Yanxia Zhang

Anti-fouling bioactive surfaces.

Bioactive surfaces refer to surfaces with immobilized bioactive molecules aimed specifically at promoting or supporting particular interactions. Such surfaces are of great importance for various biomedical and biomaterials applications. In the past few years, considerable effort has been made to create bioactive surfaces by forming specific biomolecule-modified surfaces on a non-biofouling base or background. Hydrophilic and bioinert polymers have been widely used as anti-fouling layers that resist non-specific protein interactions. They can also serve as spacers to effectively move the immobilized biomolecule away from the surface, thus enhancing its bioactivity. In this review we summarize several successful approaches for the design and preparation of bioactive surfaces based on different types of anti-fouling/spacer materials. Some perspectives on future research in this area are also presented.

Protein adsorption on poly(N-vinylpyrrolidone)-modified silicon surfaces prepared by surface-initiated atom transfer radical polymerization.

Well-controlled poly(N-vinylpyrrolidone) (PVP)-grafted silicon surfaces were prepared by surface-initiated atom transfer radical polymerization (SI-ATRP) with 1,4-dioxane/water mixtures as solvents and CuCl/5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (Me6TATD) as a catalyst. The thickness of the PVP layer on the surface increased with reaction time, suggesting that the ATRP grafting of N-vinylpyrrolidone (NVP) from the silicon surfaces was a well-controlled process. The water contact angle and X-ray photoelectron spectroscopy (XPS) were used to characterize the modified surfaces. The protein adsorption property of the PVP-grafted surfaces was evaluated using a radiolabeling method. Compared with unmodified silicon surfaces, a Si-PVP60 surface with a PVP thickness of 15.06 nm reduced the level of adsorption of fibrinogen, human serum albumin (HSA), and lysozyme by 75, 93, and 81%, respectively. Moreover, the level of fibrinogen adsorption decreases gradually with an increase in PVP thickness. However, no significant difference in fibrinogen adsorption was found when the PVP layer was thicker than the critical thickness of 13.45 nm.

Protein adsorption and cell adhesion/detachment behavior on dual-responsive silicon surfaces modified with poly(N-isopropylacrylamide)-block-polystyrene copolymer.

Diblock copolymer grafts covalently attached to surfaces have attracted considerable attention because of their special structure and novel properties. In this work, poly(N-isopropylacrylamide)-block-polystyrene (PNIPAAm-b-PS) brushes were prepared via surface-initiated consecutive atom-transfer radical polymerization on initiator-immobilized silicon. Because of the inherent thermosensitivity of PNIPAAm and the hydrophobicity difference between the two blocks, the modified surfaces were responsive to both temperature and solvent. Moreover, the diblock copolymer brushes exhibited both resistance to nonspecific protein adsorption and unique cell interaction properties. They showed strong protein resistance in both phosphate-buffered saline and blood plasma. In particular, fibrinogen adsorption from plasma at either room temperature or body temperature was less than 8 ng/cm(2), suggesting that the surfaces might possess good blood compatibility. In addition, the adhesion and detachment of L929 cells could be tuned, and the ability to control the detachment of cells thermally was restored by block polymerization of hydrophobic, cell-adhesive PS onto a thicker PNIPAAm layer. In addition to providing a simple and effective design for advanced cell-culture surfaces, these results suggest new biomedical applications for PNIPAAm.

Protein adsorption on poly(N-isopropylacrylamide)-modified silicon surfaces: Effects of grafted layer thickness and protein size

In this work, we investigated the protein adsorption on the end-tethered thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) brushes with varying grafted layer thickness prepared via surface-initiated atom transfer radical polymerization (SI-ATRP) on initiator-immobilized silicon surfaces. The thickness of a grafted layer was modulated by adjusting reaction time and/or solvent composition. The surface properties as a function of thickness were investigated by water contact angle, X-ray photoelectron spectroscopy (XPS), and atomic force microscope (AFM). The influence of PNIPAAm-grafted layer thickness on human serum albumin (HSA) adsorption in phosphate-buffered saline (PBS) (pH 7.4) at different temperature was evaluated using a radiolabeling method. In a lower thickness range (<15 nm), protein adsorption on PNIPAAm-grafted layer shows a thermoresponsive change in a certain extent, but the variation is not remarkable. However, it is interesting to observe that these surfaces exhibit good protein-resistant property. For the surface with a PNIPAAm thickness of 13.4 nm, the HSA adsorption level measured at room temperature was approximately 7 ng/cm2, corresponding to a reduction of 97% compared to the unmodified silicon surface. For thicker PNIPAAm-grafted surface with thickness of 38.1 nm, the adsorption results of three proteins (HSA, fibrinogen, and lysozyme) with different sizes and charges indicate that the PNIPAAm-modified surface exhibits a size-sensitive property of protein adsorption.

Surfaces having dual fibrinolytic and protein resistant properties by immobilization of lysine on polyurethane through a PEG spacer.

The objective of this work is to develop a blood contacting surface that possesses both resistance to nonspecific protein adsorption and clot lysing properties. Chemical modification of a polyurethane (PU) surface with poly(ethylene glycol) (PEG); and lysine was used to create a plasminogen-binding potentially fibrinolytic surface. The preparation involves modification of the PU surface with dihydroxy PEG, reaction of the unreacted distal OH with N,N-disuccinimidyl carbonate (DSC) to produce a PU-PEG-NHS surface, followed by conjugation of epsilon-amino-protected lysine (H-Lys(t-BOC)-OH) by reaction of the alpha-amino group with the NHS and deprotection. The result is a lysine-derivatized surface in which the epsilon-amino groups of the lysine are free to participate in binding plasminogen and tissue plasminogen activator (t-PA). Surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and contact angle measurements. Protein adsorption experiments showed that nonspecific protein adsorption was greatly reduced on these surfaces and that they adsorbed significant quantities of plasminogen from plasma. After incubation with plasma and treatment with t-PA the surfaces were able to dissolve nascent plasma clots formed around them.

Fibrinolytic Poly(dimethyl siloxane) Surfaces

PDMS surfaces have been modified to confer both resistance to non-specific protein adsorption and clot lyzing properties. The properties and chemical compositions of the surfaces have been investigated using water contact angle measurements, ATR FT-IR spectroscopy, and XPS. The ability of the PEG component to suppress non-specific protein adsorption was assessed by measurement of radiolabeled fibrinogen uptake from buffer. The adsorption of plasminogen from human plasma to the various surfaces was studied. In vitro experiments demonstrated that lysine-immobilized surfaces with free epsilon-amino groups were able to dissolve fibrin clots, following exposure to plasma and tissue plasminogen activator. [Figure: see text].

pH-Reversible, High-Capacity Binding of Proteins on a Substrate with Nanostructure

In this letter, a pH-switchable system for protein adsorption and release is introduced. By combining the pH sensitivity of poly(methacrylic acid) (poly(MAA) chains and the nanoeffects of 3D nanostructured silicon nanowire arrays (SiNWAs), a poly(MAA)-modified SiNWAs material showed an extremely high capacity for binding lysozyme at pH 4 (an ∼80-fold increase compared with that of smooth Si-poly(MAA)). Moreover, ∼90% of the adsorbed lysozyme was released from SiNWAs-poly(MAA) by increasing the pH from 4 to 9, without a loss of enzyme activity.

The synergistic effects of stimuli-responsive polymers with nano- structured surfaces: wettability and protein adsorption

Surface modification with stimuli-responsive polymers leads to switchable wettability and bioadhesion that varies in response to environmental stimuli. The introduction of nanoscale structure onto surfaces also results in changes to the surface properties. However, the synergistic effects of stimuli-responsive polymers with nanoscale structures are unclear. In this work, two typical stimuli-responsive polymers, thermo-responsive poly(N-isopropylacrylamide) (poly(NIPAAm)) and pH-responsive poly(methacrylic acid) (poly(MAA)), were grafted from initiator-immobilized silicon nanowire arrays (SiNWAs) with nanoscale topography via surface-initiated atom transfer radical polymerization. Because of the synergistic effects of the stimuli-responsive conformation transition of polymer chains and the nano-effects of three-dimensional nanostructured SiNWAs, these new platforms possess several unique properties. Compared with their corresponding modified flat silicon surfaces, the introduction of nanoscale roughness enhanced the thermo-responsive wettability of SiNWAs-poly(NIPAAm) but weakened the pH-responsive wettability of SiNWAs-poly(MAA). More importantly, these surfaces exhibited special protein-adsorption behavior. The SiNWAs-poly(NIPAAm) surface showed good non-specific protein resistance regardless of temperature, suggesting a weakened thermo-responsivity to protein adsorption. The SiNWAs-poly(MAA) surface showed obvious enhancement of pH-dependent protein adsorption behavior.

A surface decorated with diblock copolymer for biomolecular conjugation

A simple and attractive method was introduced to construct bioactive surfaces that exhibit non-specific protein resistant properties and high loading capacities for immobilizing various specific biomolecules. These bioactive surfaces may find wide potential biomedical applications.

Predicting Au–S bond breakage from the swelling behavior of surface tethered polyelectrolytes

Surface tethered weak polyelectrolyte, carboxylated poly(OEGMA-r-HEMA), was prepared viasurface initiated polymerization (SIP) from metal (Au, Pt and Ag) surfaces functionalized with initiators through metal–S bonds. The swelling behavior of carboxylated poly(OEGMA-r-HEMA) in phosphate buffered saline (PBS) was studied by quartz crystal microbalance (QCM) and was found to obey the following equation: Teff = (0.32ln([Na+]) + 2.53) × TCOOH,dry, where Teff and TCOOH,dry are the thicknesses of carboxylated poly(OEGMA-r-HEMA) in PBS measured by QCM and in air measured by ellipsometry, respectively. We also confirmed that the event of covalent bond breaking (CBB) of Au–S bonds occurred at a certain critical Teff, i.e. ∼255 nm. Thus, one could predict the CBB of Au–S or other metal–S bonds from the swelling behavior of surface tethered carboxylated poly(OEGMA-r-HEMA).