Yonghui Ding

Insights into the Aggregation/Deposition and Structure of a Polydopamine Film

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.

Silver/hydroxyapatite composite coatings on porous titanium surfaces by sol-gel method.

Hydroxyapatite (HA) coatings loaded with nanosilver particles is an attractive method to impart the HA coating with antibacterial properties. Producing Ag/HA coatings on porous Ti substrates have been an arduous job since commonly used line-of-sight techniques are not able to deposit uniform coatings on the inner pore surfaces of the porous Ti. In this study, porous Ti scaffolds with high porosity and interconnected structures were prepared by polymer impregnating method. A sol-gel process was used to produce uniform Ag/HA composite coatings on the surfaces of porous Ti substrates. Ca(NO(3) )(2) ·4H(2) O and P(2) O(5) in an ethyl alcohol based system was selected to prepare the sol, which ensured the homogeneous distribution of Ag in the sol. The characterization revealed that silver particles uniformly distributed in the coatings without agglomeration. High antibacterial ratio (>95%), against E. coli and S. albus was expressed by the silver-containing coatings (Ag/HA 0.8 and 1.6 wt %). The biocompatibility of the Ag/HA 0.8 surfaces was as good as that of pure HA surface, as revealed by culturing osteoblasts on them. The results indicated that Ag/HA 0.8 had the good balance between the biocompatibility and antibacterial properties of the coatings.

Nano-Ag-loaded hydroxyapatite coatings on titanium surfaces by electrochemical deposition

Hydroxyapatite (HA) coatings on titanium (Ti) substrates have attracted much attention owing to the combination of good mechanical properties of Ti and superior biocompatibility of HA. Incorporating silver (Ag) into HA coatings is an effective method to impart the coatings with antibacterial properties. However, the uniform distribution of Ag is still a challenge and Ag particles in the coatings are easy to agglomerate, which in turn affects the applications of the coatings. In this study, we employed pulsed electrochemical deposition to co-deposit HA and Ag simultaneously, which realized the uniform distribution of Ag particles in the coatings. This method was based on the use of a well-designed electrolyte containing Ag ions, calcium ions and l-cysteine, in which cysteine acted as the coordination agent to stabilize Ag ions. The antibacterial and cell culture tests were used to evaluate the antibacterial properties and biocompatibility of HA/Ag composite coatings, respectively. The results indicated the as-prepared coatings had good antibacterial properties and biocompatibility. However, an appropriate silver content should be chosen to balance the biocompatibility and antibacterial properties. Heat treatments promoted the adhesive strength and enhanced the biocompatibility without sacrificing the antibacterial properties of the HA/Ag coatings. In summary, this study provided an alternative method to prepare bioactive surfaces with bactericidal ability for biomedical devices.

A biocompatible and functional adhesive amine-rich coating based on dopamine polymerization

Amine groups physiologically play an important role in regulating the growth behavior of cells and they have technological advantages for the conjugation of biomolecules. In this work, we present a method to deposit a copolymerized coating of dopamine and hexamethylendiamine (HD) (PDAM/HD) rich in amine groups onto a target substrate. This method only consists of a simple dip-coating step of the substrate in an aqueous solution consisting of dopamine and HD. Using the technique of PDAM/HD coating, a high density of amine groups of about 30 nmol cm-2 was obtained on the target substrate surface. The PDAM/HD coating showed a high cross-linking degree that is robust enough to resist hydrolysis and swelling. As a vascular stent coating, the PDAM/HD presented good adhesion strength to the substrate and resistance to the deformation behavior of compression and expansion of a stent. Meanwhile, the PDAM/HD coating exhibited good biocompatibility and attenuated the tissue response compared with 316L stainless steel (SS). The primary amine groups of the PDAM/HD coating could be used to effectively immobilize biomolecules containing carboxylic groups such as heparin. These data suggested the promising potential of this PDAM/HD coating for application in the surface modification of biomedical devices.

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

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.

Infrared spectroscopic characterization of carbonated apatite: A combined experimental and computational study

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.

Directing Vascular Cell Selectivity and Hemocompatibility on Patterned Platforms Featuring Variable Topographic Geometry and Size

It is great challenge to generate multifunctionality of vascular grafts and stents to enable vascular cell selectivity and improve hemocompatibility. Micro/nanopatterning of vascular implant surfaces for such multifunctionality is a direction to be explored. We developed a novel patterned platform featuring two typical geometries (groove and pillar) and six pattern sizes (0.5-50 μm) in a single substrate to evaluate the response of vascular cells and platelets. Our results indicate that targeted multifunctionality can be indeed instructed by rationally designed surface topography. The pillars nonselectively inhibited the growth of endothelial and smooth muscle cells. By contrast, the grooves displayed selective effects: in a size-dependent manner, the grooves enhanced endothelialization but inhibited the growth of smooth muscle cells. Moreover, our studies suggest that topographic cues can affect response of vascular cells by regulating focal adhesion and stress fiber development, which define cytoskeleton organization and cell shape. Notably, both the grooves and the pillars at 1 μm size drastically reduced platelet adhesion and activation. Taken together, these findings suggest that the topographic pattern featuring 1 μm grooves may be the optimal design of surface multifunctionality that favors vascular cell selectivity and improves hemocompatibility.

Bacterial responses to periodic micropillar array

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 (SiPA ) with nine different feature sizes were fabricated on silicon substrates with photolithography and dry etching methods. The SiPA 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.

Control of bacterial adhesion and growth on honeycomb-like patterned surfaces

It is a great challenge to construct a persistent bacteria-resistant surface even though it has been demonstrated that several surface features might be used to control bacterial behavior, including surface topography. In this study, we develop micro-scale honeycomb-like patterns of different sizes (0.5-10 μm) as well as a flat area as the control on a single platform to evaluate the bacterial adhesion and growth. Bacteria strains, Escherichia coli and Staphylococcus aureus with two distinct shapes (rod and sphere) are cultured on the platforms, with the patterned surface-up and surface-down in the culture medium. The results demonstrate that the 1 μm patterns remarkably reduce bacterial adhesion and growth while suppressing bacterial colonization when compared to the flat surface. The selective adhesion of the bacterial cells on the patterns reveals that the bacterial adhesion is cooperatively mediated by maximizing the cell-substrate contact area and minimizing the cell deformation, from a thermodynamic point of view. Moreover, study of bacterial behaviors on the surface-up vs. surface-down samples shows that gravity does not apparently affect the spatial distribution of the adherent cells although it indeed facilitates bacterial adhesion. Furthermore, the experimental results suggest that two major factors, i.e. the availability of energetically favorable adhesion sites and the physical confinements, contribute to the anti-bacterial nature of the honeycomb-like patterns.

Hydrothermal growth of biomimetic carbonated apatite nanoparticles with tunable size, morphology and ultrastructure

Biomimetic carbonated apatite (CAp) nanoparticles with tunable size, morphology and ultrastructure were synthesized via a facile hydrothermal route. By combining the results of FTIR, XRD, ICP, TGA, TEM and HRTEM, a complete chemical, morphological and structural characterization of the nanoparticles was performed. Based on a systematic examination of the effect of the fundamental factors including pH, carbonate concentration, temperature, and reaction time that control the synthesis of CAp nanocrystals, the nucleation and growth mechanisms were proposed. The results show that CAp nanocrystals can only be synthesized in basic conditions and exhibit a distinct morphology from that of pure hydroxyapatite. Increasing the reaction temperature and time will improve the crystallinity. The resulting CAp nanocrystals have three typical facets: (001), (010) and (10). By careful manipulation of the synthesis parameters, CAp nanocrystals mimetic to biological apatite in size and morphology can be obtained. Experimental evidence is given to support the mechanism that the amorphous calcium phosphate phase is a precursor phase that gradually transforms into the mature crystalline apatite mineral. The systematic approach presented in this study provides a helpful guide for the hydrothermal growth of quality CAp nanocrystals and significant implications for understanding the mechanism of biomineralization.