film thickness, spin-assisted LbL assemblies demonstrate a transition from linear deposition of uniform 2D films to a highly developed 3D morphology for films thicker than ∼45 nm. Ellipsometry, UV–vis and mass spectrometry confirm that all coatings do not release antibiotics in phosphate buffered saline at pH 7.4 for as long as one month in the absence of bacteria and therefore do not contribute to the development of antibiotic resistance. These films do, however, release antibiotics upon pH lowering. The rate of triggered release can be controlled through the choice of assembled antibiotic and the assembly technique (spin- vs dip-deposition) and by the spinning rate used during deposition, which all affect the strength of TA–antibiotic binding. TA/antibiotic coatings as thin as 40 nm strongly inhibit S. epidermidis and E. coli bacterial growth both at surfaces and in surrounding medium, but support adhesion and proliferation of murine osteoblast cells. These coatings thus present a promising way to incorporate antibacterial agents at surfaces to prevent bacterial colonization of implanted biomedical devices.
An upper critical solution temperature (UCST) block copolymer, poly(acrylamide-co-acrylonitrile)-b-polyvinylpyrrolidone (P(AAm-co-AN)-b-PVP), was synthesized by reversible addition–fragmentation ...chain transfer (RAFT) polymerization, assembled in solution, and deposited within functional layer-by-layer (LbL) temperature-responsive films. In aqueous solutions, the polymer formed well-defined block copolymer micelles (BCMs) at ambient temperature and exhibited a reversible micelle–unimer transition within an ∼40–50 °C temperature range in a wide range of pH and ionic strengths. Temperature-induced dissociation of BCMs to unimers was completely suppressed, however, when BCMs were assembled with tannic acid (TA) within LbL films. Instead, reversible changes in micellar sizes and film swelling occurred as a result of UCST-driven uptake/release of water within/from the micellar cores. The solution pH modulated strength of hydrogen bonding between TA and PVP in the micellar corona, thus strongly affecting film growth, micelle morphology, and film swelling. Spherical micellar morphology and high film swelling degrees were observed with films at neutral pH values, where hydrogen bonding was counteracted by the negative charge of partially ionized TA. In contrast, strong hydrogen bonding and absence of charge in TA caused crumpling of BCMs and reduced the film swelling degree in acidic solutions. Film swelling was also dependent on number of assembled micellar layers, with thicker films exhibiting larger swelling amplitudes and sharper temperature transitions. The LbL assemblies were stable in phosphate buffer saline up to pH 7.5 at 50 °C and preserved their response after 55 heating/cooling cycles. The robustness of the temperature transitions in these films taken together with their occurrence in an aqueous environment in a wide range of pH and ionic strengths makes these films potentially useful for controlling responses of soft interfaces in biological environments.
Abstract We report on negatively charged layer-by-layer (LbL) hydrogel films, which turn hydrophobic and bactericidal in response to bacteria-induced acidification of the medium. Single-component ...hydrogel thin films, abbreviated as PaAALbL s, consisting of chemically crosslinked poly(2-alkylacrylic acids) (PaAAs) with varying hydrophobicity polymethacrylic acid (PMAA), poly(2-ethylacrylic acid) (PEAA), poly(2-n-propylacrylic acid) (PPAA) or poly(2-n-butylacrylic acid) (PBAA). With increasing polyacid hydrophobicity, the hydrogel films showed a decrease in water uptake and an increase in elastic modulus. Both parameters were strongly dependent on pH. At pH 7.4, hydrogels of higher hydrophobicity were more resistant to colonization by Staphylococcus epidermidis , with the PBAA coating showing almost negligible colonization. As the medium became more acidic due to bacterial proliferation, the more hydrophobic PEAALbL , PPAALbL and PBAALbL hydrogels became dehydrated and killed bacteria upon contact with the surface. The killing efficiency was strongly enhanced by the polymer hydrophobicity. The films remained cytocompatible with human osteoblasts, as indicated by the MTS assay and live/dead staining. Our approach exploits bacteria-responsive properties of the coating itself without the involvement of potentially toxic cationic polymers or the release of antimicrobial agents. These coatings thus demonstrate a novel approach to the antibacterial protection of tissue-contacting biomedical-device surfaces.
