Protein interactions with surfaces: cellular responses, complement activation, and newer methods

Cell Responses to Calcium- and Protein-Conditioned Titanium: An In Vitro Study

Dental implants have become the leading choice for patients who lose teeth; however, dental implantation is challenged by peri-implant infections. Here, calcium-doped titanium was fabricated by the combinational use of thermal evaporation and electron beam evaporation in a vacuum; then, the material was immersed in a calcium-free phosphate-buffered saline solution containing human plasma fibrinogen and incubated at 37 °C for 1 h, creating calcium- and protein-conditioned titanium. The titanium contained 12.8 ± 1.8 at.% of calcium, which made the material more hydrophilic. Calcium release by the material during protein conditioning was able to change the conformation of the adsorbed fibrinogen, which acted against the colonization of peri-implantitis-associated pathogens (Streptococcus mutans, UA 159, and Porphyromonas gingivalis, ATCC 33277), while supporting the adhesion and growth of human gingival fibroblasts (hGFs). The present study confirms that the combination of calcium-doping and fibrinogen-conditioning is a promising pathway to meeting the clinical demand for suppressing peri-implantitis.

The antimicrobial effect of calcium-doped titanium is activated by fibrinogen adsorption

Directly targeting bacterial cells is the present paradigm for designing antimicrobial biomaterial surfaces and minimizing device-associated infections (DAIs); however, such pathways may create problems in tissue integration because materials that are toxic to bacteria can also be harmful to mammalian cells. Herein, we report an unexpected antimicrobial effect of calcium-doped titanium, which itself has no apparent killing effect on the growth of pathogenic bacteria (Pseudomonas aeruginosa, Pa, ATCC 27853) while presenting strong inhibition efficiency on bacterial colonization after fibrinogen adsorption onto the material. Fine X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy analyses reported calcium-dependent shifts of the binding energy in nitrogen and oxygen involved groups and wavenumbers in the amide I and II bands of the adsorbent fibrinogen, demonstrating that locally delivered calcium can react with the carboxy-terminal regions of the Aα chains and influence their interaction with the N-termini of the Bβ chains in fibrinogen. These reactions facilitate the exposure of the antimicrobial motifs of the protein, indicating the reason for the surprising antimicrobial efficacy of calcium-doped titanium. Since protein adsorption is an immediate intrinsic step during the implantation surgery, this finding may shift the present paradigm on the design of implantable antibacterial biomaterial surfaces.

Surface Characterisation of Human Serum Albumin Layers on Activated Ti6Al4V

Adpsortion of protein layers on biomaterials plays an important role in the interactions between implants and the bio-environment. In this context, human serum albumin (HSA) layers have been deposited on modified Ti6Al4V surfaces at different ultraviolet (UV-C) irradiation times to observe possible changes in the adsorbed protein layer. Protein adsorption was done from solutions at concentraions lower than the serum protein concentration, to follow the surface modifications at the beginning of the albumin adhesion process. For this purpose, the surface of the protein-coated samples has been characterized by time of flight secondary ion mass spectrometry (ToF-SIMS), contact angle and zeta potential measurements. The results obtained show a reduction in the total surface tension and zeta potential of samples treated with UV-C light when coated with a protein layer. Furthermore, the UV-C light treatment applied to titanium alloy surfaces is able to modify the conformation, orientation and packing of the proteins arranged in the adsorbed layer. Low irradiation time generates an unstable surface with the lowest protein adsorption and the highest hydrophobic/hydrophilic protein ratio, indicating a possible denaturalization of the protein on these surfaces. However, surface changes are stabilized after 15 h or UV-C irradiation, favoring the protein adsorption through electrical interactions.

Influence of mineral coatings on fibroblast behaviour: The importance of coating formulation and experimental design

Mineral coatings manipulate surface properties such as roughness, porosity, wettability and surface energy. Properties that are known to determine cell behaviour. Therefore, mineral coatings can potentially be used to manipulate cell fate. This paper studies mineral-cell interactions through coatings in a stacked cell culture platform. Minerals were chosen according to their influence on Human Dermal Fibroblasts (HDFs): calcium carbonate, calcium sulphates, and kaolin. Mineral coatings were formulated with the additives latex, sorbitol, polyvinyl alcohol (PVOH) and TEMPO-oxidised cellulose nanofibrils (CNF-T). The coatings were placed as a bottom or top of the device, for a direct or indirect interaction with HDFs, respectively. Cells were seeded, in various densities, to the bottom of the device; and cell density and confluency were monitored in time. Overall, results show that the coating interaction is influenced at first by the cell seeding density. Scarce cell seeding density limits adaptability to the new environment, while an abundant one encourages confluency in time. In between those densities, coating formulation plays the next major role. Calcium carbonate promoted HDFs growth the most as expected, but the response to the rest of minerals depended on the coating additive. CNF-T encouraged proliferation even for kaolin, a mineral with long-term toxicity to HDFs, while PVOH induced a detrimental effect on HDF growth regardless of the mineral. At last, the placement of the coated layer provided insights on the contact-dependency of each response. This study highlights the importance of the experimental design, including coating formulation, when investigating cellular response to biomaterials.

Controlled biointerfaces with biomimetic phosphorus-containing polymers

Phosphorus is a ubiquitous and one of the most common elements found in living organisms. Almost all molecules containing phosphorus in our body exist as analogs of phosphate salts or phosphoesters. Their functions are versatile and important, being responsible for forming the genetic code, cell membrane, and mineral components of hard tissue. Several materials inspired from these phosphorus-containing biomolecules have been recently developed. These materials have shown unique properties at the biointerface, such as nonfouling ability, blood compatibility, lubricity, mineralization induction capability, and bone affinity. Several unfavorable events occur at the interface of materials and living organisms because most of these materials have not been designed while taking host responses into account. These unfavorable events are directly linked to reducing functions and shorten the usable periods of medical devices. Biomimetic phosphorus-containing polymers can improve the reliability of materials in biological systems. In addition, phosphorus-containing biomimetic polymers are useful not only for improving the biocompatibility of material surfaces but also for adding new functions due to the flexibility in molecular design. In this review, we describe the recent advances in the control of biointerfacial phenomena with phosphorus-containing polymers. We especially focus on zwitterioninc phosphorylcholine polymers and polyphosphoesters.

An Insight into the Structural Diversity and Clinical Applicability of Polyurethanes in Biomedicine

Due to their mechanical properties, ranging from flexible to hard materials, polyurethanes (PUs) have been widely used in many industrial and biomedical applications. PUs' characteristics, along with their biocompatibility, make them successful biomaterials for short and medium-duration applications. The morphology of PUs includes two structural phases: hard and soft segments. Their high mechanical resistance featuresare determined by the hard segment, while the elastomeric behaviour is established by the soft segment. The most important biomedical applications of PUs include antibacterial surfaces and catheters, blood oxygenators, dialysis devices, stents, cardiac valves, vascular prostheses, bioadhesives/surgical dressings/pressure-sensitive adhesives, drug delivery systems, tissue engineering scaffolds and electrospinning, nerve generation, pacemaker lead insulation and coatings for breast implants. The diversity of polyurethane properties, due to the ease of bulk and surface modification, plays a vital role in their applications.

Tuning the biomimetic behavior of scaffolds for regenerative medicine through surface modifications

Tissue engineering and regenerative medicine rely extensively on biomaterial scaffolds to support cell adhesion, proliferation, and differentiation physically and chemically in vitro and in vivo. Changes to the surface characteristics of the scaffolds have the greatest impact on cell response. Here, we discuss five dominant surface modification approaches used to biomimetically improve the most common scaffolds for tissue engineering, those based on aliphatic polyesters. Scaffolds of aliphatic polyesters such as poly(l-lactic acid), poly(l-lactic-co-glycolic acid), and poly(ε-caprolactone) are often used in tissue engineering because they provide desirable, tunable properties such as ease of manufacturing, good mechanical properties, and nontoxic degradation products. However, cell-surface interactions necessary for tissue engineering are limited on these materials by their smooth postfabrication surfaces, hydrophobicity, and lack of recognizable biochemical binding sites. The surface modification techniques that have been developed for synthetic polymer scaffolds reduce initial barriers to cell adhesion, proliferation, and differentiation. Topographical modification, protein adsorption, mineral coating, functional group incorporation, and biomacromolecule immobilization each contribute through varying mechanisms to improving cell interactions with aliphatic polyester scaffolds. Furthermore, rational combination of methods from these categories can provide nuanced, specific environments for targeted tissue development.

POLYMERIC BIOMATERIALS FOR SCAFFOLD-BASED BONE REGENERATIVE ENGINEERING

Reconstruction of large bone defects resulting from trauma, neoplasm, or infection is a challenging problem in reconstructive surgery. The need for bone grafting has been increasing steadily partly because of our enhanced capability to salvage limbs after major bone loss. Engineered bone graft substitutes can have advantages such as lack of antigenicity, high availability, and varying properties depending on the applications chosen for use. These favorable attributes have contributed to the rise of scaffold-based polymeric tissue regeneration. Critical components in the scaffold-based polymeric regenerative engineering approach often include 1. The existence of biodegradable polymeric porous structures with properties selected to promote tissue regeneration and while providing appropriate mechanical support during tissue regeneration. 2. Cellular populations that can influence and enhance regeneration. 3. The use of growth and morphogenetic factors which can influence cellular migration, differentiation and tissue regeneration in vivo. Biodegradable polymers constitute an attractive class of biomaterials for the development of scaffolds due to their flexibility in chemistry and their ability to produce biocompatible degradation products. This paper presents an overview of polymeric scaffold-based bone tissue regeneration and reviews approaches as well as the particular roles of biodegradable polymers currently in use.

Blood-Compatible Surfaces with Phosphorylcholine-Based Polymers for Cardiovascular Medical Devices

For the acquisition of blood-compatible materials, various hydrophilic polymers for surface modification have been examined. Among them, polymers with a representative phospholipid polar group, the phosphorylcholine (PC) group, are a successful example. These polymers were designed from inspiration of the cell membrane surface and provide protein adsorption resistance even following contact with plasma. This important property is based on the unique hydration state of water molecules surrounding hydrated polymer; in other words, water molecules weakly interact with the polymers and maintain their favorable cluster structure through hydrogen bonding. These polymers are not only hydrophilic, but also electrically neutral, important characteristics which make hydrogen bonding with water molecules less likely to occur and avoid hydrophobic interactions. Phosphorylcholine groups and other zwitterionic structures are significant as hydrophilic functional groups meeting these important requirements. In this review, blood compatibility of a polymer having a PC group is introduced in relation to its hydration structure, followed by a description of the applications of this polymer to cardiovascular medical devices.

Biomaterials: Foreign Bodies or Tuners for the Immune Response?

The perspectives of regenerative medicine are still severely hampered by the host response to biomaterial implantation, despite the robustness of technologies that hold the promise to recover the functionality of damaged organs and tissues. In this scenario, the cellular and molecular events that decide on implant success and tissue regeneration are played at the interface between the foreign body and the host inflammation, determined by innate and adaptive immune responses. To avoid adverse events, rather than the use of inert scaffolds, current state of the art points to the use of immunomodulatory biomaterials and their knowledge-based use to reduce neutrophil activation, and optimize M1 to M2 macrophage polarization, Th1 to Th2 lymphocyte switch, and Treg induction. Despite the fact that the field is still evolving and much remains to be accomplished, recent research breakthroughs have provided a broader insight on the correct choice of biomaterial physicochemical modifications to tune the reaction of the host immune system to implanted biomaterial and to favor integration and healing.

Biological Response to Carbon-Family Nanomaterials: Interactions at the Nano-Bio Interface

During the last few decades, several studies have suggested that carbon-based nanomaterials, owing to their unique properties, could act as promising candidates in biomedical engineering application. Wide-ranging research efforts have investigated the cellular and molecular responses to carbon-based nanomaterials at the nano-bio interfaces. In addition, a number of surface functionalization strategies have been introduced to improve their safety profile in the biological environment. The present review discusses the general principles of immunological responses to nanomaterials. Then, it explains essential physico-chemical properties of carbon-familynanomaterials, including carbon nanotubes (CNTs), graphene, fullerene, carbon quantum dots (CDs), diamond-like carbon (DLC), and mesoporous carbon biomaterials (MCNs), which significantly affect the immunological cellular and molecular responses at the nano-bio interface. The discussions also briefly highlight the recent studies that critically investigated the cellular and molecular responses to various carbon-based nanomaterials. It is expected that the most recent perspective strategies for improving the biological responses to carbon-based nanomaterials can revolutionize their functions in emerging biological applications.

Influence of nanoporous titanium niobium alloy surfaces produced via hydrogen peroxide oxidative etching on the osteogenic differentiation of human mesenchymal stromal cells

Titanium niobium alloys exhibit a lower stiffness compared to Ti6Al4V, the 'gold standard' for load-bearing bone implants. Thus, the critical mismatch in stiffness between the implant and adjacent bone tissue could be addressed with TiNb alloys and thereby reduce stress shielding, which can result in bone resorption and subsequent implant loosening; however, the cellular response on the specific material is crucial for sufficient osseointegration. We therefore hypothesize that the response of human mesenchymal stromal cells (hMSC) and osteoblast-like cells on Ti45Nb surfaces can be improved by a novel nanoporous surface structure. For this purpose, an etching technique using hydrogen peroxide electrolyte solution was applied to Ti45Nb. The treated surfaces were characterized using SEM, LSM, AFM, nanoindentation, and contact angle measurements. Cell culture experiments using hMCS and MG-63 were conducted. The H₂O₂ treatment resulted in surface nanopores, an increase in surface wettability and a reduction in surface hardness. The proliferation of MG-63 was enhanced on TiNb45 compared to Ti6Al4V. MG-63 focal adhesion complexes were detected on all Ti45Nb surfaces, whereas the nanostructures notably increased the cell area and decreased cell solidity, indicating stimulated cell spreading and pseudopodia formation. Alizarin red stainings indicated that the nanoporous surfaces stimulated the osteogenic differentiation of hMSC. It can be concluded that the proposed surface treatment could potentially help to stimulate the osseointegration behaviour of the advantageous low stiff Ti45Nb alloy.

Understanding interactions between biomaterials and biological systems using proteomics

The role that biomaterials play in the clinical treatment of damaged organs and tissues is changing. While biomaterials used in permanent medical devices were required to passively take over the function of a damaged tissue in the long term, current biomaterials are expected to trigger and harness the self-regenerative potential of the body in situ and then to degrade, the foundation of regenerative medicine. To meet these different requirements, it is imperative to fully understand the interactions biomaterials have with biological systems, in space and in time. This knowledge will lead to a better understanding of the regenerative capabilities of biomaterials aiding their design with improved functionalities (e.g. biocompatibility, bioactivity). Proteins play a pivotal role in the interaction between biomaterials and cells or tissues. Protein adsorption on the material surface is the very first event of this interaction, which is determinant for the subsequent processes of cell growth, differentiation, and extracellular matrix formation. Against this background, the aim of the current review is to provide insight in the current knowledge of the role of proteins in cell-biomaterial and tissue-biomaterial interactions. In particular, the focus is on proteomics studies, mainly using mass spectrometry, and the knowledge they have generated on protein adsorption of biomaterials, protein production by cells cultured on materials, safety and efficacy of new materials based on nanoparticles and the analysis of extracellular matrices and extracellular matrix-derived products. In the outlook, the potential and limitations of this approach are discussed and mass spectrometry imaging is presented as a powerful technique that complements existing mass spectrometry techniques by providing spatial molecular information about the material-biological system interactions.

VV extracorporeal life support for the Third Millennium: will we need anticoagulation?

Since the late 1600's medicine and science have entertained the idea of extracorporeal circulation. With this technology to allow for cardiac and pulmonary support came the development of anticoagulation. Although this advanced the technology and capabilities of extracorporeal life support, it was not without complications and risks. The most common complications in extracorporeal life support (ECLS) present day are related to hemorrhage and thrombus due to the need for systemic anticoagulation and the challenges associated with it. This review focuses on present day techniques for anticoagulation for ECLS and what future surface modifications may do to obviate the use of systemic anticoagulation entirely.

Islet encapsulation

Review of emerging advances and persisting challenges in the engineering and translation of islet encapsulation technologies.

The pathology of the foreign body reaction against biomaterials

The healing process after implantation of biomaterials involves the interaction of many contributing factors. Besides their in vivo functionality, biomaterials also require characteristics that allow their integration into the designated tissue without eliciting an overshooting foreign body reaction (FBR). The targeted design of biomaterials with these features, thus, needs understanding of the molecular mechanisms of the FBR. Much effort has been put into research on the interaction of engineered materials and the host tissue. This elucidated many aspects of the five FBR phases, that is protein adsorption, acute inflammation, chronic inflammation, foreign body giant cell formation, and fibrous capsule formation. However, in practice, it is still difficult to predict the response against a newly designed biomaterial purely based on the knowledge of its physical-chemical surface features. This insufficient knowledge leads to a high number of factors potentially influencing the FBR, which have to be analyzed in complex animal experiments including appropriate data-based sample sizes. This review is focused on the current knowledge on the general mechanisms of the FBR against biomaterials and the influence of biomaterial surface topography and chemical and physical features on the quality and quantity of the reaction. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 927-940, 2017.

Understanding Polymer-Cell Attachment

The development of polymeric materials with cell adhesion abilities requires an understanding of cell-surface interactions which vary with cell type. To investigate the correlation between cell attachment and the nature of the polymer, a series of random and block copolymers composed of 2-(dimethylamino)ethyl acrylate and ethyl acrylate are synthesized through single electron transfer living radical polymerization. The polymers are synthesized with highly defined and controlled monomer compositions and exhibited narrow polydispersity indices. These polymers are examined for their performance in the attachment and growth of HeLa and HEK cells, with attachment successfully modeled on monomer composition and polymer chain length, with both cell lines found to preferentially attach to moderately hydrophobic functional materials. The understanding of the biological-material interactions assessed in this study will underpin further investigations of engineered polymer scaffolds with predictable cell binding performance.

Photo-assisted generation of phospholipid polymer substrates for regiospecific protein conjugation and control of cell adhesion

Novel photo-reactive phospholipid polymers were synthesized for use in the preparation of nonfouling surfaces with protein conjugation capacity. Poly[2-methacryloyloxyethyl phosphorylcholine (MPC)-ran-N-methacryloyl-(l)-tyrosinemethylester (MAT)] (P(MPC/MAT)) was synthesized by conventional radical polymerization, with the MAT units capable of being oxidized by 254nm UV irradiation. Because of this photo-oxidation, active species such as catechol and quinone were alternately generated in the copolymer. A silicon wafer was subjected to surface modification through spin coating of P(MPC/MAT) from an aqueous solution for use as a model substrate. The surface was then irradiated several times with UV light. The thickness of the polymer layers formed on the Si wafers was influenced by various parameters such as polymer concentration, UV irradiation time, and composition of the MAT units in P(MPC/MAT). Oxidized MAT units were advantageous not only for polymer adhesion to a solid surface but also for protein conjugation with the adhered polymers. The amount of protein immobilized on UV-irradiated P(MPC/MAT) was dependent on the composition of the MAT units in the polymer. Furthermore, it was confirmed that protein immobilization on the polymer occurred through the oxidized MAT units because the protein adsorption was significantly reduced upon blocking these units through pretreatment with glycine. Conjugation of regiospecific protein could also be achieved through the use of a photomask. In addition, nonspecific protein adsorption was reduced on the non-irradiated regions whose surface was covered with physisorbed P(MPC/MAT). Therefore, P(MPC/MAT) can be used in the preparation of nonfouling substrates, which enable micrometer-sized manipulation of proteins through photo-irradiation. Function of proteins immobilized on MPC copolymers was also confirmed by cell adhesion test. As such, photo-reactive MPC copolymers are suitable for performing controlled protein conjugation and preparing polymer-protein hybrid platforms for use in biomedical and diagnostic devices.

STATEMENT OF SIGNIFICANCE

Novel photo-reactive phospholipid polymers have been synthesized for immobilization on solid surfaces and regiospecific protein conjugation. Tyrosine residues embedded in 2-methacryloyloxyethyl phosphorylcholine (MPC) copolymers could be photo-oxidized, resulting in polymers able to form layers on a solid surface and conjugate with proteins. Moreover, nonspecific biofouling on the surface significantly reduced when the oxidized tyrosine units in the polymer layers were blocked. Upon UV irradiation through a photomask, the UV-exposed tyrosine units were selectively oxidized, forming the only specific regions in which protein conjugation could occur.

Protein selective adsorption properties of a polyethylene terephtalate artificial ligament grafted with poly(sodium styrene sulfonate) (polyNaSS): correlation with physicochemical parameters of proteins

Immediately after surgical placement of biomaterials, a first step consists in the adsorption of proteins from the biological environment on the artificial surfaces. Because the composition of the adsorbed protein layer modulates the cell response to the implanted material, researchers in the biomaterials field have focused on coating proteins or peptides onto surfaces to improve cell response and therefore the long-term compatibility of the implant. However, some materials used in tissue engineering, mainly synthetic polymers, are too hydrophobic to allow the optimal adsorption of proteins and have to be first submitted to physical or chemical treatments. In our laboratory, we have demonstrated that grafting of poly(sodium styrene sulfonate) (polyNaSS) onto biomaterials can strongly modulate the protein adsorption and the cellular response compared to unmodified surfaces. In this study, we used a liquid chromatography strategy coupled to proteomics to evaluate the adsorptive properties of a polyethylene terephtalate (PET) artificial ligament grafted with polyNaSS, and to identify and analyse proteins adsorbed on PET fibers. Results obtained with platelet rich plasma (PRP) proteins demonstrated that grafting significantly increases the protein adsorption of the PET and also selectively modulates the adsorption of proteins on PET fibers. Finally, regarding physicochemical parameters calculated from the amino acid sequence of identified proteins, we found that the aliphatic index is highly correlated with the selective adsorption of proteins onto the polyNaSS/PET surface. Therefore, the proteomic approach complemented with physicochemical property evaluation could provide a powerful tool for the elaboration of new biomaterials based on protein layer deposition.

Evaluation of the Effectiveness of Surfactants and Denaturants to Elute and Denature Adsorbed Protein on Different Surface Chemistries

The elution and/or denaturation of proteins from material surfaces by chemical excipients such as surfactants and denaturants is important for numerous applications including medical implant reprocessing, bioanalyses, and biodefense. The objective of this study was to develop and apply methods to quantitatively assess how surface chemistry and adsorption conditions influence the effectiveness of three commonly used surfactants (sodium dodecyl sulfate, n-octyl-β-d-glucoside, and 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate) and two denaturants (guanidium hydrochloride and urea) to elute protein (hen egg white lysozyme and bovine pancreatic ribonuclease A) from three different surface chemistries (silica glass, poly(methyl methacrylate), and high-density polyethylene). The structure and bioactivity of residual protein on the surface following elution were characterized using circular dichroism spectropolarimetry and enzyme assays to assess the extent of protein denaturation. Our results indicate that the denaturants were generally more effective than the surfactants in removing the adsorbed proteins from each type of surface. Also, the denaturing capacity of these excipients on the residual proteins on the surfaces was distinctly different from their influence on the proteins in solution and was unique for each of the adsorption conditions. Taken altogether, these results reveal that the effectiveness of surfactants and denaturants to elute and denature adsorbed protein is significantly influenced by surface chemistry and the conditions from which the protein was adsorbed. These results provide a basis for the selection, design, and further development of chemical agents for protein elution and surface decontamination.

Fibrinogen matrix deposited on the surface of biomaterials acts as a natural anti-adhesive coating

Adsorption of fibrinogen on the luminal surface of biomaterials is a critical early event during the interaction of blood with implanted vascular graft prostheses which determines their thrombogenicity. We have recently identified a nanoscale process by which fibrinogen modifies the adhesive properties of various surfaces for platelets and leukocytes. In particular, adsorption of fibrinogen at low density promotes cell adhesion while its adsorption at high density results in the formation of an extensible multilayer matrix, which dramatically reduces cell adhesion. It remains unknown whether deposition of fibrinogen on the surface of vascular graft materials produces this anti-adhesive effect. Using atomic force spectroscopy, single cell force spectroscopy, and standard adhesion assays with platelets and leukocytes, we have characterized the adhesive and physical properties of the contemporary biomaterials, before and after coating with fibrinogen. We found that uncoated PET, PTFE and ePTFE exhibited high adhesion forces developed between the AFM tip or cells and the surfaces. Adsorption of fibrinogen at the increasing concentrations progressively reduced adhesion forces, and at ≥2 μg/ml all surfaces were virtually nonadhesive. Standard adhesion assays performed with platelets and leukocytes confirmed this dependence. These results provide a better understanding of the molecular events underlying thrombogenicity of vascular grafts.

Innate Immunity and Biomaterials at the Nexus: Friends or Foes

Biomaterial implants are an established part of medical practice, encompassing a broad range of devices that widely differ in function and structural composition. However, one common property amongst biomaterials is the induction of the foreign body response: an acute sterile inflammatory reaction which overlaps with tissue vascularisation and remodelling and ultimately fibrotic encapsulation of the biomaterial to prevent further interaction with host tissue. Severity and clinical manifestation of the biomaterial-induced foreign body response are different for each biomaterial, with cases of incompatibility often associated with loss of function. However, unravelling the mechanisms that progress to the formation of the fibrotic capsule highlights the tightly intertwined nature of immunological responses to a seemingly noncanonical “antigen.” In this review, we detail the pathways associated with the foreign body response and describe possible mechanisms of immune involvement that can be targeted. We also discuss methods of modulating the immune response by altering the physiochemical surface properties of the biomaterial prior to implantation. Developments in these areas are reliant on reproducible and effective animal models and may allow a “combined” immunomodulatory approach of adapting surface properties of biomaterials, as well as treating key immune pathways to ultimately reduce the negative consequences of biomaterial implantation.

Molecular interaction forces generated during protein adsorption to well-defined polymer brush surfaces

The molecular interaction forces generated during the adsorption of proteins to surfaces were examined by the force-versus-distance (f-d) curve measurements of atomic force microscopy using probes modified with appropriate molecules. Various substrates with polymer brush layers bearing zwitterionic, cationic, anionic, and hydrophobic groups were systematically prepared by surface-initiated atom transfer radical polymerization. Surface interaction forces on these substrates were analyzed by the f-d curve measurements using probes with the same polymer brush layer as the substrate. Repulsive forces, which decreased depending on the ionic strength, were generated between cationic or anionic polyelectrolyte brush layers; these were considered to be electrostatic interaction forces. A strong adhesive force was detected between hydrophobic polymer brush layers during retraction; this corresponded to the hydrophobic interaction between two hydrophobic polymer layers. In contrast, no significant interaction forces were detected between zwitterionic polymer brush layers. Direct interaction forces between proteins and polymer brush layers were then quantitatively evaluated by the f-d curve measurements using protein-immobilized probes consisting of negatively charged albumin and positively charged lysozyme under physiological conditions. In addition, the amount of protein adsorbed on the polymer brush layer was quantified by surface plasmon resonance measurements. Relatively large amounts of protein adsorbed to the polyelectrolyte brush layers with opposite charges. It was considered that the detachment of the protein after contact with the polymer brush layer hardly occurred due to salt formation at the interface. Both proteins adsorbed significantly on the hydrophobic polymer brush layer, which was due to hydrophobic interactions at the interface. In contrast, the zwitterionic polymer brush layer exhibited no significant interaction force with proteins and suppressed protein adsorption. Taken together, our results suggest that to obtain the protein-repellent surfaces, the surface should not induce direct interaction forces with proteins after contact with them.

Ligand engagement on material surfaces is discriminated by cell mechanosensoring

Peptide or protein ligands can be used for molecular decoration to enhance the functionality of synthetic materials. However, some skepticism has arisen about the efficacy of such strategy in practical contexts since serum proteins largely adsorb. To address this issue, it is crucial to ascertain whether a chemically conjugated integrin-binding peptide is fully recognized by a cell even if partially covered by a physisorbed layer of serum protein; in more general terms, if competitive protein fragments physisorbed onto the surface are distinguishable from those chemically anchored to it. Here, we engraft an RGD peptide on poly-ε-caprolactone (PCL) surfaces and follow the dynamics of focal adhesion (FA) and cytoskeleton assembly at different times and culture conditions using a variety of analytical tools. Although the presence of serum protein covers the bioconjugated RGD significantly, after the first adhesion phase cells dig into the physisorbed layer and reach the submerged signal to establish a more stable adhesion structure (mature FAs). Although the spreading area index is not substantially affected by the presence of the RGD peptide, cells attached to chemically bound signals develop a stronger adhesive interaction with the materials and assemble a mechanically stable cytoskeleton. This demonstrates that cells are able to discriminate, via mechanosensoring, between adhesive motives belonging to physisorbed proteins and those firmly anchored on the material surface.

Extracellular matrix elasticity and topography: material-based cues that affect cell function via conserved mechanisms

Chemical, mechanical, and topographic extracellular matrix (ECM) cues have been extensively studied for their influence on cell behavior. These ECM cues alter cell adhesion, cell shape, and cell migration and activate signal transduction pathways to influence gene expression, proliferation, and differentiation. ECM elasticity and topography, in particular, have emerged as material properties of intense focus based on strong evidence these physical cues can partially dictate stem cell differentiation. Cells generate forces to pull on their adhesive contacts, and these tractional forces appear to be a common element of cells' responses to both elasticity and topography. This review focuses on recently published work that links ECM topography and mechanics and their influence on differentiation and other cell behaviors. We also highlight signaling pathways typically implicated in mechanotransduction that are (or may be) shared by cells subjected to topographic cues. Finally, we conclude with a brief discussion of the potential implications of these commonalities for cell based therapies and biomaterial design.

The influence of fluorocarbon chain and phosphorylcholine on the improvement of hemocompatibility: a comparative study in polyurethanes

The synergistic effect of a fluorocarbon chain and phosphorylcholine groups on the improvement of hemocompatibility in polyurethanes was investigated.

Zeta potential, contact angles, and AFM imaging of protein conformation adsorbed on hybrid nanocomposite surfaces

The sputtering deposition of gold (Au) and poly(tetrafluoroethylene) (PTFE) was used to prepare a nanocomposite hybrid thin film suitable for protein adsorption while maintaining the native conformation of the biological material. The monolithic PTFE and the nanocomposite PTFE/Au thin films, with Au content up to 1 at %, were co-deposited by r.f. magnetron sputtering using argon as a discharge gas and deposited onto 316L stainless steel substrates, the most commonly used steel in biomaterials. The deposited thin films, before and after bovine serum albumin (BSA) adsorption, were thoroughly characterized with special emphasis on the surface properties/characteristics by atomic force microscopy (AFM), zeta potential, and static and dynamic contact angle measurements, in order to assess the relationship between structure and conformational changes. The influence of a pre-adsorbed peptide (RGD) was also evaluated. The nanotopographic and chemical changes induced by the presence of gold in the nanocomposite thin films enable RGD bonding, which is critical for the maintenance of the BSA native conformation after adsorption.

Quantification of the influence of protein-protein interactions on adsorbed protein structure and bioactivity

While protein-surface interactions have been widely studied, relatively little is understood at this time regarding how protein-surface interaction effects are influenced by protein-protein interactions and how these effects combine with the internal stability of a protein to influence its adsorbed-state structure and bioactivity. The objectives of this study were to develop a method to study these combined effects under widely varying protein-protein interaction conditions using hen egg-white lysozyme (HEWL) adsorbed on silica glass, poly(methyl methacrylate), and polyethylene as our model systems. In order to vary protein-protein interaction effects over a wide range, HEWL was first adsorbed to each surface type under widely varying protein solution concentrations for 2h to saturate the surface, followed by immersion in pure buffer solution for 15h to equilibrate the adsorbed protein layers in the absence of additionally adsorbing protein. Periodic measurements were made at selected time points of the areal density of the adsorbed protein layer as an indicator of the level of protein-protein interaction effects within the layer, and these values were then correlated with measurements of the adsorbed protein's secondary structure and bioactivity. The results from these studies indicate that protein-protein interaction effects help stabilize the structure of HEWL adsorbed on silica glass, have little influence on the structural behavior of HEWL on HDPE, and actually serve to destabilize HEWL's structure on PMMA. The bioactivity of HEWL on silica glass and HDPE was found to decrease in direct proportion to the degree of adsorption-induce protein unfolding. A direct correlation between bioactivity and the conformational state of adsorbed HEWL was less apparent on PMMA, thus suggesting that other factors influenced HEWL's bioactivity on this surface, such as the accessibility of HEWL's bioactive site being blocked by neighboring proteins or the surface itself. The developed methods provide an effective means to characterize the influence of protein-protein interaction effects and provide new molecular-level insights into how protein-protein interaction effects combine with protein-surface interaction and internal protein stability effects to influence the structure and bioactivity of adsorbed protein.

Oxidation of tertiary amine-derivatized surfaces to control protein adhesion

Selective oxidation of ω-tertiary amine self-assembled thiol monolayers to tertiary amine N-oxides is shown to transform the adhesion of model proteins lysozyme and fibrinogen upon them. Efficient preparation of both secondary and tertiary linker amides as judged by X-ray photoelectron spectroscopy (XPS) and water droplet contact angle was achieved with an improved amide bond formation on gold quartz crystal microbalance (QCM) sensors using 2-(1H-7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyl hexafluorophosphate methanaminium uronium (HATU). Oxidation with hydrogen peroxide was similarly assessed, and adhesion of lysozyme and fibrinogen from phosphate buffered saline was then assayed by QCM and imaged by AFM. Tertiary amine-functionalized sensors adsorbed multilayers of aggregated lysozyme, whereas tertiary amine N-oxides and triethylene glycol-terminated monolayers are consistent with small protein aggregates. The surface containing a dimethylamine N-oxide headgroup and ethyl secondary amide linker showed the largest difference in adsorption of both proteins. Oxidation of tertiary amine decorated surfaces therefore holds the potential for selective deposition of proteins and cells through masking and other patterning techniques.

Facets of protein assembly on nanostructured titanium oxide surfaces

One key for the successful integration of implants into the human body is the control of protein adsorption by adjusting the surface properties at different length scales. This is particularly important for titanium oxide, one of the most common biomedical interfaces. As for titania (TiO(2)) the interface is largely defined by its crystal surface structure, it is crucial to understand how the surface crystallinity affects the structure, properties and function of protein layers mediating subsequent biological reactions. For rutile TiO(2) we demonstrate that the conformation and relative amount of human plasma fibrinogen (HPF) and the structure of adsorbed HPF layers depend on the crystal surface nanostructure by employing thermally etched multi-faceted TiO(2) surfaces. Thermal etching of polycrystalline TiO(2) facilitates a nanoscale crystal faceting and, thus, the creation of different surface nanostructures on a single specimen surface. Atomic force microscopy shows that HPF arranges into networks and thin globular layers on flat and irregular crystal grain surfaces, respectively. On a third, faceted category we observed an alternating conformation of HPF on neighboring facets. The bulk grain orientation obtained from electron backscatter diffraction and thermodynamic mechanisms of surface reconstruction during thermal etching suggest that the grain and facet surface-specific arrangement and relative amount of adsorbed proteins depend on the associated free crystal surface energy. The implications for potentially favorable TiO(2) crystal facets regarding the inflammatory response and hemostasis are discussed with a view to the advanced surface design of future implants.

Expanded applications, shifting paradigms and an improved understanding of host-biomaterial interactions

The conventional approach to biomaterial design and development typically focuses upon the mechanical and material properties with long-term objectives that include an inert host immune response and long-lasting mechanical and structural support. The emergence of and interest in tissue engineering and regenerative medicine have driven the development of novel cell-friendly biomaterials, materials with tailored degradation rates, materials with highly specific architectures and surfaces, and vehicles for delivery of bioactive molecules, among numerous other advancements. Each of these biomaterial developments supports specific strategies for tissue repair and reconstruction. These advancements in biomaterial form and function, combined with new knowledge of innate and acquired immune system biology, provide an impetus for re-examination of host-biomaterial interactions, including host-biomaterial interface events, spatial and temporal patterns of in vivo biomaterial remodeling, and related downstream functional outcomes. An examination of such issues is provided herein with a particular focus on macrophage polarization and its implications in tissue engineering and regenerative medicine.

Modelling human embryoid body cell adhesion to a combinatorial library of polymer surfaces

Designing materials to control biology is an intense focus of biomaterials and regenerative medicine research. Discovering and designing materials with appropriate biological compatibility or active control of cells and tissues is being increasingly undertaken using high throughput synthesis and assessment methods. We report a relatively simple but powerful machine-learning method of generating models that link microscopic or molecular properties of polymers or other materials to their biological effects. We illustrate the potential of these methods by developing the first robust, predictive, quantitative, and purely computational models of adhesion of human embryonic stem cell embryoid bodies (hEB) to the surfaces of a 496-member polymer micro array library.