Dual stimuli-responsive coating designed through layer-by-layer assembly of PAA-b-PNIPAM block copolymers for the control of protein adsorption

Self-Assembled Block Copolymers as a Facile Pathway to Create Functional Nanobiosensor and Nanobiomaterial Surfaces

Block copolymer (BCP) surfaces permit an exquisite level of nanoscale control in biomolecular assemblies solely based on self-assembly. Owing to this, BCP-based biomolecular assembly represents a much-needed, new paradigm for creating nanobiosensors and nanobiomaterials without the need for costly and time-consuming fabrication steps. Research endeavors in the BCP nanobiotechnology field have led to stimulating results that can promote our current understanding of biomolecular interactions at a solid interface to the never-explored size regimes comparable to individual biomolecules. Encouraging research outcomes have also been reported for the stability and activity of biomolecules bound on BCP thin film surfaces. A wide range of single and multicomponent biomolecules and BCP systems has been assessed to substantiate the potential utility in practical applications as next-generation nanobiosensors, nanobiodevices, and biomaterials. To this end, this Review highlights pioneering research efforts made in the BCP nanobiotechnology area. The discussions will be focused on those works particularly pertaining to nanoscale surface assembly of functional biomolecules, biomolecular interaction properties unique to nanoscale polymer interfaces, functionality of nanoscale surface-bound biomolecules, and specific examples in biosensing. Systems involving the incorporation of biomolecules as one of the blocks in BCPs, i.e., DNA-BCP hybrids, protein-BCP conjugates, and isolated BCP micelles of bioligand carriers used in drug delivery, are outside of the scope of this Review. Looking ahead, there awaits plenty of exciting research opportunities to advance the research field of BCP nanobiotechnology by capitalizing on the fundamental groundwork laid so far for the biomolecular interactions on BCP surfaces. In order to better guide the path forward, key fundamental questions yet to be addressed by the field are identified. In addition, future research directions of BCP nanobiotechnology are contemplated in the concluding section of this Review.

Sustained growth factor delivery from bioactive PNIPAM-grafted-chitosan/heparin multilayers as a tool to promote growth and migration of cells

Delivery of growth factors (GFs) is challenging for regulation of cell proliferation and differentiation due to their rapid inactivation under physiological conditions. Here, a bioactive polyelectrolyte multilayer (PEM) is engineered by the combination of thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) and glycosaminoglycans to be used as reservoir for GF storage. PNIPAM-grafted-chitosan (PChi) with two degrees of substitution (DS) are synthesized, namely LMW* (DS 0.14) and HMW (DS 0.03), by grafting low (2 kDa) and high (10 kDa) molecular weight of PNIPAM on the backbone of chitosan (Chi) to be employed as polycations to form PEM with the polyanion heparin (Hep) at pH 4. Subsequently, PEMs are chemically crosslinked to improve their stability at physiological pH 7.4. Resulting surface and mechanical properties indicate that PEM containing HMW is responsive to temperature at 20 °C and 37 °C, while LMW is not. More importantly, Hep as terminal layer combined with HMW allows not only a better retention of the adhesive protein vitronectin but also a sustained release of FGF-2 at 37 °C. With the synergistic effect of vitronectin and matrix-bound FGF-2, significant promotion on adhesion, proliferation, and migration of 3T3 mouse embryonic fibroblasts is achieved on HMW-containing PEM compared to Chi-containing PEM and exogenously added FGF-2. Thus, PEM containing PNIPAM in combination with bioactive glycosaminoglycans like Hep represents a versatile approach to fabricate a GF delivery system for efficient cell culture, which can be potentially served as cell culture substrate for production of (stem) cells and bioactive wound dressing for tissue regeneration.

Engineering of Stable Cross-Linked Multilayers Based on Thermo-Responsive PNIPAM-Grafted-Chitosan/Heparin to Tailor Their Physiochemical Properties and Biocompatibility

The thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) is ubiquitously applied in controlled drug release and tissue engineering. However, the lack of bioactivity of PNIPAM restricts its use in cell-containing systems being a thermo-responsive adhesive substratum with no regulating effect on cell growth and differentiation. In this study, integrating PNIPAM with chitosan into PNIPAM-grafted-chitosan (PNIPAM-Chi) allows a layer-by-layer assembly with bioactive heparin to fabricate PNIPAM-modified polyelectrolyte multilayers (PNIPAM-PEMs). Grafting PNIPAM chains of either 2 (LMW) or 10 kDa (HMW) on the chitosan backbone influences the cloud point (CP) temperature in the range from 31 to 33 °C. PNIPAM-Chi with either a higher molecular weight or a higher degree of substitution of PNIPAM chains exhibiting a significant increase in diameter above CP as ensured by dynamic light scattering is selected to fabricate PEM with heparin as a polyanion at pH 4. Little difference of layer growth is detected between the chosen PNIPAM-Chi used as polycations by surface plasmon resonance, while multilayers formed with HMW-0.02 are more hydrated and show striking swelling-and-shrinking abilities when studied with quartz crystal microbalance with dissipation monitoring. Subsequently, the multilayers are covalently cross-linked using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide to strengthen the stability of the systems under physiological conditions. Ellipsometry results confirm the layer integrity after exposure to the physiological buffer at pH 7.4 compared to those without cross-linking. Moreover, significantly higher adhesion and more spreading of C3H10T1/2 multipotent embryonic mouse fibroblasts on cross-linked PEMs, particularly with heparin terminal layers, are observed owing to the bioactivity of heparin. The slightly more hydrophobic surfaces of cross-linked PNIPAM-PEMs at 37 °C also increase cell attachment and growth. Thus, layer-by-layer constructed PNIPAM-PEM with cross-linking represents an interesting cell culture system that can be potentially employed for thermally uploading and controlled release of growth factors that further promotes tissue regeneration.

Oscillating Reactions Meet Polymers at Interfaces

Chemo-mechanical phenomena, including oscillations and peristaltic motions, are widespread in nature-just think of heartbeats-thanks to the ability of living organisms to convert directly chemical energy into mechanical work. Their imitation with artificial systems is still an open challenge. Chemical clocks and oscillators (such as the popular Belousov-Zhabotinsky (BZ) reaction) are reaction networks characterized by the emergence of peculiar spatiotemporal dynamics. Their application to polymers at interfaces (grafted chains, layer-by-layer assemblies, and polymer brushes) offers great opportunities for developing novel smart biomimetic materials. Despite the wide field of potential applications, limited research has been carried out so far. Here, we aim to showcase the state-of-the-art of this fascinating field of investigation, highlighting the potential for future developments and providing a personal outlook.

Well-controlled polymerization of tri-vinyl dynamic covalent boroxine monomer: one dynamic covalent boroxine moiety toward a tunable penta-responsive polymer

Attributed to dynamic characteristics of dynamic covalent boroxine, well-controlled polymerization of tri-vinyl monomer and molecular design of penta-responsive polymer with only one functional moiety are achieved.

Thermal and pH dual-responsive cellulose microfilament spheres for dye removal in single and binary systems

Cellulose microfilaments/poly(N-Isopropylacrylamide-co-acrylic acid) spheres (MPNAA) were prepared via the in-situ synthesis of semi-interpenetrating networks (semi-IPN). The free radical copolymerization of acrylic acid (AA) (for pH-sensitive chain segments) and N-isopropylacrylamide (NIPAM) (for temperature-sensitive chain segments) was conducted in a microwave-reactor in the presence of porous cellulose/microfilament composite spherical beads pre-prepared. The surface morphology and adsorption properties of the as-prepared spheres were systematically characterized. The adsorption behaviors of resulting MPNAA towards dyes, methylene blue (MB) and methyl violet (MV), were pH sensitive; and the optimal adsorption occurred at pH 9. The dynamic adsorption processes could be well fitted with pseudo-second-order kinetic, Elovich and simplified intraparticle diffusion models. Meanwhile, Langmuir, Temkin, Freundlich, and Dubinin-Raduskevich models were used to fit the adsorption isotherms at 25, 40, and 55 °C, respectively. The results indicated that the adsorption capacities of MPNAA towards MB and MV could reach as high as 497.5 and 840.3 mg g^-1, respectively, in single systems; and high adsorption capacity was maintain in binary systems with the favorable adsorption of MV. Overall, the semi-IPN MPNAA spheres are promising as novel pH- and temperature-responsive adsorbents, facilitating the controllable adsorption/desorption processes.

Functional Surfaces through Controlled Assemblies of Upper Critical Solution Temperature Block and Star Copolymers

Endowing surfaces with multiple advanced functionalities, such as temperature-controlled swelling or the triggered release of functional small molecules, is attractive for a large variety of applications ranging from smart textiles to advanced biomedical applications. This Invited Feature Article summarizes recent advances in the development of upper critical solution temperature (UCST) behavior of copolymers in aqueous solutions and compares the fundamental differences between lower critical solution temperature (LCST) and UCST transitions. The effect of polymer chemistry and architecture on UCST transitions is discussed for block copolymer micelles (BCMs) and star polymers in solution and assembled at surfaces. The inclusion of such nanocontainers (i.e., BCMs and star polymers) in layer-by-layer (LbL) coatings and how to control their responsive behavior through deposition conditions and binding partners is explored. Finally, the inclusion and temperature-triggered release of functional small molecules is explored for nanocontainers in LbL coatings. Taken together, UCST nanocontainers containing LbL films are promising building blocks for the development of new generations of practical, functional surface coatings.

Host-guest self-assembly toward reversible visible-light-responsive switching for bacterial adhesion

Here we report a facile method to construct reversible visible-light-responsive switching from antibacterial to bioadhesion by host-guest self-assembly of β-cyclodextrin (β-CD) and azobenzene functionalized polycation/polyanion. The visible-light-responsible azobenzene functionalized polycation, poly{6-[(2,6-dimethoxyphenyl)azo-4-(2',6'-dimethoxy)phenoxy]propyl dimethylaminoethyl methacrylate-random-poly(2-(N,N-dimethylaminoethyl) methacrylate) (Azo-PDMAEMA), was synthesized via quaternization reaction between 2,6,2',6'-tetramethoxy-4-(3-bromopropoxy)azobenzene (AzoOMeBr) and poly(2-(N,N-dimethylaminoethyl) methacrylate) (PDMAEMA), and the polyanion, poly{6-[(2,6-dimethoxyphenyl)azo-4-(2',6'-dimethoxy) phenoxy]hexyl acrylate-random-acrylic acid} (Azo-PAA), was synthesized via esterification reaction between 2,6,2',6'-tetramethoxy-4-(6-hydroxyhexyloxy) azobenzene (AzoOMeOH) and poly(acryloyl chloride) (PAC) and subsequent hydrolysis reactions. The switch surface could be achieved via the alternate host-guest assembly of Azo-PDMAEMA and Azo-PAA onto a β-CD-terminated substratum (Sub-CD) through visible light irradiation. The positively charged Azo-PDMAEMA with quaternary ammonium groups exhibited antimicrobial properties and few bacteria were adhered on the surface, while the negatively charged Azo-PAA with carboxyl acid groups exhibited excellent bioadhesive properties and a large number of bacteria were adhered. Interestingly, the switch between antibacterial and bioadhesive could be realized upon visible light irradiation via alternate assembly of Azo-PDMAEMA and Azo-PAA. The proposed approach to manufacturing visible-light-responsive surface with reversible and alterable biofunctionality switching between antibacterial and bioadhesive is simple and efficient, which is promising for preparation of multifunctional polymeric surfaces to encounter multifarious demands for the biomedical and biotechnological applications.

STATEMENT OF SIGNIFICANCE

Light has attracted great attention in building biointerfaces for its precise spatiotemporal control and convenient operation. However, UV light may damage to biological samples and living tissues, which will limit its applications. This study demonstrates a novel visible-light-responsive surface fabricated through reversible assembly of azobenzene functionalized polycations/polyanions on cyclodextrin (CD)-terminated substrate by host-guest interactions between the visible-light-responsive azobenzene mAzo and CD, which has not been examined previously. It is noted that the azobenzene functionalized polycations show strong antibacterial activities, while the polyanions show excellent bioadhesive properties, as can be switched through the alternate assembly upon visible-light irradiation. This facile and versatile approach to visible-light-responsive surfaces holds great potential for switching of bioadhesion.

Enzyme immobilization on protein-resistant PNIPAAm brushes: impact of biotin linker length on enzyme amount and catalytic activity

Thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) brushes with terminal click functionality can be used to selectively immobilize enzymes. Exploiting their inherent protein-repellent and thus non-fouling properties, surfaces with specific bioactivity can be created in this way. This report describes the functionalization of alkyne-PNIPAAm brushes with two biotin linkers of different poly(ethylene glycol) (PEG) spacer length via click chemistry and the subsequent immobilization of streptavidin-conjugated horseradish peroxidase (SA-HRP) by the strong interaction between biotin and streptavidin. Spectroscopic ellipsometry is used to quantify the biotin and the SA-HRP amount on the PNIPAAm brushes. Enzyme activities are determined by UV‑vis spectroscopy. A better accessibility to the alkyne-functionalized chain ends for the short biotin-PEG₃ linker leads to a higher biotin amount on these PNIPAAm brushes, which in turn results in higher SA‑HRP amounts on biotin-PEG₃-modified brushes in the swollen (20 °C) and collapsed state (37 °C) compared to biotin-PEG₂₃-modified ones. For both linkers, an increased immobilization temperature leads to higher SA-HRP amounts due to an enhanced reaction kinetics and mobility. All immobilized SA-HRP amounts are in range of 81-98% monolayer coverage, except for SA-HRP immobilized on biotin-PEG₂₃-modified PNIPAAm brushes at 20 °C, exhibiting only 43% monolayer coverage. A high mobility of the biotin linker combined with a low surface loading of SA-HRP is found to be beneficial for the activity of SA-HRP. Hence, the highest specific activity as measured by HRP-catalyzed 3,3´,5,5´‑tetramethylbenzidine (TMB) oxidation is found for a low SA-HRP amount on biotin-PEG₂₃-modified brushes.

Synthesis of chitosan-based micelles for pH responsive drug release and antibacterial application

The over- and inefficient release of antibiotics from common delivery systems causes the development of drug-resistant bacteria. In the present work, methoxy poly(ethylene glycol)-poly(ε-caprolactone)-chitosan/montmorillonite (MPC/MMT) hybrid multilayer films were constructed for bacterial infections and pH-dependent release of the hydrophobic drug triclosan (TCA). The thickness of the (MPC-T/MMT)₁₀ multilayer films was 384.4 ± 26.5 nm, and the TCA loading dosage was 2.4 μg/cm². Staphylococcus aureus, Escherichia coli and Staphylococcus epidermidis were used in the antibacterial tests. pH responsive TCA release from the prepared multilayer films was examined by measuring the bactericidal activity of the films after immersion in PBS (pH 7.4) or MES (pH 5.5) and zone of inhibition on nutrient agar. In vitro bacterial shake-flask, zone of inhibition and live/dead staining results demonstrated the high sterilization efficiency of the films. Furthermore, cell biocompatibility measurements toward L929 fibroblasts and human lens epithelial cells showed no adverse effects of the multilayer film.

A thermo-/pH-responsive hydrogel (PNIPAM-PDMA-PAA) with diverse nanostructures and gel behaviors as a general drug carrier for drug release

The aim of this research was to develop thermo- and pH-responsive hydrogels based on H-bonds for the sustained release of the small-molecule model drug Methylene Blue (MB).

Immobilization of Aluminum Hydroxide Particles on Quartz Crystal Microbalance Sensors to Elucidate Antigen-Adjuvant Interaction Mechanisms in Vaccines

Aluminum hydroxide (AH) salts are the most widely used adjuvants in vaccine formulation. They trigger immunogenicity from antigenic subunits that would otherwise suffer from a lack of efficiency. Previous studies focusing on antigen-AH interaction mechanisms, performed with model proteins, suggested that electrostatic interactions and phosphate-hydroxyl ligand exchanges drive protein adsorption on AH. We however recently evidenced that NaCl, used in vaccine formulation, provokes AH particle aggregation. This must be taken into account to interpret data related to protein adsorption on AH. Here, we report on the successful development and use of a stable AH-coated surface to explore the mechanisms of protein adsorption by means of ultrasensitive surface analysis tools. Bovine serum albumin (BSA) adsorption was studied at different pHs and ionic strengths (I) using quartz crystal microbalance. The results show that protein adsorption on the AH adjuvant cannot be explained solely by electrostatic interactions and ligand exchanges. Hence, a higher adsorption was observed at pH 3 compared to pH 7, although AH and BSA respectively undergo repulsive and attractive electrostatic interactions at these pH values. Almost no effect of I on adsorption was moreover noted at pH 7. These new developments and observations not only suggest that other mechanisms govern protein adsorption on AH but also offer a new platform for the study of antigen adsorption in the context of vaccine formulation. Immobilizing particles on QCM sensors also enriches the range of applications for which QCM can be exploited, especially in colloid science.