Yang Leng

Surface-adherent polydopamine (PDA) films as multifunctional coatings can be easily deposited onto a wide range of materials through dopamine self-polymerization. However, a lack of in-depth understanding of PDA aggregation and deposition processes and definite structure elucidation of PDA make it challenging to tailor the surface characteristic and functionality of the PDA films. Herein, we demonstrate that the surface characteristics of the PDA films can be readily tuned by controlling the competitive interplay between PDA aggregation in solution and deposition on the substrate. Moreover, a structural investigation of the PDA films using analytical tools such as X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) allows us to propose a new structure model for the PDA building block. The (DHI)₂/PCA trimer complex, which consists of two 5,6-dihydroxyindole (DHI) units and one pyrrolecarboxylic acid (PCA) moiety, is definitely identified as a primary building block of PDA, and its formation is steered by covalent interactions in the initial stages of polymerization. In latter stages, the (DHI)₂/PCA trimer complexes are further linked primarily through noncovalent interactions to build up the supramolecular structure of PDA. This study provides new insights into the mechanisms of PDA buildup.

A mussel-inspired surface functionalization of the polydopamine (PDA) coating has been demonstrated to be a promising strategy to ensure the biocompatibility of various biomaterials. To explore the multifunctionality of the PDA coating for vascular stents and elucidate the mechanisms by which the PDA coating modulates vascular cell behavior, this study examined the protein adsorption, the responses of endothelial cells (ECs) and smooth muscle cells (SMCs), and platelet adhesion to various PDA-coated surfaces synthesized at varied initial dopamine concentrations. Our results indicate that various PDA coatings present distinct and varied functionalities. The quinone group on the PDA coating induces a substantially higher amount of protein adsorption, which subsequently plays a key role in promoting EC attachment and proliferation by regulating their focal adhesion and stress fiber formation. Meanwhile, the reactive phenolic hydroxyl group on the PDA coating potently inhibits SMC proliferation. In addition, the quinone-regulated fibrinogen adsorption to the PDA coating may increase platelet adhesion. Notably, the PDA coating synthesized at an initial dopamine concentration of 1.0 g L⁻¹ shows the most favorable vascular cell selectivity. These findings shed light on the relationships between surface characteristics, protein adsorption, vascular cell behavior, and platelet adhesion of the PDA coating, which may guide better design of PDA application in vascular stents.

For a basic understanding and potential biomedical applications of surface topographical effects on bacterial responses, this study focuses on not only the bacterial retention but also the bacterial growth, proliferation, and viability that are significant post-retentive behaviors playing critical roles in infections of medical implants. Specifically, periodic micropillar arrays (Si_PA) with nine different feature sizes were fabricated on silicon substrates with photolithography and dry etching methods. The Si_PA was cultured with Staphylococcus aureus or Escherichia coli for different periods to investigate the bacterial retention, growth and proliferation behavior on a patterned surface. The experimental results show that a significant reduction of bacterial retention, growth, and proliferation can be achieved when the pillar size is reduced to the submicrometer level. However, micropillars have no obvious influence on the viability of the bacteria within 24 h. On the basis of the bacterial experiment results, it is inferred that the topographical effects may have resulted from bacterial confinement by micropillars, either limiting the attachment area for individual bacterium or trapping a bacterium between pillars. Furthermore, the extended Derjaguin–Landau–Verwey–Overbeek theoretical analysis indicates the effects might have come from the topographic induced surface property changes, mainly hydrophobicity, which is represented by the changes in the interaction free energy of Lifshitz–van der Waals among different periodic micropillar arrays. This study could help to deepen the understanding about the surface topographical effects on bacterial responses and may provide a guidance for the future medical implant surface design to decrease the infection risk by avoiding the surface topography which could attract more bacteria. © 2014 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 103A: 384–396, 2015.

A combined experimental and computational approach was employed to investigate the feasibility and effectiveness of characterizing carbonated apatite (CAp) by infrared (IR) spectroscopy. First, an experimental comparative study was conducted to identify characteristic IR vibrational bands of carbonate substitution in the apatite lattice. The IR spectra of pure hydroxyapatite (HA), carbonate adsorbed on the HA surface, a physical mixture of HA and sodium carbonate monohydrate, a physical mixture of HA and calcite, synthetic CAps prepared using three methods (precipitation method, hydrothermal route, and solid‐gas reaction at high temperature) and biological apatites (human enamel, human cortical bone, and two animal bones) were compared. Then, the IR vibrational bands of carbonate in CAp were calculated with density functional theory. The experimental study identified characteristic IR bands of carbonate that cannot be generated from surface adsorption or physical mixtures and the results show that the bands at ∼880, 1413, and 1450 cm⁻¹ should not be used as characteristic bands of CAp since they could result from carbonate adsorbed on the apatite crystals surface or present as a separate phase. The combined experimental and computational study reveals that the carbonate v₃ bands at ∼1546 and 1465 cm⁻¹ are, respectively, the IR signature bands for type A CAp and type B CAp. © 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 102A: 496–505, 2014.

This study systematically investigated the effects of microtopographic patterns of titanium oxide on platelet adhesion and activation in order to reveal the mechanisms of interactions between platelet and surface topography. Periodic arrays of groove and pillar patterns with dimensions ranging from submicron to several micrometers were fabricated by photolithography and deep reactive‐ion etching on silicon substrates, followed by the sputter deposition of titanium oxide (TiO₂). Platelet adhesion and activation on TiO₂ patterned surfaces were evaluated by lactate dehydrogenase (LDH) and GMP‐140 assays, respectively. The morphology of adherent platelets was examined by scanning electron microscope (SEM). The results showed that the microtopographic patterns were able to effectively manipulate the platelet response by varying geometry and size of patterns. A groove pattern resulted in much higher levels of platelet adhesion and activation than a pillar pattern. The study revealed that a difference in pattern size led to two distinctive modes of platelet adhesion: the “bridging” mode in which platelets can bridge over spacing between adjacent patterns when spacing size is smaller than 1.5 μm; and the “full‐contact” mode in which platelets cannot bridge but fully contact the entire surface when spacing size is larger than 3 μm. Our analysis indicates good correlations between platelet behavior and hydrophobicity/wetting anisotropy in “bridging” mode, and effective surface contact area in “full‐contact” mode. © 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 2013:101A:622–632.

The effects of protein on the nucleation and growth rate of calcium phosphate precipitation, including hydroxyapatite (HA) and octacalcium phosphate (OCP) precipitation, in simulated body fluid were analyzed based on the classical nucleation and diffusion-controlled growth theory. To obtain quantitative results for analysis, a model system of bovine serum albumin and lysozyme added to revised simulated body fluid was studied. The parameters, such as the concentration of protein binding to calcium and phosphorous ions and the viscosity of the revised simulated body fluid with protein additions, were experimentally measured to assist quantitative analysis. The analysis focused on the effects of protein on calcium phosphate nucleation including changes in the ion concentration, interfacial energies and pH values of the simulated body fluid system. The protein effects on calcium phosphate growth were also analyzed based on a diffusion-controlled growth model. The analysis results indicate that proteins present in simulated body fluid act to increase the free energy of nucleation and reduce the nucleation rates by reducing the effective level of supersaturation in simulated body fluid. The proteins also affect calcium phosphate nucleation by changing the interfacial energies in simulated body fluid. However, the analysis indicates that the effects of protein on the interfacial energies could be to either promote or retard the nucleation. The protein effects on the nucleation induced by altering the pH value of the simulated body fluid can be neglected. The analysis also indicates that protein addition in revised simulated body fluid does not change the basic thermodynamic nature of HA and OCP formation; that is, HA precipitation is more thermodynamically favourable than OCP while OCP precipitation is more kinetically favourable than HA. Proteins presenting in simulated body fluid always exhibit an inhibitory effect on the growth rate of calcium phosphate precipitates because proteins increase the diffusion resistance of calcium and phosphate ions by increasing the revised simulated body fluid viscosity.

This study numerically studies absorption of human serum albumin (HSA) and basic protein lysozyme (LSZ) on crystallographic planes of octacalcium phosphate (OCP), an essential bioactive calcium phosphate. The molecular simulations include constructing atomic structure of OCP crystallographic planes and representative segments of HSA and LSZ with three different initiate orientations respect to OCP planes. The simulation reveals the dynamic process of the protein absorption. The absorption behavior of proteins is quantified by the interaction energy between proteins and OCP planes and the strain energy of proteins in absorption. The results show that absorption interaction energy of basic LSZ is higher than that of acidic HSA, which indicates that LSZ is more favorable to adsorb onto OCP surface than HSA. The interaction energies change with the OCP crystallographic planes, the trend of changes for both proteins are similar, that is OCP (001) > OCP (111) > OCP (110) > OCP (100), which is corrected with surface energy variation of crystallographic planes. The strain energy strongly depends on the orientations of the proteins before absorption, but weakly depends on crystallographic planes. The simulation results provide useful significant information for predicting/designing interface between bioceramic materials and organic tissues as well as for understanding the mechanism of the osteoinductivity at an atomic level.

Percutaneous orthopedic and dental implants require not only good adhesion with bone but also the ability to attach and form seals with connective tissues and the skin. To solve the skin‐seal problem of such implants, an electrochemical deposition method was used to modify the surfaces of metallic implants to improve their antibacterial ability and skin seals around them. A dense and uniform fluoridated calcium phosphate coating with a thickness of about 200 nm was deposited on an acid‐etched pure titanium substrate by controlling the current density and reaction duration of the electrochemical process. The as‐deposited amorphous fluoridated calcium phosphate transformed to fluoridated hydroxyapatite (FHA) after heat treatment at 600°C in a water vapor environment for 3 h. Both single crystal diffraction patterns and high‐resolution transmission electron microscope (HRTEM) images confirmed the phase of the fluoridated calcium phosphate after the heat treatment. The antibacterial activities of FHA coatings were tested against Staphylococcus aureus (S. aureus), Escherichia coli (E. coli), and Porphyromonas gingivalis (P. gingivalis) with the film attachment method. The antibacterial activity of FHA coating is much higher than that of pure hydroxyapatite (HA) coating and acid‐etched pure titanium surface. The promising features of FHA coating make it suitable for orthopedic and dental applications. © 2010 Wiley Periodicals, Inc. J Biomed Mater Res Part A, 2010.