Investigation of the hydration of nonfouling material poly(ethylene glycol) by low-field nuclear magnetic resonance

Simple Thermal Pretreatment Strategy to Tune Mechanical and Antifouling Properties of Zwitterionic Hydrogels

Zwitterionic hydrogels are promising biomaterials because of their high water content, three-dimensional network structure, and antifouling property. However, it still remains unclear about how mechanical properties of zwitterionic hydrogels affect their antifouling property. In this work, we propose a simple, thermal-pretreatment method to fabricate poly(sulfobetaine methacrylate) (pSBMA) hydrogels with varied mechanical properties that can be readily tuned by thermal pretreatment time and cross-linker density, as well as to correlate their mechanical property with antifouling property. The resulting thermal-treated pSBMA hydrogels show significantly enhanced mechanical properties with tunable compressive modulus and elastic modulus as compared to the untreated hydrogels. A combination of ELISA investigations and short-term cell adhesion assays also confirm that pSBMA hydrogels exhibit superior antifouling properties to resist protein adsorption and cell adhesion. Further analysis shows a linear inversion correlation between elastic modulus and protein adsorption of pSBMA hydrogels, i.e., the hydrogel with the higher elastic modulus exhibits the lower protein adsorption (the better antifouling property). This work not only provides a simple thermal-pretreatment strategy for fabricating pSBMA hydrogels, but also demonstrates multifunctional properties of the pSBMA hydrogels, which possess a great potential to fulfill some biomedical applications.

A negative correlation between water content and protein adsorption on polymer brushes

A negative correlation between the water content inside polymer brushes and protein adsorption.

Distribution of tightly and loosely bound water in biological macromolecules and age-related diseases

This mini-review article is focused on publications devoted to the changes in water binding energy and content of bound water in biological tissues during aging processes, when bound water lost from the hydration layer becomes free water. Bound water is released during cataractogenesis. In skin, water bound to proteins and other biomacromolecules becomes more mobile with increasing skin age. Extracellular to intracellular water ratio increases with age and was associated with muscle cell atrophy. Bound water concentration decreases with age in hydrated human bone and can be correlated with the strength and toughness of the bone. Higher fraction of free water in malignant tissues compared to normal tissues was observed. Hydration water mobility is enhanced around tau amyloid fibers. Water plays a decisive role in amyloid formation as entropic driving force. In the natural aging processes dehydration and glycation may be considered as subsequent steps.

Design and preparation of bi-functionalized short-chain modified zwitterionic nanoparticles

An ideal nanomaterial for use in the bio-medical field should have a distinctive surface capable of effectively preventing nonspecific protein adsorption and identifying target bio-molecules. Recently, the short-chain zwitterion strategy has been suggested as a simple and novel approach to create outstanding anti-fouling surfaces. In this paper, the carboxyl end group of short-chain zwitterion-coated silica nanoparticles (SiO₂-ZWS) was found to be difficult to functionalize via a conventional EDC/NHS strategy due to its rapid hydrolysis side-reactions. Hence, a series of bi-functionalized silica nanoparticles (SiO₂-ZWS/COOH) were designed and prepared by controlling the molar ratio of 3-aminopropyltriethoxysilane (APTES) to short-chain zwitterionic organosiloxane (ZWS) in order to achieve above goal. The synthesized SiO₂-ZWS/COOH had similar excellent anti-fouling properties compared with SiO₂-ZWS, even in 50% fetal bovine serum characterized by DLS and turbidimetric titration. Subsequently, SiO₂-ZWS/COOH_5/1 was chosen as a representative and then demonstrated higher detection signal intensity and more superior signal-to-noise ratios compare with the pure SiO₂-COOH when they were used as a bio-carrier for chemiluminescence enzyme immunoassay (CLEIA). These unique bi-functionalized silica nanoparticles have many potential applications in the diagnostic and therapeutic fields.

STATEMENT OF SIGNIFICANCE

Reducing nonspecific protein adsorption and enhancing the immobilized efficiency of specific bio-probes are two of the most important issues for bio-carriers, particularly for a nanoparticle based bio-carrier. Herein, we designed and prepared a bi-functional nanoparticle with anti-fouling property and bio conjugation capacity for further bioassay by improving the short-chain zwitterionic modification strategy we have proposed previously. The heterogeneous surface of this nanoparticle showed effective anti-fouling properties both in model protein solutions and fetal bovine serum (FBS). The modified nanoparticles can also be successfully functionalized with a specific antibody for CLEIA assay with a prominent bio-detection performance even in 50% FBS. In this paper, we also investigated an unexpectedly fast hydrolysis behavior of NHS-activated carboxylic groups within the pure short-chain zwitterionic molecule that led to no protein binding in the short-chain zwitterion modified nanoparticle. Our findings pave a new way for the designing of high performance bio-carriers, demonstrating their strong potential as a robust platform for diagnosis and therapy.

Development of pH-responsive polymer and citrate aqueous two-phase system for extractive bioconversion of cefprozil

A pH-responsive aqueous two-phase system (pH-ATPS) has been developed by sodium citrate and a recyclable pH-responsive polymer PADB6.8 that can response to the change of pH values. Phase separation mechanism is studied through Low field-NMR. All variables affecting the phase separation are evaluated. Phase characteristics (viscosity, density, interfacial tension) and phase separation kinetic are studied for understanding of separation process and operational parameters in applications. This pH-ATPS has the characters of low interfacial tension, high recovery leading efficient mass transfer and low cost. The proposed system can be used as a mild medium for extractive bioconversion with low cost. We applied this pH-ATPS in extractive bioconversion of cefprozil. Cefprozil is partitioned towards the polymer-rich phase while the substrates tended to be partitioned in the salt-rich phase. Extractive bioconversion of cefprozil in this pH-ATPS can improve yield of the enzymatic process and reduce the product hydrolysis in optimal conditions. The maximal conversion yield of cefprozil in the studied system is 91.0%.

Facilitating Proton Transport in Nafion-Based Membranes at Low Humidity by Incorporating Multifunctional Graphene Oxide Nanosheets

Nafion, as a state-of-the-art solid electrolyte for proton exchange membrane fuel cells (PEMFCs), suffers from drastic decline in proton conductivity with decreasing humidity, which significantly restricts the efficient and stable operation of the fuel cell system. In this study, the proton conductivity of Nafion at low relative humidity (RH) was remarkably enhanced by incorporating multifunctional graphene oxide (GO) nanosheets as multifunctional fillers. Through surface-initiated atom transfer radical polymerization of sulfopropyl methacrylate (SPM) and poly(ethylene glycol) methyl ether methacrylate, the copolymer-grafted GO was synthesized and incorporated into the Nafion matrix, generating efficient paths at the Nafion-GO interface for proton conduction. The Lewis basic oxygen atoms of ethylene oxide (EO) units and sulfonated acid groups of SPM monomers served as additional proton binding and release sites to facilitate the proton hopping through the membrane. Meanwhile, the hygroscopic EO units enhanced the water retention property of the composite membrane, conferring a dramatic increase in proton conductivity under low humidity. With 1 wt % filler loading, the composite membrane displayed the highest proton conductivity of 2.98 × 10^-2 S cm^-1 at 80 °C and 40% RH, which was 10 times higher than that of recast Nafion. Meanwhile, the Nafion composite exhibited a 135.5% increase in peak power density at 60 °C and 50% RH, indicating its great application potential in PEMFCs.

Channel-facilitated molecule and ion transport across polymer composite membranes

This tutorial review highlights transport channels within polymer composite membranes and focuses on the regulation of channel microenvironments through bio-inspiration.

The dynamics of solvation dictates the conformation of polyethylene oxide in aqueous, isobutyric acid and binary solutions

The dynamics of solvation dictates the conformation of polyethylene oxide in water and isobutyric acid causing a helix–coil transition in a mixed isobutyric acid/water solvent.

Protein diffusion characteristics in the hydrogels of poly(ethylene glycol) and zwitterionic poly(sulfobetaine methacrylate) (pSBMA)

Nonfouling materials such as neutral poly(ethylene glycol) (PEG) and zwitterionic poly(sulfobetaine methacrylate) (pSBMA) are ideal biocompatible materials for drug, especially protein drug delivery. The interaction behavior of protein between the nonfouling materials could cause great impact on their future applications, such as controlled release drug delivery systems. In this work, we investigated the diffusion behavior of the fluorescence-labeled model proteins (bovine serum albumin (BSA) and lysozyme (LYZ)) in nonfouling PEG, pSBMA and mixed PEG-SBMA hydrogels (SBMA:PEG 4:1, SBMA:PEG 1:4). It was observed that these four hydrogels showed varied diffusion behavior for either negatively charged BSA or positively charged LYZ due to protein-polymer interaction and the free water content in hydrogel matrix. The relatively stronger interaction between protein-PEG than protein-pSBMA could increase protein loading efficiency and control release rate by changing ratio of PEG to SBMA in the hybrid hydrogel. Moreover, it is further demonstrated the free water (freezable water) content in low cross-linked hydrogel, not the equilibrium water content (EWC), is a more accurate parameter to reflect the diffusion behavior of protein molecules. Thus, these results together provide new insights of the interactions between protein molecules and nonfouling polymers as well as the bio applications of the nonfouling polymeric hydrogels.

STATEMENT OF SIGNIFICANCE

This work shows that the relative stronger interaction between protein-PEG than protein-pSBMA could increase protein loading efficiency and control release rate by the change ratio of PEG to SBMA in the hydrogel, while the free water (freezable water) content in low cross-linked hydrogel, not the equilibrium water content (EWC), is a more accurate parameter to reflect the diffusion behavior of protein molecules. The impact of this work (i) gains some new insights of the interactions between protein molecules and nonfouling polymer matrixes for protein drug delivery; (ii) prompts to apply the weak PEG-protein interactions to protein drug loading and release; (iii) provides a new fundamental understanding of free water in hydrogel for protein diffusion.

Development of polypeptide-based zwitterionic amphiphilic micelles for nanodrug delivery

Protein molecules, which typically have a hydrophobic core and a zwitterionic shell with a polypeptide backbone, could be ideal materials for nanodrug vehicles (NDVs) with low side effects. Here, we synthesized poly(l-aspartic acid(lysine))-b-poly(l-lysine(Z)) (PAsp(Lys)-b-PLys(Z)) (PALLZ), a novel amphiphilic block polypeptide with key structures of protein to investigate the possibility for use as a NDV. This polypeptide can spontaneously self-assemble into micelles in aqueous solution with a zwitterionic brush (the PAsp(Lys) part) to provide the nonfouling shell and a hydrophobic core (the PLys(Z) part) for loading hydrophobic drugs. The doxorubicin (DOX) loaded PALLZ micelles showed excellent resistance to nonspecific protein adsorption in FBS, which leads to very low internalization. Moreover, PALLZ micelles showed no cytotoxicity to MCF7, HeLa and HepG-2 cells up to 500 μg mL^-1. All these results indicated that zwitterionic amphiphilic block polypeptides could be promising materials for NDVs.

Bicontinuous Fluid Structure with Low Cohesive Energy: Molecular Basis for Exceptionally Low Interfacial Tension of Complex Coacervate Fluids

An exceptionally low interfacial tension of a dense fluid of concentrated polyelectrolyte complexes, phase-separated from a biphasic fluid known as complex coacervates, represents a unique and highly sought-after materials property that inspires novel applications from superior coating to wet adhesion. Despite extensive studies and broad interest, the molecular and structural bases for the unique properties of complex coacervates are unclear. Here, a microphase-separated complex coacervate fluid generated by mixing a recombinant mussel foot protein-1 (mfp-1) as the polycation and hyaluronic acid (HA) as the polyanion at stoichiometric ratios was macroscopically phase-separated into a dense complex coacervate and a dilute supernatant phase to enable separate characterization of the two fluid phases. Surprisingly, despite up to 4 orders of magnitude differing density of the polyelectrolytes, the diffusivity of water in these two phases was found to be indistinguishable. The presence of unbound, bulk-like, water in the dense fluid can be reconciled with a water population that is only weakly perturbed by the polyelectrolyte interface and network. This hypothesis was experimentally validated by cryo-TEM of the macroscopically phase-separated dense complex coacervate phase that was found to be a bicontinuous and biphasic nanostructured network, in which one of the phases was confirmed by staining techniques to be water and the other polyelectrolyte complexes. We conclude that a weak cohesive energy between water-water and water-polyelectrolytes manifests itself in a bicontinuous network, and is responsible for the exceptionally low interfacial energy of this complex fluid phase with respect to virtually any surface within an aqueous medium.

Development of Long-Circulating Zwitterionic Cross-Linked Micelles for Active-Targeted Drug Delivery

Blood stability, active targeting, and controlled drug release are the most important features to design desirable drug carriers. Here, we demonstrate a zwitterionic biodegradable cross-linked micelle based on a penta-block copolymer, which utilizes poly(carboxybetaine methacrylate) as hydrophilic segment, poly(ε-caprolactone) as biodegradable hydrophobic segment, poly(S-2-hydroxyethyl-O-ethyl dithiocarbonate methacrylate) (PSODMA) block as thiol protecting segment for cross-linking, and cyclic Arg-Gly-Asp-d-Tyr-Lys [c(RGDyK)] as targeting ligand. As a result, this micelle possessed excellent colloidal stability at high dilution and in 50% fetal bovine serum. In vitro drug release experiment showed no burst release under physiological conditions but accelerated drug release in mimicking tumor tissue environment. In vivo tests showed that the drug-loaded micelles had prolonged half-life in bloodstream, enhanced therapeutic efficiency, and reduced cardiac toxicity and biotoxicity compared with free drug formulation. Taken together, the reported c(RGDyK)-modified zwitterionic interfacially cross-linked micelle has emerged as an appealing platform for cancer therapy.

Molecular Understanding and Structural-Based Design of Polyacrylamides and Polyacrylates as Antifouling Materials

Design and synthesis of highly bioinert and biocompatible antifouling materials are crucial for a broad range of biomedical and engineering applications. Among antifouling materials, polyacrylamides and polyacrylates have proved so promising because of cheap raw materials, ease of synthesis and applicability, and abundant functional groups. The strong surface hydration and the high surface packing density of polyacrylamides and polyacrylates are considered to be the key contributors to their antifouling property. In this article, we review our studies on the design and synthesis of a series of polyacrylamides and polyacrylates with different molecular structures. These polymers can be fabricated into different architectural forms (brushes, nanoparticles, nanogels, and hydrogels), all of which are highly resistant to the attachment of proteins, cells, and bacteria. We find that small structural changes in the polymers can lead to large enhancement in surface hydration and antifouling performance, both showing a positive correlation. This reveals a general design rule for effective antifouling materials. Furthermore, polyacrylamides and polyacrylates are readily functionalized with other bioactive compounds to achieve different new multifunctionalities.

Characterization of water state and distribution in fibre materials by low-field nuclear magnetic resonance

The states of absorbed water in the cotton and PET fibres materials characterized by LF-NMR method.

Drug carriers based on highly protein-resistant materials for prolonged in vivo circulation time

Long-circulating drug carriers are highly desirable in drug delivery system. However, nonspecific protein adsorption leaves a great challenge in drug delivery of intravenous administration and significantly affects both the pharmacokinetic profiles of the carrier and drugs, resulting in negatively affect of therapeutic efficiency. Therefore, it is important to make surface modification of drug carriers by protein-resistant materials to prolong the blood circulation time and increase the targeted accumulation of therapeutic agents. In this review, we highlight the possible mechanism of protein resistance and recent progress of the alternative protein-resistant materials and their drug carriers, such as poly(ethylene glycol), oligo(ethylene glycol), zwitterionic materials, and red blood cells adhesion.

Construction of 3D micropatterned surfaces with wormlike and superhydrophilic PEG brushes to detect dysfunctional cells

Detection of dysfunctional and apoptotic cells plays an important role in clinical diagnosis and therapy. To develop a portable and user-friendly platform for dysfunctional and aging cell detection, we present a facile method to construct 3D patterns on the surface of styrene-b-(ethylene-co-butylene)-b-styrene elastomer (SEBS) with poly(ethylene glycol) brushes. Normal red blood cells (RBCs) and lysed RBCs (dysfunctional cells) are used as model cells. The strategy is based on the fact that poly(ethylene glycol) brushes tend to interact with phosphatidylserine, which is in the inner leaflet of normal cell membranes but becomes exposed in abnormal or apoptotic cell membranes. We demonstrate that varied patterned surfaces can be obtained by selectively patterning atom transfer radical polymerization (ATRP) initiators on the SEBS surface via an aqueous-based method and growing PEG brushes through surface-initiated atom transfer radical polymerization. The relatively high initiator density and polymerization temperature facilitate formation of PEG brushes in high density, which gives brushes worm-like morphology and superhydrophilic property; the tendency of dysfunctional cells adhered on the patterned surfaces is completely different from well-defined arrays of normal cells on the patterned surfaces, providing a facile method to detect dysfunctional cells effectively. The PEG-patterned surfaces are also applicable to detect apoptotic HeLa cells. The simplicity and easy handling of the described technique shows the potential application in microdiagnostic devices.

Investigation of the interaction between poly(ethylene glycol) and protein molecules using low field nuclear magnetic resonance

A comprehensive insight into the interaction between proteins and poly(ethylene glycol) (PEG) is crucial to understand the behavior of PEG, which is widely used in pharmaceutical and medical applications. Although PEG is believed to be an excellent material to resist non-specific protein adsorption, there is a lack of quantitative information about the interactions between proteins and PEG. In this paper the interactions of bovine serum albumin (BSA) and lysozyme (LYZ) with different molecular weight (MW) PEGs were investigated through the T2 relaxation time of PEGs measured by low field nuclear magnetic resonance spectroscopy. The integrated signal intensity of PEGs was quantified under various conditions from the concentrations and MWs of PEG, and ionic strength of solutions, as well as the molar ratios of PEG to protein. The results show that a large number of PEG molecules could associate with protein molecules with association constants in the range ~10(4)-10(5) M(-1). The association constant is insensitive to the ionic strength change in the physiological range and the lowest associate constant occurs at the medium MW PEG with protein. This suggests that the interaction between PEG and protein molecules might not be negligible in investigations of the resistance to non-specific protein adsorption. Long chain PEG coatings might cause modest protein adsorption, which could interfere with any weak specific interaction between ligand and receptor. Thus, it is necessary to reconsider the popular accepted method of protecting nanoparticles (NP) in blood with long chain PEG coatings since these NPs might be surrounded by a layer of weakly adsorbed plasma protein in the circulatory system.

Probing structure-antifouling activity relationships of polyacrylamides and polyacrylates

We have synthesized two different polyacrylamide polymers with amide groups (polySBAA and polyHEAA) and two corresponding polyacrylate polymers without amide groups (polySBMA and polyHEA), with particular attention to the evaluation of the effect of amide group on the hydration and antifouling ability of these systems using both computational and experimental approaches. The influence of polymer architectures of brushes, hydrogels, and nanogels, prepared by different polymerization methods, on antifouling performance is also studied. SPR and ELISA data reveal that all polymers exhibit excellent antifouling ability to repel proteins from undiluted human blood serum/plasma, and such antifouling ability can be further enhanced by presenting amide groups in polySBAA and polyHEAA as compared to polySBMA and polyHEA. The antifouling performance is positively correlated with the hydration properties. Simulations confirm that four polymers indeed have different hydration characteristics, while all presenting a strong hydration overall. Integration of amide group with pendant hydroxyl or sulfobetaine group in polymer backbones is found to increase their surface hydration of polymer chains and thus to improve their antifouling ability. Importantly, we present a proof-of-concept experiment to synthesize polySBAA nanogels, which show a switchable property between antifouling and pH-responsive functions driven by acid-base conditions, while still maintaining high stability in undiluted fetal bovine serum and minimal toxicity to cultured cells. This work provides important structural insights into how very subtle structural changes in polymers can yield great improvement in biological activity, specifically the inclusion of amide group in polymer backbone/sidechain enables to obtain antifouling materials with better performance for biomedical applications.

Dual functionality of antimicrobial and antifouling of poly(N-hydroxyethylacrylamide)/salicylate hydrogels

The emergence and reemergence of microbial infection demand an urgent response to develop effective biomaterials that prevent biofilm formation and associated bacterial infection. In this work, we have synthesized and characterized hybrid poly(N-hydroxyethylacrylamide) (polyHEAA)/salicylate (SA) hydrogels with integrated antifouling and antimicrobial capacities. The antifouling efficacy of polyHEAA hydrogels was examined via exposure to proteins, cells, and bacteria, while the antimicrobial activity of SA-treated polyHEAA hydrogels was investigated against both gram-negative Escherichia coli RP437 and gram-positive Staphylococcus epidermidis. The results showed that polyHEAA/SA hydrogels exhibited high surface resistance to protein adsorption, cell adhesion, and bacteria attachment. The polyHEAA hydrogels were also characterized by their water content and state of water, revealing a strong ability to contain and retain high nonfreezable water content. This work demonstrates that the hybrid polyHEAA/SA hydrogels can be engineered to possess both antifouling and antimicrobial properties, which can be used for different in vitro and in vivo applications against bacterial infection.

Generic top-functionalization of patterned antifouling zwitterionic polymers on indium tin oxide

This paper presents a novel surface engineering approach that combines photochemical grafting and surface-initiated atom transfer radical polymerization (SI-ATRP) to attach zwitterionic polymer brushes onto indium tin oxide (ITO) substrates. The photochemically grafted hydroxyl-terminated organic layer serves as an excellent platform for initiator attachment, and the zwitterionic polymer generated via subsequent SI-ATRP exhibits very good antifouling properties. Patterned polymer coatings can be obtained when the surface with covalently attached initiator was subjected to photomasked UV-irradiation, in which the C-Br bond that is present in the initiator was broken upon exposure to UV light. A further, highly versatile top-functionalization of the zwitterionic polymer brush was achieved by a strain-promoted alkyne-azide cycloaddition, without compromising its antifouling property. The attached bioligand (here: biotin) enables the specific immobilization of target proteins in a spatially confined fashion, pointing to future applications of this approach in the design of micropatterned sensing platforms on ITO substrates.

Investigation of the hydration of nonfouling material poly(sulfobetaine methacrylate) by low-field nuclear magnetic resonance

The strong surface hydration layer of nonfouling materials plays a key role in their resistance to nonspecific protein adsorption. Poly(sulfobetaine methacrylate) (polySBMA) is an effective material that can resist nonspecific protein adsorption and cell adhesion. About eight water molecules are tightly bound with one sulfobetaine (SB) unit, and additional water molecules over 8:1 ratio mainly swell the polySBMA matrix, which is obtained through the measurement of T(2) relaxation time by low-field nuclear magnetic resonance (LF-NMR). This result was also supported by the endothermic behavior of water/polySBMA mixtures measured by differential scanning calorimetry (DSC). Furthermore, by comparing both results of polySBMA and poly(ethylene glycol) (PEG), it is found that (1) the hydrated water molecules on the SB unit are more tightly bound than on the ethylene glycol (EG) unit before saturation, and (2) the additional water molecules after forming the hydration layer in polySBMA solutions show higher freedom than those in PEG. These results might illustrate the reason for higher resistance of zwitterionic materials to nonspecific protein adsorptions compared to that of PEGs.