A mouse model to evaluate the interface between skin and a percutaneous device

Cells resident to precision templated 40-µm pore scaffolds generate small extracellular vesicles that affect CD4+ T cell phenotypes through regulatory TLR4 signaling

Precision porous templated scaffolds (PTS) are a hydrogel construct of uniformly sized interconnected spherical pores that induce a pro-healing response (reducing the foreign body reaction, FBR) exclusively when the pores are 30-40µm in diameter. Our previous work demonstrated the necessity of Tregs in the maintenance of PTS pore size specific differences in CD4+ T cell phenotype. Work here characterizes the role of Tregs in the responses to implanted 40µm and 100µm PTS using WT and FoxP3+ cell (Treg) depleted mice. Proteomic analyses indicate that integrin signaling, monocytes/macrophages, cytoskeletal remodeling, inflammatory cues, and vesicule endocytosis may participate in Treg activation and the CD4+ T cell equilibrium modulated by PTS resident cell-derived small extracellular vesicles (sEVs). The role of MyD88-dependent and MyD88-independent TLR4 activation in PTS cell-derived sEV-to-T cell signaling is quantified by treating WT, TLR4ko, and MyD88ko splenic T cells with PTS cell-derived sEVs. STAT3 and mTOR are identified as mechanisms for further study for pore-size dependent PTS cell-derived sEV-to-T cell signaling. STATEMENT OF SIGNIFICANCE: Unique cell populations colonizing only within 40µm pore size PTS generate sEVs that resolve inflammation by modifying CD4+ T cell phenotypes through TLR4 signaling.

Plasma polymerized bio-interface directs fibronectin adsorption and functionalization to enhance "epithelial barrier structure" formation via FN-ITG β1-FAK-mTOR signaling cascade

BACKGROUND

Transepithelial medical devices are increasing utilized in clinical practices. However, the damage of continuous natural epithelial barrier has become a major risk factor for the failure of epithelium-penetrating implants. How to increase the "epithelial barrier structures" (focal adhesions, hemidesmosomes, etc.) becomes one key research aim in overcoming this difficulty. Directly targeting the in situ "epithelial barrier structures" related proteins (such as fibronectin) absorption and functionalization can be a promising way to enhance interface-epithelial integration.

METHODS

Herein, we fabricated three plasma polymerized bio-interfaces possessing controllable surface chemistry. Their capacity to adsorb and functionalize fibronectin (FN) from serum protein was compared by Liquid Chromatography-Tandem Mass Spectrometry. The underlying mechanisms were revealed by molecular dynamics simulation. The response of gingival epithelial cells regarding the formation of epithelial barrier structures was tested.

RESULTS

Plasma polymerized surfaces successfully directed distinguished protein adsorption profiles from serum protein pool, in which plasma polymerized allylamine (ppAA) surface favored adsorbing adhesion related proteins and could promote FN absorption and functionalization via electrostatic interactions and hydrogen bonds, thus subsequently activating the ITG β1-FAK-mTOR signaling and promoting gingival epithelial cells adhesion.

CONCLUSION

This study offers an effective perspective to overcome the current dilemma of the inferior interface-epithelial integration by in situ protein absorption and functionalization, which may advance the development of functional transepithelial biointerfaces. Tuning the surface chemistry by plasma polymerization can control the adsorption of fibronectin and functionalize it by exposing functional protein domains. The functionalized fibronectin can bind to human gingival epithelial cell membrane integrins to activate epithelial barrier structure related signaling pathway, which eventually enhances the formation of epithelial barrier structure.

Junctional epithelium and hemidesmosomes: Tape and rivets for solving the "percutaneous device dilemma" in dental and other permanent implants

The percutaneous device dilemma describes etiological factors, centered around the disrupted epithelial tissue surrounding non-remodelable devices, that contribute to rampant percutaneous device infection. Natural percutaneous organs, in particular their extracellular matrix mediating the "device"/epithelium interface, serve as exquisite examples to inspire longer lasting long-term percutaneous device design. For example, the tooth's imperviousness to infection is mediated by the epithelium directly surrounding it, the junctional epithelium (JE). The hallmark feature of JE is formation of hemidesmosomes, cell/matrix adhesive structures that attach surrounding oral gingiva to the tooth's enamel through a basement membrane. Here, the authors survey the multifaceted functions of the JE, emphasizing the role of the matrix, with a particular focus on hemidesmosomes and their five main components. The authors highlight the known (and unknown) effects dental implant - as a model percutaneous device - placement has on JE regeneration and synthesize this information for application to other percutaneous devices. The authors conclude with a summary of bioengineering strategies aimed at solving the percutaneous device dilemma and invigorating greater collaboration between clinicians, bioengineers, and matrix biologists.

Medical Applications of Porous Biomaterials: Features of Porosity and Tissue-Specific Implications for Biocompatibility

Porosity is an important material feature commonly employed in implants and tissue scaffolds. The presence of material voids permits the infiltration of cells, mechanical compliance, and outward diffusion of pharmaceutical agents. Various studies have confirmed that porosity indeed promotes favorable tissue responses, including minimal fibrous encapsulation during the foreign body reaction (FBR). However, increased biofilm formation and calcification is also described to arise due to biomaterial porosity. Additionally, the relevance of host responses like the FBR, infection, calcification, and thrombosis are dependent on tissue location and specific tissue microenvironment. In this review, the features of porous materials and the implications of porosity in the context of medical devices is discussed. Common methods to create porous materials are also discussed, as well as the parameters that are used to tune pore features. Responses toward porous biomaterials are also reviewed, including the various stages of the FBR, hemocompatibility, biofilm formation, and calcification. Finally, these host responses are considered in tissue specific locations including the subcutis, bone, cardiovascular system, brain, eye, and female reproductive tract. The effects of porosity across the various tissues of the body is highlighted and the need to consider the tissue context when engineering biomaterials is emphasized.

Systems of conductive skin for power transfer in clinical applications

The primary aim of this article is to review the clinical challenges related to the supply of power in implanted left ventricular assist devices (LVADs) by means of transcutaneous drivelines. In effect of that, we present the preventive measures and post-operative protocols that are regularly employed to address the leading problem of driveline infections. Due to the lack of reliable wireless solutions for power transfer in LVADs, the development of new driveline configurations remains at the forefront of different strategies that aim to power LVADs in a less destructive manner. To this end, skin damage and breach formation around transcutaneous LVAD drivelines represent key challenges before improving the current standard of care. For this reason, we assess recent strategies on the surface functionalization of LVAD drivelines, which aim to limit the incidence of driveline infection by directing the responses of the skin tissue. Moreover, we propose a class of power transfer systems that could leverage the ability of skin tissue to effectively heal short diameter wounds. In this direction, we employed a novel method to generate thin conductive wires of controllable surface topography with the potential to minimize skin disruption and eliminate the problem of driveline infections. Our initial results suggest the viability of the small diameter wires for the investigation of new power transfer systems for LVADs. Overall, this review uniquely compiles a diverse number of topics with the aim to instigate new research ventures on the design of power transfer systems for IMDs, and specifically LVADs.

Implantable Sensors Based on Gold Nanoparticles for Continuous Long-Term Concentration Monitoring in the Body

Implantable sensors continuously transmit information on vital values or biomarker concentrations in bodily fluids, enabling physicians to survey disease progression and monitor therapeutic success. However, currently available technologies still face difficulties with long-term operation and transferability to different analytes. We show the potential of a generalizable platform based on gold nanoparticles embedded in a hydrogel for long-term implanted biosensing. Using optical imaging and an intelligent sensor/reference-design, we assess the tissue concentration of kanamycin in anesthetized rats by interrogating our implanted sensor noninvasively through the skin. Combining a tissue-integrating matrix, robust aptamer receptors, and photostable gold nanoparticles, our technology has strong potential to extend the lifetime of implanted sensors. Because of the easy adaptability of gold nanoparticles toward different analytes, our concept will find versatile applications in personalized medicine or pharmaceutical development.

Uniform 40-µm-pore diameter precision templated scaffolds promote a pro-healing host response by extracellular vesicle immune communication

Implanted porous precision templated scaffolds (PTS) with 40-µm spherical pores reduce inflammation and foreign body reaction (FBR) while increasing vascular density upon implantation. Larger or smaller pores, however, promote chronic inflammation and FBR. While macrophage (MØ) recruitment and polarization participates in perpetuating this pore-size-mediated phenomenon, the driving mechanism of this unique pro-healing response is poorly characterized. We hypothesized that the primarily myeloid PTS resident cells release small extracellular vesicles (sEVs) that induce pore-size-dependent pro-healing effects in surrounding T cells. Upon profiling resident immune cells and their sEVs from explanted 40-µm- (pro-healing) and 100-µm-pore diameter (inflammatory) PTS, we found that PTS pore size did not affect PTS resident immune cell population ratios or the proportion of myeloid sEVs generated from explanted PTS. However, quantitative transcriptomic assessment indicated cell and sEV phenotype were pore size dependent. In vitro experiments demonstrated the ability of PTS cell-derived sEVs to stimulate T cells transcriptionally and proliferatively. Specifically, sEVs isolated from cells inhabiting explanted 100 μm PTS significantly upregulated T_h1 inflammatory gene expression in immortalized T cells. sEVs isolated from cell inhabiting both 40- and 100-μm PTS upregulated essential T_reg transcriptional markers in both primary and immortalized T cells. Finally, we investigated the effects of T_reg depletion on explanted PTS resident cells. FoxP3+ cell depletion suggests T_regs play a unique role in balancing T cell subset ratios, thus driving host response in 40-μm PTS. These results indicate that predominantly 40-µm PTS myeloid cell-derived sEVs affect T cells through a distinct, pore-size-mediated modality.

Si substituted hydroxyapatite nanorods on Ti for percutaneous implants

An ideal intraosseous transcutaneous implant should form a tight seal with soft tissue, besides a requirement of osseointegration at the bone-fixed position. Si substituted hydroxyapatite (Si-HA) nanorods releasing Si ion and simulating nanotopography of natural tissue were designed on Ti to enhance fibroblast response in vitro and biosealing with soft tissue in vivo. Si-HA nanorods were fabricated by alkali-heat treatment followed with hydrothermal treatment. The hydrothermal formation mechanism of Si-HA nanorods was explored. The surface characteristic of Si-HA nanorods was compared with pure HA nanorods. Fibroblast behaviors in vitro and skin response in vivo on different surfaces were also evaluated. The obtained results show that the substitution of Si did not significantly alter the phase component, morphology, roughness and wettability of HA, but additional Si and more Ca were released from Si-HA into medium. Comparing to pure HA nanrods and Ti substrate, Si-HA nanrods enhanced cell behaviors including proliferation, fibrotic phenotype and collagen secretion in vitro, and reduced epithelial down growth in vivo. The enhanced cell response and biosealing should be due to the releasing of Ca, Si and nanotopography of Si-HA nanorods. Si-HA nanorods can be a potential coating to accelerate skin integration for percutaneous implants in clinic.

Bijel-templated implantable biomaterials for enhancing tissue integration and vascularization

Mitigation of the foreign body response (FBR) and successful tissue integration are essential to ensuring the longevity of implanted devices and biomaterials. The use of porous materials and coatings has been shown to have an impact, as the textured surfaces can mediate macrophage interactions with the implant and influence the FBR, and the pores can provide space for vascularization and tissue integration. In this study, we use a new class of implantable porous biomaterials templated from bicontinuous interfacially jammed emulsion gels (bijels), which offer a fully percolating, non-constricting porous network with a uniform pore diameter on the order of tens of micrometers, and surfaces with consistent curvature. We demonstrate that these unique morphological features, inherent to bijel-templated materials (BTMs), can enhance tissue integration and vascularization, and reduce the FBR. Cylindrical polyethylene glycol diacrylate (PEGDA) BTMs, along with PEGDA particle-templated materials (PTMs), and non-templated materials (NTMs), were implanted into the subcutaneous space of athymic nude mice. After 28 days, implants were retrieved and analyzed via histological techniques. Within BTMs, blood vessels of increased size and depth, changes in collagen deposition, and increased presence of pro-healing macrophages were observed compared to that of PTM and NTM implants. Bijel templating offers a new route to biomaterials that can improve the function and longevity of implantable devices. STATEMENT OF SIGNIFICANCE: All implanted biomaterials are subject to the foreign body response (FBR) which can have a detrimental effect on their efficacy. Altering the surface chemistry can decrease the FBR by limiting the amount of proteins adsorbed to the implant. This effect can be enhanced by including pores in the biomaterial to allow new tissue growth as the implant becomes integrated in the body. Here, we introduce a new class of self-assembled biomaterials comprising a fully penetrating, non-constricting pore phase with hyperbolic (saddle) surfaces for enhanced tissue integration. These unique morphological characteristics result in dense blood vessel formation and favorable tissue response properties demonstrated in a four-week implantation study.

Influence of negative pressure wound therapy on peri-prosthetic tissue vascularization and inflammation around porous titanium percutaneous devices

Negative Pressure Wound Therapy (NPWT) has been shown to limit downgrowth around percutaneous devices in a guinea pig model. However, the influence of NPWT on peri-prosthetic tissue characteristics leading to limited downgrowth is still unclear. In order to investigate this, 12 CD hairless rats were assigned into two groups, NPWT and Untreated (n = 6/group). Each animal was implanted with a porous coated titanium percutaneous device and was dressed with a gauze and semi-occlusive base dressing. Post-surgery, animals in the NPWT Group received a regimen of NPWT treatment (-70 to -90 mmHg). After 4 weeks, tissue was collected over the device and stained with CD31 and CD68 to quantify blood vessel density and inflammation, respectively. The device with the surrounding tissue was also collected to quantify downgrowth. NPWT treatment led to a 1.6-fold increase in blood vessel densities compared to untreated tissues (p < 0.05). NPWT treatment also resulted in half the downgrowth as the Untreated Group, although not statistically significant (p = 0.19). Additionally, the results showed a trend toward increased CD68 cell densities in the NPWT Group compared to the Untreated Group (p = 0.09). These findings suggest that NPWT may influence wound healing responses in percutaneous devices by increasing blood vessel densities, limiting downgrowth and potentially increasing inflammation. Overall, NPWT may enhance tissue vascularity around percutaneous devices, especially in patients with impaired wound healing. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 2091-2101, 2019.

Surface-Functionalized Model Contact Lenses with a Bioinspired Proteoglycan 4 (PRG4)-Grafted Layer

Ocular dryness and discomfort are the primary reasons for the discontinuation of contact lens wear. This is mainly due to poorly hydrated contact lens surfaces and increased friction, particularly at the end of the day and can potentially cause reduced vision or even inflammation. Proteoglycan 4 (PRG4) is a mucinous glycoprotein with boundary lubricating properties, naturally found in the eye, able to prevent tear film evaporation and protect the ocular surface during blinking. Aiming to improve the interfacial interactions between the ocular surface and the contact lens, the synthesis and characterization of surface-modified model contact lenses with PRG4 is described. Full-length recombinant human PRG4 (rhPRG4) was successfully grafted onto the surface of model conventional and silicone hydrogel (SiHy) contact lenses via its somatomedin B-like end-domain using N, N'-carbonyldiimidazole linking chemistry. Grafting was assessed by Fourier transform infrared spectroscopy-attenuated total reflectance, X-ray photoelectron spectroscopy, and radioactive (I¹³¹) labeling. Surface immobilization of rhPRG4 led to model conventional and SiHy materials with improved antifouling properties, without impacting optical transparency or causing any toxic effects to human corneal epithelial cells in vitro. The surface wettability and the boundary friction against human corneal tissue were found to be substrate-dependent, with only the rhPRG4-grafted model SiHy exhibiting a reduced contact angle and kinetic friction coefficient compared to the unmodified surfaces. Hence, clinical grade rhPRG4 can be an attractive candidate for the development of novel bioinspired SiHy contact lenses, providing improved comfort and overall lens performance.

Modification of silicone elastomers with Bioglass 45S5® increases in ovo tissue biointegration

Silicone is an important material family used for various medical implants. It is biocompatible, but its bioinertness prevents cell attachment, and thus tissue biointegration of silicone implants. This often results in constrictive fibrosis and implant failure. Bioglass 45S5® (BG) could be a suitable material to alter the properties of silicone, render it bioactive and improve tissue integration. Therefore, BG micro- or nanoparticles were blended into medical-grade silicone and 2D as well as 3D structures of the resulting composites were analyzed in ovo by a chick chorioallantoic membrane (CAM) assay. The biomechanical properties of the composites were measured and the bioactivity of the composites was verified in simulated body fluid. The bioactivity of BG-containing composites was confirmed visually by the formation of hydroxyapatite through scanning electron microscopy as well as by infrared spectroscopy. BG stiffens as prepared non-porous composites by 13% and 36% for micro- and nanocomposites respectively. In particular, after implantation for 7 days, the Young's modulus had increased significantly from 1.20 ± 0.01 to 1.57 ± 0.03 MPa for microcomposites and 1.44 ± 0.03 to 1.69 ± 0.29 MPa to for nanocpmosites. Still, the materials remain highly elastic and are comparably soft. The incorporation of BG into silicone overcame the bioinertness of the pure polymer. Although the overall tissue integration was weak, it was significantly improved for BG-containing porous silicones (+72% for microcomposites) and even further enhanced for composites containing nanoparticles (+94%). These findings make BG a suitable material to improve silicone implant properties. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1180-1188, 2019.

Strategies for Optimizing the Soft Tissue Seal around Osseointegrated Implants

Percutaneous and permucosal devices such as catheters, infusion pumps, orthopedic, and dental implants are commonly used in medical treatments. However, these useful devices breach the soft tissue barrier that protects the body from the outer environment, and thus increase bacterial infections resulting in morbidity and mortality. Such associated infections can be prevented if these devices are effectively integrated with the surrounding soft tissue, and thus creating a strong seal from the surrounding environment. However, so far, there are no percutaneous/permucosal medical devices able to prevent infection by achieving strong integration at the soft tissue-device interface. This review gives an insight into the current status of research into soft tissue-implant interface and the challenges associated with these interfaces. Biological soft/hard tissue interfaces may provide insights toward engineering better soft tissue interfaces around percutaneous devices. In this review, focus is put on the history and current findings as well as recent progress of the strategies aiming to develop a strong soft tissue seal around osseointegrated implants, such as orthopedic and dental implants.

Inverse Opal Scaffolds and Their Biomedical Applications

Three-dimensional porous scaffolds play a pivotal role in tissue engineering and regenerative medicine by functioning as biomimetic substrates to manipulate cellular behaviors. While many techniques have been developed to fabricate porous scaffolds, most of them rely on stochastic processes that typically result in scaffolds with pores uncontrolled in terms of size, structure, and interconnectivity, greatly limiting their use in tissue regeneration. Inverse opal scaffolds, in contrast, possess uniform pores inheriting from the template comprised of a closely packed lattice of monodispersed microspheres. The key parameters of such scaffolds, including architecture, pore structure, porosity, and interconnectivity, can all be made uniform across the same sample and among different samples. In conjunction with a tight control over pore sizes, inverse opal scaffolds have found widespread use in biomedical applications. In this review, we provide a detailed discussion on this new class of advanced materials. After a brief introduction to their history and fabrication, we highlight the unique advantages of inverse opal scaffolds over their non-uniform counterparts. We then showcase their broad applications in tissue engineering and regenerative medicine, followed by a summary and perspective on future directions.

Biomaterial surface proteomic signature determines interaction with epithelial cells

Cells interact with biomaterials indirectly through extracellular matrix (ECM) proteins adsorbed onto their surface. Accordingly, it could be hypothesized that the surface proteomic signature of a biomaterial might determine its interaction with cells. Here, we present a surface proteomic approach to test this hypothesis in the specific case of biomaterial-epithelial cell interactions. In particular, we determined the surface proteomic signature of different biomaterials exposed to the ECM of epithelial cells (basal lamina). We revealed that the biomaterial surface chemistry determines the surface proteomic profile, and subsequently the interaction with epithelial cells. In addition, we found that biomaterials with surface chemistries closer to that of percutaneous tissues, such as aminated PMMA and aminated PDLLA, promoted higher selective adsorption of key basal lamina proteins (laminins, nidogen-1) and subsequently improved their interactions with epithelial cells. These findings suggest that mimicking the surface chemistry of natural percutaneous tissues can improve biomaterial-epithelial integration, and thus provide a rationale for the design of improved biomaterial surfaces for skin regeneration and percutaneous medical devices.

STATEMENT OF SIGNIFICANCE

Failure of most biomaterials originates from the inability to predict and control the influence of their surface properties on biological phenomena, particularly protein adsorption, and cellular behaviour, which subsequently results in unfavourable host response. Here, we introduce a surface-proteomic screening approach using a label-free mass spectrometry technique to decipher the adsorption profile of extracellular matrix (ECM) proteins on different biomaterials, and correlate it with cellular behaviour. We demonstrated that the way a biomaterial selectively interacts with specific ECM proteins of a given tissue seems to determine the interactions between the cells of that tissue and biomaterials. Accordingly, this approach can potentially revolutionize the screening methods for investigating the protein-cell-biomaterial interactions and pave the way for deeper understanding of these interactions.

Activation of Macrophages in Response to Biomaterials

Macrophages are the initial biologic responders to biomaterials. These highly plastic immune sentinels control and modulate responses to materials, foreign or natural. The responses may vary from immune stimulatory to immune suppressive. Several parameters have been identified that influence macrophage response to biomaterials, specifically size, geometry, surface topography, hydrophobicity, surface chemistry, material mechanics, and protein adsorption. In this review, the influence of these parameters is supported with examples of both synthetic and naturally derived materials and illustrates that a combination of these parameters ultimately influences macrophage responses to the biomaterial. Having an understanding of these properties may lead to highly efficient design of biomaterials with desirable biologic response properties.

Accelerated tissue integration into porous materials by immobilizing basic fibroblast growth factor using a biologically safe three-step reaction

Soft tissue integration into a porous structure is important to prevent bacterial infection of percutaneous devices and improve tissue regeneration using porous scaffolds. Here, basic fibroblast growth factor (bFGF) was immobilized on porous polymer materials using a mild and biologically safe three-step reaction: (1) modification with a novel surface-modification peptide (penta-lysine-mussel adhesive sequence, which reacts with various matrices), (2) electrostatic binding of heparin with introduced penta-lysine, and (3) biologically specific binding of bFGF to heparin. Porous polyethylene specimens (PPSs) (D = 6.0 mm, H = 2.0 mm) with a good size for tissue integration were selected as a base material, immobilized with bFGF, and subcutaneously implanted into mice. Half of the unmodified PPSs extruded out of the body on day 112 postimplantation; however, the three-step reaction completely prevented sample rejection. Tissue integration was greatly accelerated by immobilizing bFGF. Direct physical coating of bFGF on PPS resulted in greater immobilization but lesser tissue integration than that after the three-step bFGF immobilization, indicating that heparin binds and enhances bFGF efficacy. This three-step bFGF immobilization reaction will be applicable to various polymeric, metallic, and ceramic materials and is a simple strategy to integrate tissue on porous medical devices or scaffolds for tissue regeneration.

A novel vacuum assisted closure therapy model for use with percutaneous devices

Long-term maintenance of a dermal barrier around a percutaneous prosthetic device remains a common clinical problem. A technique known as Negative Pressure Wound Therapy (NPWT) uses negative pressure to facilitate healing of impaired and complex soft tissue wounds. However, the combination of using negative pressure with percutaneous prosthetic devices has not been investigated. The goal of this study was to develop a methodology to apply negative pressure to the tissues surrounding a percutaneous device in an animal model; no tissue healing outcomes are presented. Specifically, four hairless rats received percutaneous porous coated titanium devices implanted on the dorsum and were bandaged with a semi occlusive film dressing. Two of these animals received NPWT; two animals received no NPWT and served as baseline controls. Over a 28-day period, both the number of dressing changes required between the two groups as well as the pressures were monitored. Negative pressures were successfully applied to the periprosthetic tissues in a clinically relevant range with a manageable number of dressing changes. This study provides a method for establishing, maintaining, and quantifying controlled negative pressures to the tissues surrounding percutaneous devices using a small animal model.

Capillary force seeding of sphere-templated hydrogels for tissue-engineered prostate cancer xenografts

Biomaterial-based tissue-engineered tumor models are now widely used in cancer biology studies. However, specific methods for efficient and reliable cell seeding into these and tissue-engineering constructs used for regenerative medicine often remain poorly defined. Here, we describe a capillary force-based method for seeding the human prostate cancer cell lines M12 and LNCaP C4-2 into sphere-templated poly(2-hydroxyethyl methacrylate) hydrogels. The capillary force seeding method improved the cell number and distribution within the porous scaffolds compared to well-established protocols such as static and centrifugation seeding. Seeding efficiency was found to be strongly dependent on the rounded cell diameter relative to the pore diameter and pore interconnect size, parameters that can be controllably modulated during scaffold fabrication. Cell seeding efficiency was evaluated quantitatively using a PicoGreen DNA assay, which demonstrated some variation in cell retention using the capillary force method. When cultured within the porous hydrogels, both cell lines attached and proliferated within the network, but histology showed the formation of a necrotic zone by 7 days likely due to oxygen and nutrient diffusional limitations. The necrotic zone thickness was decreased by dynamically culturing cells in an orbital shaker. Proliferation analysis showed that despite a variable seeding efficiency, by 7 days in culture, scaffolds contained a roughly consistent number of cells as they proliferated to fill the pores of the scaffold. These studies demonstrate that sphere-templated polymeric scaffolds have the potential to serve as an adaptable cell culture substrate for engineering a three-dimensional prostate cancer model.

Engineering biomaterials to integrate and heal: the biocompatibility paradigm shifts

This article focuses on one of the major failure routes of implanted medical devices, the foreign body reaction (FBR)--that is, the phagocytic attack and encapsulation by the body of the so-called "biocompatible" biomaterials comprising the devices. We then review strategies currently under development that might lead to biomaterial constructs that will harmoniously heal and integrate into the body. We discuss in detail emerging strategies to inhibit the FBR by engineering biomaterials that elicit more biologically pertinent responses.

Cutaneous and inflammatory response to long-term percutaneous implants of sphere-templated porous/solid poly(HEMA) and silicone in mice

This study investigates mouse cutaneous responses to long-term percutaneously implanted rods surrounded by sphere-templated porous biomaterials engineered to mimic medical devices surrounded by a porous cuff. We hypothesized that keratinocytes would migrate through the pores and stop, permigrate, or marsupialize along the porous/solid interface. Porous/solid-core poly(2-hydroxyethyl methacrylate) [poly(HEMA)] and silicone rods were implanted in mice for 14 days, and for 1, 3, and 6 months. Implants with surrounding tissue were analyzed (immuno)histochemically by light microscopy. Poly(HEMA)/skin implants yielded better morphologic data than silicone implants. Keratinocytes at the poly(HEMA) interface migrated in two different directions. "Ventral" keratinocytes contiguous with the dermal-epidermal junction migrated into the outermost pores, forming an integrated collar surrounding the rods. "Dorsal" keratinocytes appearing to emanate from the differentiated epithelial layer, extended upward along and into the exterior portion of the rod, forming an integrated sheath. Leukocytes persisted in poly(HEMA) and silicone pores for the duration of the study. Vascular and collagen networks within the poly(HEMA) pores matured as a function of time up to 3-months implantation. Nerves were not observed within the pores. Poly(HEMA) underwent morphological changes by 6 months of implantation. Marsupialization, foreign body encapsulation, and infection were not observed in any implants.

Glucose sensor membranes for mitigating the foreign body response

Continuous glucose monitoring devices remain limited in their duration of use due to difficulties presented by the foreign body response (FBR), which impairs sensor functionality immediately following implantation via biofouling and leukocyte infiltration. The FBR persists through the life of the implant, culminating with fibrous encapsulation and isolation from normal tissue. These issues have led researchers to develop strategies to mitigate the FBR and improve tissue integration. Studies have often focused on abating the FBR using various outer coatings, thereby changing the chemical or physical characteristics of the sensor surface. While such strategies have led to some success, they have failed to fully integrate the sensor into surrounding tissue. To further address biocompatibility, researchers have designed coatings capable of actively releasing biological agents (e.g., vascular endothelial growth factor, dexamethasone, and nitric oxide) to direct the FBR to induce tissue integration. Active release approaches have proven promising and, when combined with biocompatible coating materials, may ultimately improve the in vivo lifetime of subcutaneous glucose biosensors. This article focuses on strategies currently under development for mitigating the FBR.

Quantifying the effect of pore size and surface treatment on epidermal incorporation into percutaneously implanted sphere-templated porous biomaterials in mice

The sinus between skin and a percutaneous medical device is often a portal for infection. Epidermal integration into an optimized porous biomaterial could seal this sinus. In this study, we measured epithelial ingrowth into rods of sphere-templated porous poly(2-hydroxyethyl methacrylate) implanted percutaneously in mice. The rods contained spherical 20-, 40-, or 60-μm pores with and without surface modification. Epithelial migration was measured 3, 7, and 14 days post-implantation utilizing immunohistochemistry for pankeratins and image analysis. Our global results showed average keratinocyte migration distances of 81 ± 16.85 μm (SD). Migration was shorter through 20-μm pores (69.32 ± 21.73) compared with 40 and 60 μm (87.04 ± 13.38 μm and 86.63 ± 8.31 μm, respectively). Migration was unaffected by 1,1' carbonyldiimidazole surface modification without considering factors of pore size and healing duration. Epithelial integration occurred quickly showing an average migration distance of 74.13 ± 12.54 μm after 3 days without significant progression over time. These data show that the epidermis closes the sinus within 3 days, migrates into the biomaterial (an average of 11% of total rod diameter), and stops. This process forms an integrated epithelial collar without evidence of marsupialization or permigration.

Percutaneous implants with porous titanium dermal barriers: an in vivo evaluation of infection risk

Osseointegrated percutaneous implants are a promising prosthetic alternative for a subset of amputees. However, as with all percutaneous implants, they have an increased risk of infection since they breach the skin barrier. Theoretically, host tissues could attach to the metal implant creating a barrier to infection. When compared with smooth surfaces, it is hypothesized that porous surfaces improve the attachment of the host tissues to the implant, and decrease the infection risk. In this study, four titanium implants, manufactured with a percutaneous post and a subcutaneous disk, were placed subcutaneously on the dorsum of eight New Zealand White rabbits. Beginning at four weeks post-op, the implants were inoculated weekly with 10(8) CFU Staphylococcus aureus until signs of clinical infection presented. While we were unable to detect a difference in the incidence of infection of the porous metal implants, smooth surface (no porous coating) percutaneous and subcutaneous components had a 7-fold increased risk of infection compared to the implants with a porous coating on one or both components. The porous coated implants displayed excellent tissue ingrowth into the porous structures; whereas, the smooth implants were surrounded with a thick, organized fibrotic capsule that was separated from the implant surface. This study suggests that porous coated metal percutaneous implants are at a significantly lower risk of infection when compared to smooth metal implants. The smooth surface percutaneous implants were inadequate in allowing a long-term seal to develop with the soft tissue, thus increasing vulnerability to the migration of infecting microorganisms.

Epidermal and dermal integration into sphere-templated porous poly(2-hydroxyethyl methacrylate) implants in mice

Percutaneous medical devices remain susceptible to infection and failure. We hypothesize that healing of the skin into the percutaneous device will provide a seal, preventing bacterial attachment, biofilm formation, and subsequent device failure. Porous poly(2-hydroxyethyl methacrylate) [poly(HEMA)] with sphere-templated pores (40 microm) and interconnecting throats (16 microm) were implanted in normal C57BL/6 mice for 7, 14, and 28 days. Poly(HEMA) was either untreated, keeping the surface nonadhesive for cells and proteins, or modified with carbonyldiimidazole (CDI) or CDI reacted with laminin 332 to enhance adhesion. No clinical signs of infection were observed. Epidermal and dermal response within the poly(HEMA) pores was evaluated using light and transmission electron microscopy. Cells (keratinocytes, fibroblasts, endothelial cells, inflammatory cells) and basement membrane proteins (laminin 332, beta4 integrin, type VII collagen) could be demonstrated within the poly(HEMA) pores of all implants. Blood vessels and dermal collagen bundles were evident in all of the 14- and 28-day implants. Fibrous capsule formation and permigration were not observed. Sphere-templated polymers with 40 microm pores demonstrate an ability to recapitulate key elements of both the dermal and the epidermal layers of skin. Our morphological findings indicate that the implant model can be used to study the effects of biomaterial pore size, pore interconnect (throat) size, and surface treatments on cutaneous biointegration. Further, this model may be used for bacterial challenge studies.

Proangiogenic scaffolds as functional templates for cardiac tissue engineering

We demonstrate here a cardiac tissue-engineering strategy addressing multicellular organization, integration into host myocardium, and directional cues to reconstruct the functional architecture of heart muscle. Microtemplating is used to shape poly(2-hydroxyethyl methacrylate-co-methacrylic acid) hydrogel into a tissue-engineering scaffold with architectures driving heart tissue integration. The construct contains parallel channels to organize cardiomyocyte bundles, supported by micrometer-sized, spherical, interconnected pores that enhance angiogenesis while reducing scarring. Surface-modified scaffolds were seeded with human ES cell-derived cardiomyocytes and cultured in vitro. Cardiomyocytes survived and proliferated for 2 wk in scaffolds, reaching adult heart densities. Cardiac implantation of acellular scaffolds with pore diameters of 30-40 microm showed angiogenesis and reduced fibrotic response, coinciding with a shift in macrophage phenotype toward the M2 state. This work establishes a foundation for spatially controlled cardiac tissue engineering by providing discrete compartments for cardiomyocytes and stroma in a scaffold that enhances vascularization and integration while controlling the inflammatory response.

Models for the histologic study of the skin interface with percutaneous biomaterials

Percutaneous devices are critical for health care. Access to tissue, vessels and internal organs afforded by these devices provides the means to treat and monitor many diseases. Unfortunately, such access is not restricted, and infection may compromise the usefulness of the device and even the life of the patient. New biomaterials offer the possibility of maintaining internal access while limiting microbial access, but understanding of the cutaneous/biomaterial interface and models to study this area are limited. This paper focuses on models useful for studying the morphology and biology of the intersection of skin and percutaneous biomaterials. An organ culture and a mouse model are described that offer promising possibilities for improved understanding of this critical interface.