Shengfu Chen

Zwitterionic Polymers Exhibiting High Resistance to Nonspecific Protein Adsorption from Human Serum and Plasma

This study examined six different polymer and self-assembled monolayer (SAM) surface modifications for their interactions with human serum and plasma. It was demonstrated that zwitterionic polymer surfaces are viable alternatives to more traditional surfaces based on poly(ethylene glycol) (PEG) as nonfouling surfaces. All polymer surfaces were formed using atom transfer radical polymerization (ATRP) and they showed an increased resistance to nonspecific protein adsorption compared to SAMs. This improvement is due to an increase in the surface packing density of nonfouling groups on the surface, as well as a steric repulsion from the flexible polymer brush surfaces. The zwitterionic polymer surface based on carboxybetaine methacrylate (CBMA) also incorporates functional groups for protein immobilization in the nonfouling background, making it a strong candidate for many applications such as in diagnostics and drug delivery.

Zwitterionic carboxybetaine polymer surfaces and their resistance to long-term biofilm formation

In this work, we report a systematic study of zwitterionic poly(carboxybetaine methacrylate) (pCBMA) grafted from glass surfaces via atom transfer radical polymerization (ATRP) for their resistance to long-term bacterial biofilm formation. Results show that pCBMA-grafted surfaces are highly resistant to non-specific protein adsorption (fibrinogen and undiluted blood plasma) at 25, 30 and 37 degrees C. Long-term (over 24 h) colonization of two bacterial strains (Pseudomonas aeruginosa PAO1 and Pseudomonas putida strain 239) on pCBMA surface was studied using a parallel flow cell at 25, 30 and 37 degrees C. Uncoated glass cover slips were chosen as the positive reference. Results show that pCBMA coatings reduced long-term biofilm formation of P. aeruginosa up to 240 h by 95% at 25 degrees C and for 64 h by 93% at 37 degrees C, and suppressed P. putida biofilm accumulation up to 192 h by 95% at 30 degrees C, with respect to the glass reference. The ability of pCBMA coatings to resist non-specific protein adsorption and significantly retard bacterial biofilm formation makes it a very promising material for biomedical and industrial applications.

Spectral surface plasmon resonance biosensor for detection of staphylococcal enterotoxin B in milk

This work evaluates a newly developed wavelength modulation-based SPR biosensor for the detection of staphylococcal enterotoxin B (SEB) in milk. Two modes of operation of the SPR biosensor are described: direct detection of SEB and sandwich assay. In the sandwich assay detection mode, secondary antibodies are bound to the already captured toxin to amplify sensor response. Samples including SEB in buffer and SEB in milk were analyzed in this work. The SPR biosensor has been shown to be capable of directly detecting concentrations of SEB in buffer as low as 5 ng/ml. In sandwich detection mode, the lowest detection limit was determined to be 0.5 ng/ml for both buffer and milk samples. The reported wavelength modulation-based SPR sensor provides a generic platform which can be tailored for detection of various foodborne pathogens and agents for food analysis and testing.

Molecular Simulation Studies of Protein Interactions with Zwitterionic Phosphorylcholine Self-Assembled Monolayers in the Presence of Water

Molecular simulations were performed to study the interactions between a protein (lysozyme, LYZ) and phosphorylcholine-terminated self-assembled monolayers (PC-SAMs) in the presence of explicit water molecules and ions. The results show that the water molecules above the PC-SAM surface create a strong repulsive force on the protein as it approaches the surface. The structural and dynamic properties of the water molecules above the PC-SAM surface were analyzed to provide information regarding the role of hydration in surface resistance to protein adsorption. It can be seen from residence time dynamics that the water molecules immediately above the PC-SAM surface are significantly slowed down as compared to bulk water, suggesting that the PC-SAM surface generates a tightly bound, structured water layer around its head groups. Moreover, the orientational distribution and reorientational dynamics of the interfacial water molecules near the PC-SAM surface were found to have the ionic solvation nature of the PC head groups. These properties were also compared to those obtained previously for an oligo(ethylene glycol) (OEG) SAM system and bulk water.

Film Thickness Dependence of Protein Adsorption from Blood Serum and Plasma onto Poly(sulfobetaine)-Grafted Surfaces

In this work, we investigate protein adsorption from single protein solutions and complex media such as 100% blood serum and plasma onto poly(sulfobetaine methacrylate) (polySBMA)-grafted surfaces via atom transfer radical polymerization (ATRP) at varying film thicknesses. It is interesting to observe that protein adsorption exhibits a minimum at a medium film thickness. Results show that the surface with 62 nm polySBMA brushes presents the best nonfouling character in 100% blood serum and plasma although all of these surfaces are highly resistant to nonspecific protein adsorption from single fibrinogen and lysozyme solutions. Surface resistance to 100% blood serum or plasma is necessary for many applications from blood-contacting devices to drug delivery. This work provides a new in vitro evaluation standard for the application of biomaterials in vivo.

Ultra low fouling zwitterionic polymers with a biomimetic adhesive group

Biomimetic polymers with a zwitterionic moiety for ultra low fouling and a catechol end group for surface anchoring have been developed. Binding tests of the adhesive polymers on various surfaces, including amino (NH(2)), hydroxyl (OH), and methyl (CH(3)) terminated self-assembled monolayers (SAMs) along with bare gold, were performed under acidic and basic conditions. Protein adsorption from single protein solutions of fibrinogen, lysozyme, and complex media of 10-100% blood plasma and serum was measured using a surface plasmon resonance (SPR) sensor. Under optimized conditions, the coated surfaces are highly resistant to non-specific protein adsorption from both single protein solutions and blood serum/plasma. Furthermore, the 3-day accumulation of Pseudomonas aeruginosa on the coated surfaces was evaluated in situ in a laminar flow chamber. Results show that the coated surfaces are highly resistant to bacterial adhesion and biofilm formation. This work demonstrates a convenient and efficient method for using zwitterionic polymers with a catechol anchor group to achieve ultra low fouling surfaces via surface modification, for applications in complex media.

Ultra-low fouling peptide surfaces derived from natural amino acids.

This work demonstrated the ultra-low fouling natural peptides composed of certain negatively and positively charged residues such as glutamic acid (E) or aspartic acid (D) and lysine (K), in the form of either alternating or randomly mixed charge. These peptide-based materials are major candidates as biodegradable nonfouling materials since their final metabolized products are natural amino acids. Although hydrophilic materials can generally reduce nonspecific binding to a certain extent, it is very challenging to achieve ultra-low fouling, which is critical for many biomedical applications, such as medical implants, drug delivery carriers, and biosensors. Based on the design principle of uniformly mixed charges and the selection of appropriate amino acid residues, the natural peptides developed exhibit high resistance to nonspecific protein adsorption (<0.3 ng/cm(2) adsorbed proteins) comparable to what is achieved by poly(ethylene glycol) (PEG)-based materials. Mixed charged groups, when uniformly distributed at the molecular level, can achieve ultra-low fouling properties similar to zwitterionic groups due to their strong hydration ability.

Protein interactions with oligo(ethylene glycol) (OEG) self-assembled monolayers: OEG stability, surface packing density and protein adsorption

We present a study of protein adsorption on oligo(ethylene glycol) (OEG) self-assembled monolayers (SAMs) at a range of OEG surface densities. OEG SAMs were formed in mixed ethanol and water solutions at different assembly temperatures to adjust the packing density of EG4-SAMs. These SAMs were characterized using X-ray photoelectron spectroscopy (XPS). Fibrinogen adsorption on these surfaces was measured by a surface plasmon resonance (SPR) sensor at different temperatures. This work is aimed at addressing three important issues for protein–OEG interactions, i.e., (i) OEG stability, (ii) the correlation between OEG surface densities and surface non-fouling properties, and (iii) protein adsorption on OEG surfaces at different temperatures.

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.

The hydrolysis of cationic polycarboxybetaine esters to zwitterionic polycarboxybetaines with controlled properties

In this work, we report a new class of materials, cationic polycarboxybetaine esters, which have unique properties when they interact with proteins, DNAs, and bacteria. These cationic polymers can be converted to nontoxic and nonfouling zwitterionic polymers upon their hydrolysis. Due to their unique properties, they are very promising for a wide range of applications, such as highly effective gene delivery carriers and environmentally friendly antimicrobial coatings. Three positively charged polyacrylamides, of which the pedant groups bear carboxybetaine ester groups, were synthesized. These three polymers have different spacer groups between the quaternary ammonium and the ester groups. Their hydrolysis behaviors were studied using proton NMR under different NaOH concentrations. Their interactions with biomolecules and microorganisms before and after hydrolysis were demonstrated by protein adsorption/resistance, DNA condensation/release, and antimicrobial properties. The polymers were grafted onto a gold-coated surface covered with initiators using surface-initiated atom transfer radical polymerization (ATRP). Fibrinogen adsorption was measured by surface plasmon resonance (SPR) sensors. While the polymer-grafted surfaces have high protein adsorption, the surfaces became nonfouling after hydrolysis. Linear polymers were also synthesized and DNA/polymer complexes were evaluated. Agarose gel electrophoresis shows that DNA can be condensed into nanoparticles by the cationic polymers before hydrolysis and released from the DNA/polymer complexes upon the hydrolysis of the cationic polymers into zwitterionic polymers. The complexes formed were characterized by dynamic light scattering measurements. In addition, the interactions of linear polymers with bacteria were also evaluated. The polycarboxybetaine ester with a pentene spacer exhibits evident antimicrobial properties when they are incubated with Gram negative bacteria (Escherichia Coli). The polymer can be converted to a nontoxic polycarboxybetaine after hydrolysis. This work shows that the biological properties of polycarboxybetaine esters can be dramatically changed via controlled hydrolysis.