Spatial variation of the charge and sulfur oxidation state in a surface gradient affects plasma protein adsorption

Fabrication of Polysulfobetaine Gradient Coating via Oxidation Polymerization of Pyrogallol To Modulate Biointerfaces

A surface with a gradient physical or chemical feature, such as roughness, hardness, wettability, and chemistry, serves as a powerful platform for high-throughput investigation of cell responses to a biointerface. In this work, we developed a continuous antifouling gradient surface using pyrogallol (PG) chemistry. A copolymer of a zwitterionic monomer, sulfobetaine methacrylate, and an amino monomer, aminoethyl methacrylate, were synthesized (pSBAE) and deposited on glass slides via the deposition of self-polymerized PG. A gradient of pSBAE was fabricated on glass slides in 7 min in the presence of an oxidant, ammonium persulfate, by withdrawing the reaction solution. The modified glass slide showed a wettability gradient, determined by measuring the water contact angle. Cell adhesion and protein adsorption were well correlated with surface wettability. We expect that this simple and faster method for the fabrication of a continuous chemical gradient is applicable for high-throughput screening of surface properties to modulate biointerfaces.

Microparticle Deposition on Human Serum Albumin Layers: Unraveling Anomalous Adsorption Mechanism

Human serum albumin (HSA) layers are adsorbed on mica under controlled diffusion transport at pH 3.5 and various ionic strengths. The surface concentration of HSA is directly determined by AFM imaging of single molecules. It is shown that the adsorption kinetics derived in this way is quantitatively described using the random sequential (RSA) adsorption model. The electrokinetic characteristics of the HSA layers at various pHs comprising their zeta potential are acquired in situ while using the streaming potential method. It is shown that at pH 3.5 the zeta potential of mica becomes positive for HSA concentrations above 3000 μm−2. At larger pHs, HSA layers exhibit negative zeta potential for the entire range of coverage. Thorough characteristics of these monolayers at various pHs were performed applying the colloid deposition method involving negatively charged polystyrene microparticles. The kinetics of their deposition and their maximum coverage are determined as a function of the HSA layer surface concentration, pH, and ionic strength. An anomalous deposition of microparticles on substrates also exhibiting a negative zeta potential is observed, which contradicts the Derjaguin, Landau, Vervey, Overbeek (DLVO) theory. This effect is interpreted in terms of heterogeneous charge distribution that results from molecule concentration fluctuations. It is also shown that the maximum concentration of microparticles abruptly decreases with the electric double-layer thickness that is regulated by changing ionic strength, which indicates that their deposition is governed by electrostatic interactions. One can argue that the results obtained in this work can be exploited as useful reference data for the analysis of deposition phenomena of bioparticles on protein layers.

Functionally graded biomaterials for use as model systems and replacement tissues

The heterogeneity of native tissues requires complex materials to provide suitable substitutes for model systems and replacement tissues. Functionally graded materials have the potential to address this challenge by mimicking the gradients in heterogeneous tissues such as porosity, mineralization, and fiber alignment to influence strength, ductility, and cell signaling. Advancements in microfluidics, electrospinning, and 3D printing enable the creation of increasingly complex gradient materials that further our understanding of physiological gradients. The combination of these methods enables rapid prototyping of constructs with high spatial resolution. However, successful translation of these gradients requires both spatial and temporal presentation of cues to model the complexity of native tissues that few materials have demonstrated. This review highlights recent strategies to engineer functionally graded materials for the modeling and repair of heterogeneous tissues, together with a description of how cells interact with various gradients.

On the Importance of Silane Infusion Order on the Microscopic and Macroscopic Properties of Multifunctional Charge Gradients

Four multicomponent charge gradients containing acidic and basic functionalities were prepared via sol-gel processes and the controlled-rate infusion (CRI) method to more clearly understand how preparation conditions influence macroscopic properties. CRI is used to form gradients by infusing reactive alkoxysilanes into a glass vial housing a vertically oriented modified silicon wafer. The concentration and time of infusion of the silane solutions were kept constant. Only the sequence of infusion of the silane solutions was changed. The first set of samples was prepared by initially infusing a solution containing 3-aminopropyltriethoxysilane (APTES) followed by a mercaptopropyltrimethoxysilane (MPTMS) solution. The individual gradients were formed either in an aligned or opposed fashion with respect to the initial gradient. The second set of samples was prepared by infusing the MPTMS solution first followed by the APTES solution, again in either an aligned or opposed fashion. To create charge gradients (NH3 +, SO3 -), the samples were immersed into H2O2. The extent of modification, the degree of protonation of the amine, and the thicknesses of the individual layers were examined by X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. The wettability of the individual gradients was assessed via static contact angle measurements. The results demonstrate the importance of infusion order and how it influences the macroscopic and microscopic properties of gradient surfaces including the surface concentration, packing density, degree of protonation, and ultimately wettability. When the gradient materials are prepared via infusion of the APTES sol first, it results in increased deposition of both the amine and thiol groups as evidenced by XPS. Interestingly, the total thickness evaluated from ellipsometry was independent of the infusion order for the aligned gradients, indicative of significant differences in the film density. For the opposed gradients, however, the infusion of APTES first leads to a significantly thicker composite film. Furthermore, it also leads to a more pronounced gradient in the protonation of the amine, which introduces a very different surface wettability. The use of aminosilanes provides a viable approach to create gradient surfaces with different functional group distributions. These studies demonstrate that the controlled placement of functional groups on a surface can provide a new route to prepare gradient materials with improved performance.

Pullulan-Based Nanoparticle-HSA Complex Formation and Drug Release Influenced by Surface Charge

The nanomaterial composition of nanoparticles and their protein adsorption in the blood is of great significance in the design of drug-loaded nanoparticles. To explore the interaction between the different surface components of nanoparticles (NPs) and protein, we synthesized three kinds of pullulan NP polymers: cholesteric hydrophobically (CH) modified pullulan (CHP), CH-modified animated pullulan (CHAP), and CH-modified carboxylated pullulan (CHSP). Pullulan NPs were prepared by the dialysis method. Dynamic light scattering was used to determine the charge and size of the three NPs. The size of NPs was altered by the number of charge groups when polymers contain the same degree of cholesterol substitution. The zeta potentials were + 12.9, - 15.4, and - 0.698 mV for CHAP, CHSP, and CHP, respectively, and the dimensions were 116.9, 156.9, and 73.1 nm, respectively. Isothermal titration calorimetry was used to determine the thermodynamic changes of NPs with different surface charge, and the effect of human serum albumin (HSA) on the titration was investigated. The changes of enthalpy and entropy demonstrated an interaction between NPs and HSA; the binding constant (K_b) for CHSP, CHP, and CHAP was 1.41, 27.7, and 412 × 10⁴ M^-1, respectively, with the positive charge for CHAP-HSA, uncharged for CHP-HSA, and negative charge for CHSP-HSA complex. Fluorescence and circular dichroism spectroscopy were used to determine the protein structure change after the complexation between NPs and HSA. The NP and HSA complexation is a complicated process composed of protein α-helical content reduction and the peptide chain extension; CHP NPs had the largest reduction in HSA α-helical content. The drug release rates of all compounds of NP and HSA were significantly lower than those of free drug and drug-loaded NPs after 48 h. The highest and lowest rates were observed in CHSP-HSA and CHP-HSA, respectively. The drug release was significantly influenced by the adsorption of HSA on NPs, and the size and surface charge of NPs played an important role in this process.

pH and Surface Charge Switchability on Bifunctional Charge Gradients

Multifunctionalized pH-sensitive silica gradients containing acidic and basic functional groups have been prepared to evaluate how the spatial arrangement of active sites on a surface influences the surface charge and pH switchability. The gradient surfaces were prepared using controlled rate infusion in such a manner that the individual gradients in the strong acid (sulfonic acid) and in the weak base (propylamine) align, whereas a gradient in the weakly acidic silanol groups opposes them. The relative amounts of the three species were varied by controlling the composition of the deposition solution, whereas the hydrophobicity of the underlying surface was set by using base layer-coated substrates prepared from either tetramethoxysilane or tetramethoxysilane/octyltrimethoxysilane mixtures. Results from X-ray photoelectron spectroscopy confirm that aligned gradients are formed in both amine and sulfonic acid groups, and the relative amounts bound to the surface follow that expected from the solution composition. Water contact angle measurements show a 40°-50° change across the length of the gradient, the exact values being dependent on the hydrophobicity of the base layer. Zeta potential measurements on gradient mimics reveal that there is a pH where the net charge on the gradient surface is predicted to have a constant but nonzero value. Static contact angle measurements and modeling confirm this prediction. At a pH acidic of this value, the gradient in charge runs in one direction, whereas at a pH basic of this value, the gradient in charge runs in the other direction. This point can be strategically moved from acidic values to basic values by changing the relative amounts of acidic and basic functionalities on the surface. The origin of this unique pH switchability can be found in acid-base chemistry. By modeling the charge along the gradient surface using a simple equilibrium model, a distribution of pK_a values were noted in these materials.

Cooperative Effects in Aligned and Opposed Multicomponent Charge Gradients Containing Strongly Acidic, Weakly Acidic, and Basic Functional Groups

Bifunctionalized surface charge gradients in which the individual component gradients either align with or oppose each other have been prepared. The multicomponent gradients contain strongly acidic, weakly acidic, and basic functionalities that cooperatively interact to define surface wettability, nanoparticle binding, and surface charge. The two-step process for gradient formation begins by modifying a siloxane coated silicon wafer in a spatially dependent fashion first with an aminoalkoxysilane and then with a mercapto-functionalized alkoxysilane. Immersion in hydrogen peroxide leads to oxidation of the surface immobilized sulfhydryl groups and subsequent protonation of the surface immobilized amines. Very different surface chemistries were obtained from gradients that either align with or oppose each other. X-ray photoelectron spectroscopy (XPS) data show that the degree of amine group protonation depends on the local concentration of sulfonate groups, which form ion pairs with the resulting ammonium ions. Contact angle measurements show that these ion pairs greatly enhance the wettability of the gradient surface. Finally, studies of colloidal gold binding show that the presence of both amine and thiol moieties enhance colloid binding, which is also influenced by surface charge. Cooperativity is also revealed in the distribution of charges on uniform samples used as models of the gradient surfaces, as evaluated via zeta potential measurements. Most significantly, the net surface charge and how it changes with distance and solution pH strongly depend on whether the gradients in amine and thiol align or oppose each other. The aligned multicomponent gradients show the most interesting behavior in that there appears to be a point at pH ∼ 6.5 where surface charge remains constant with distance. Setting the pH above or below this transition point leads to changes in the direction of charge variation along the length of the substrate.

Revealing deposition mechanism of colloid particles on human serum albumin monolayers

Colloid particle deposition was applied in order to characterize human serum albumin (HSA) monolayers on mica adsorbed under diffusion transport at pH 3.5. The surface concentration of HSA was determined by a direct AFM imaging of single molecules. The electrokinetic characteristics of the monolayers for various ionic strength were done by in situ streaming potential measurements. In this way the mean-field zeta potential of monolayers was determined. It was shown that the initially negative potential changed its sign for HSA surface concentrations above 2800μm(-2) that was interpreted as overcharging effect. The monolayers were also characterized by the colloid deposition method where negatively charged polystyrene particles, 810nm in diameter were used. The kinetics of particle deposition and their maximum coverage were determined as a function of the HSA monolayer surface concentration. An anomalous deposition of particles on substrates exhibiting a negative zeta potential was observed, which contradicts the mean-field theoretical predictions. This effect was quantitatively interpreted in terms of the random site sequential adsorption model. It was shown that efficient immobilization of particles only occurs at adsorption sites formed by three and more closely adsorbed HSA molecules. These results can be exploited as useful reference data for the analysis of deposition phenomena of bioparticles at protein monolayers that has practical significance for the regulation of the bioadhesive properties of surfaces.

Versatile thiol-based reactions for micrometer- and nanometer-scale photopatterning of polymers and biomolecules

Thiol-based reactions were applied to enable the photochemical patterning of polymer brushes and green fluorescent protein on silicon oxide surfaces.

Bi-ligand surfaces with oriented and patterned protein for real-time tracking of cell migration

A bioactive platform for the quantitative observation of cell migration is presented by (1) presenting migration factors in a well-defined manner on 2-D substrates, and (2) enabling continuous cell tracking. Well-defined substrate presentation is achieved by correctly orienting immobilized proteins (chemokines and cell adhesion molecules), such that the active site is accessible to cell surface receptors. A thiol-terminated self-assembled monolayer on a silica slide was used as a base substrate for subsequent chemistry. The thiol-terminated surface was converted to an immobilized metal ion surface using a maleimido-nitrilotriacetic acid (NTA) cross-linker that bound Histidine-tagged recombinant proteins on the surface with uniform distribution and specific orientation. This platform was used to study the influence of surface-immobilized chemokine SDF-1α and cell adhesion molecule ICAM-1 on murine splenic B lymphocyte migration. While soluble SDF-1α induced trans-migration in a Boyden Chamber type chemotaxis assay, immobilized SDF-1α alone did not elicit significant surface-migration on our test-platform surface. Surface-immobilized cell adhesion protein, ICAM-1, in conjunction with activation enabled migration of this cell type on our surface. Controlled exposure to UV light was used to produce stable linear gradients of His-tagged recombinant SDF-1α co-immobilized with ICAM-1 following our surface chemistry approach. XPS and antibody staining showed defined gradients of outwardly oriented SDF-1α active sites. This test platform can be especially valuable for investigators interested in studying the influence of surface-immobilized factors on cell behavior and may also be used as a cell migration enabling platform for testing the effects of various diffusible agents.

Mussel-inspired one-step adherent coating rich in amine groups for covalent immobilization of heparin: hemocompatibility, growth behaviors of vascular cells, and tissue response

Heparin, an important polysaccharide, has been widely used for coatings of cardiovascular devices because of its multiple biological functions including anticoagulation and inhibition of intimal hyperplasia. In this study, surface heparinization of a commonly used 316L stainless steel (SS) was explored for preparation of a multifunctional vascular stent. Dip-coating of the stents in an aqueous solution of dopamine and hexamethylendiamine (HD) (PDAM/HD) was presented as a facile method to form an adhesive coating rich in primary amine groups, which was used for covalent heparin immobilization via active ester chemistry. A heparin grafting density of about 900 ng/cm(2) was achieved with this method. The retained bioactivity of the immobilized heparin was confirmed by a remarkable prolongation of the activated partial thromboplastin time (APTT) for about 15 s, suppression of platelet adhesion, and prevention of the denaturation of adsorbed fibrinogen. The Hep-PDAM/HD also presented a favorable microenvironment for selectively enhancing endothelial cell (EC) adhesion, proliferation, migration and release of nitric oxide (NO), and at the same time inhibiting smooth muscle cell (SMC) adhesion and proliferation. Upon subcutaneous implantation, the Hep-PDAM/HD exhibited mitigated tissue response, with thinner fibrous capsule and less granulation formation compared to the control 316L SS. This number of unique functions qualifies the heparinized coating as an attractive alternative for the design of a new generation of stents.

Modulation of protein adsorption, vascular cell selectivity and platelet adhesion by mussel-inspired surface functionalization

The “surface property–protein adsorption–cell behavior” relationship of polydopamine was investigated and the mechanism of polydopamine selectively modulating vascular cell behavior was explored.

Mechanism of HSA adsorption on mica determined by streaming potential, AFM and XPS measurements

Adsorption of human serum albumin (HSA) on mica at pH 3.5 (0.15M NaCl) was studied using in situ streaming potential measurements, AFM imaging and XPS. Results obtained by streaming potential were consistent with AFM measurements and theoretical predictions based on the random sequential adsorption model. These results in combination with complementary data derived from XPS allowed one to determine both the kinetics of adsorption and the maximum coverage of irreversibly bound HSA, which was close to 1.6 mg m(-2) (dimensionless coverage 0.45). It was concluded that HSA adsorption on mica at pH 3.5 occurred under side-on, electrostatically controlled mechanisms with no tendency to multilayer formation. Such irreversible bound HSA monolayers of well-defined coverage can find applications for performing efficient immunological tests, designing biomaterials surfaces and biosensors.

Gradient biomaterials and their influences on cell migration

Cell migration participates in a variety of physiological and pathological processes such as embryonic development, cancer metastasis, blood vessel formation and remoulding, tissue regeneration, immune surveillance and inflammation. The cells specifically migrate to destiny sites induced by the gradually varying concentration (gradient) of soluble signal factors and the ligands bound with the extracellular matrix in the body during a wound healing process. Therefore, regulation of the cell migration behaviours is of paramount importance in regenerative medicine. One important way is to create a microenvironment that mimics the in vivo cellular and tissue complexity by incorporating physical, chemical and biological signal gradients into engineered biomaterials. In this review, the gradients existing in vivo and their influences on cell migration are briefly described. Recent developments in the fabrication of gradient biomaterials for controlling cellular behaviours, especially the cell migration, are summarized, highlighting the importance of the intrinsic driving mechanism for tissue regeneration and the design principle of complicated and advanced tissue regenerative materials. The potential uses of the gradient biomaterials in regenerative medicine are introduced. The current and future trends in gradient biomaterials and programmed cell migration in terms of the long-term goals of tissue regeneration are prospected.

Competitive Adsorption of Three Human Plasma Proteins onto Sulfhydryl-to-sulfonate Gradient Surfaces

Competitive adsorption of three human plasma proteins: albumin (HSA), fibrinogen (Fgn), and immunoglobulin G (IgG) from their ternary solution mixtures onto a sulfhydryl-to-sulfonate gradient surface was investigated using spatially-resolved total internal reflection fluorescence (TIRF) and autoradiography. The concentration of each protein in the ternary solution mixture was kept at an equivalent of 1/100 of its physiological concentration in blood plasma. The three proteins displayed different adsorption and desorption characteristics. Each protein adsorbed less to the sulfonate region than to the sulfhydryl region of the gradient. The adsorption-desorption kinetics revealed large differences in the adsorption and desorption rates of three proteins. By fitting the experimental data to a simple model of competitive protein adsorption, the affinity of each protein to the surface at the gradient center position was ranked as: Fgn > HSA ≫ IgG. Competitive exchange of adsorbed proteins was related to the magnitude of desorption rate constants. Such competitive adsorption of the three major human plasma proteins illustrates the complex dynamics of blood proteins - biomaterials interactions.