Biomaterials in tissue engineering

Collagen-reinforced electrospun silk fibroin tubular construct as small calibre vascular graft

None of the replacements proposed in the literature for small-calibre blood vessels (SCBV) fully satisfies the stringent requirements that these grafts have to fulfil. Here, an electrospun silk fibroin tubular construct is hybridized with type I collagen gel to produce a biomimetic SCBV graft with physiologically relevant compliance and burst pressure and optimal cytocompatibility. The hybridization of the two polymers results in the formation of a nanofibrillar hydrated matrix, where the collagen gel enhances the mechanical properties of the SF tubular construct and improves the early response of the material to in vitro cell adhesion and proliferation.

Advances in biomimetic regeneration of elastic matrix structures

Elastin is a vital component of the extracellular matrix, providing soft connective tissues with the property of elastic recoil following deformation and regulating the cellular response via biomechanical transduction to maintain tissue homeostasis. The limited ability of most adult cells to synthesize elastin precursors and assemble them into mature crosslinked structures has hindered the development of functional tissue-engineered constructs that exhibit the structure and biomechanics of normal native elastic tissues in the body. In diseased tissues, the chronic overexpression of proteolytic enzymes can cause significant matrix degradation, to further limit the accumulation and quality (e.g., fiber formation) of newly deposited elastic matrix. This review provides an overview of the role and importance of elastin and elastic matrix in soft tissues, the challenges to elastic matrix generation in vitro and to regenerative elastic matrix repair in vivo, current biomolecular strategies to enhance elastin deposition and matrix assembly, and the need to concurrently inhibit proteolytic matrix disruption for improving the quantity and quality of elastogenesis. The review further presents biomaterial-based options using scaffolds and nanocarriers for spatio-temporal control over the presentation and release of these biomolecules, to enable biomimetic assembly of clinically relevant native elastic matrix-like superstructures. Finally, this review provides an overview of recent advances and prospects for the application of these strategies to regenerating tissue-type specific elastic matrix structures and superstructures.

A crimp-like microarchitecture improves tissue production in fibrous ligament scaffolds in response to mechanical stimuli

The aim of this study was to determine the influence of a crimp-like microarchitecture within electrospun polymer scaffolds on fibroblast extracellular matrix (ECM) production when cultured under dynamic conditions. Electrospun poly(L-lactide-co-D,L-lactide) scaffolds possessing a wave pattern similar to collagen crimp (amplitude: 5 μm and wavelength: 46 μm) were seeded with bovine fibroblasts and mechanically stimulated under dynamic uniaxial tension. The effect of strain amplitude (5%, 10% and 20%) was investigated in a short-term stimulation study. The 10% strain amplitude in the stimulated crimp-like fibre scaffold increased only collagen synthesis, while the 20% strain amplitude increased both collagen and sulphated proteoglycan synthesis compared to stimulated uncrimped (straight) fibre scaffolds and unloaded controls (crimp-like static fibre scaffolds). Alternatively, mechanical stimulation of fibroblasts seeded on uncrimped fibre scaffolds induced significant fibroblast proliferation compared to the stimulated crimp-like fibre scaffolds and no-load controls. Long-term, dynamic mechanical stimulation of fibroblasts seeded on crimp-like fibre scaffolds at 10% strain amplitude resulted in significantly up-regulated collagen accumulation and down-regulated sulphated proteoglycan accumulation. Additionally, the fibroblasts seeded on dynamically stimulated crimp-like fibre scaffolds appeared to form bundles that resembled fascicles, a characteristic hierarchical feature of the native ligament. Our findings demonstrate that fibroblasts seeded on crimp-like fibrous scaffolds respond more favourably (increased ECM synthesis and fascicle formation) to dynamic mechanical loading compared to those grown on scaffolds containing uncrimped (straight) fibres.

Fibrin-loaded porous poly(ethylene glycol) hydrogels as scaffold materials for vascularized tissue formation

Vascular network formation within biomaterial scaffolds is essential for the generation of properly functioning engineered tissues. In this study, a method is described for generating composite hydrogels in which porous poly(ethylene glycol) (PEG) hydrogels serve as scaffolds for mechanical and structural support, and fibrin is loaded within the pores to induce vascularized tissue formation. Porous PEG hydrogels were generated by a salt leaching technique with 100-150-μm pore size and thrombin (Tb) preloaded within the scaffold. Fibrinogen (Fg) was loaded into pores with varying concentrations and polymerized into fibrin due to the presence of Tb, with loading efficiencies ranging from 79.9% to 82.4%. Fibrin was distributed throughout the entire porous hydrogels, lasted for greater than 20 days, and increased hydrogel mechanical stiffness. A rodent subcutaneous implant model was used to evaluate the influence of fibrin loading on in vivo response. At weeks 1, 2, and 3, all hydrogels had significant tissue invasion, but no difference in the depth of invasion was found with the Fg concentration. Hydrogels with fibrin loading induced more vascularization, with a significantly higher vascular density at 20 mg/mL (week 1) and 40 mg/mL (weeks 2 and 3) Fg concentration compared to hydrogels without fibrin. In conclusion, we have developed a composite hydrogel that supports rapid vascularized tissue ingrowth, and thus holds great potential for tissue engineering applications.

Acceleration of segmental bone regeneration in a rabbit model by strontium-doped calcium polyphosphate scaffold through stimulating VEGF and bFGF secretion from osteoblasts

The development of suitable bioactive three-dimensional scaffold for the promotion of bone regeneration is critical in bone tissue engineering. The purpose of this study was to investigate in vivo osteogenesis of the porous strontium-doped calcium polyphosphate (SCPP) scaffolds for bone repair, as well as the relationship between osteogenic properties of SCPP scaffolds and the secretion of bFGF and VEGF from osteoblasts stimulated by SCPP. Besides, the advantages of scaffolds seeded with mesenchymal stem cells (MSCs) for bone repair were also studied. Firstly, the bone repair evaluation of scaffolds was performed on a rabbit segmental bony defects model over a period of 16 weeks by histology combined with X-ray microradiography. And then, in order to avoid the influence from the other factors such as hypoxia which emerge in vivo study and affect the secretion of VEGF and bFGF from host cells, human osteoblast-like cells (MG63) were seeded to SCPP, CPP and HA scaffolds in vitro to determine the ability of these scaffolds to stimulate the secretion of angiogenic growth factors (VEGF and bFGF) from MG63 and further explore the reason for the better osteogenic properties of SCPP scaffolds. The histological and X-ray microradiographic results showed that the SCPP scaffolds presented better osteogenic potential than CPP and HA scaffolds, when combined with MSCs, the SCPP scaffolds could further accelerate the bone repair. And the amounts of VEGF measured by ELISA assay in SCPP, CPP and HA groups after cultured for 7 days were about 364.989 pg/mL, 244.035 pg/mL and 232.785 pg/mL, respectively. Accordingly, the amounts of bFGF were about 27.085 pg/mL, 15.727 pg/mL and 8.326 pg/mL. The results revealed that the SCPP scaffolds significantly enhanced the bFGF and VEGF secretion compared with other scaffolds. The results presented in vivo and in vitro study demonstrated that the SCPP could accelerate bone formation through stimulating the secretion of VEGF and bFGF from osteoblasts, making it attractive for bone regeneration.

Mineralization Potential of Electrospun PDO-Hydroxyapatite-Fibrinogen Blended Scaffolds

The current bone autograft procedure for cleft palate repair presents several disadvantages such as limited availability, additional invasive surgery, and donor site morbidity. The present preliminary study evaluates the mineralization potential of electrospun polydioxanone:nano-hydroxyapatite : fibrinogen (PDO : nHA : Fg) blended scaffolds in different simulated body fluids (SBF). Scaffolds were fabricated by blending PDO : nHA : Fg in the following percent by weight ratios: 100 : 0 : 0, 50 : 25 : 25, 50 : 50 : 0, 50 : 0 : 50, 0 : 0 : 100, and 0 : 50 : 50. Samples were immersed in (conventional (c), revised (r), ionic (i), and modified (m)) SBF for 5 and 14 days to induce mineralization. Scaffolds were characterized before and after mineralization via scanning electron microscopy, Alizarin Red-based assay, and modified burnout test. The addition of Fg resulted in scaffolds with smaller fiber diameters. Fg containing scaffolds also induced sheet-like mineralization while individual fiber mineralization was noticed in its absence. Mineralized electrospun Fg scaffolds without PDO were not mechanically stable after 5 days in SBF, but had superior mineralization capabilities which produced a thick bone-like mineral (BLM) layer throughout the scaffolds. 50 : 50 : 0 scaffolds incubated in either r-SBF for 5 days or c-SBF for 14 days produced scaffolds with high mineral content and individual-mineralized fibers. These mineralized scaffolds were still porous and will be further optimized as an effective bone substitute in future studies.

A reproducible, high throughput method for fabricating fibrin gels

Background

Fibrin gels are a promising biomaterial for tissue engineering. However, current fabrication methods are time intensive with inherent variation. There is a pressing need to develop new and consistent approaches for producing fibrin-based hydrogels for examination.

Findings

We developed a high throughput method for creating fibrin gels using molds fabricated from polydimethylsiloxane (PDMS). Fibrin gels were produced by adding solutions of fibrinogen and thrombin to cylindrical defects in a PDMS sheet. Undisturbed gels were collected by removing the sheet, and fibrin gels were characterized. The characteristics of resulting gels were compared to published data by measuring compressive stiffness and osteogenic response of entrapped human mesenchymal stem cells (MSCs). Gels exhibited compressive moduli nearly identical to our previously reported fabrication method. Trends in alkaline phosphatase activity, an early marker of osteogenic differentiation in MSCs, were also consistent with previous data.

Conclusions

These findings demonstrate a streamlined approach to fibrin gel production that drastically reduces the time required to make fibrin gels, while also reducing variability between gel batches. This fabrication technique provides a valuable tool for generating large numbers of gels in a cost-effective manner.

Protein polymer hydrogels: effects of endotoxin on biocompatibility

Protein polymer-based hydrogels have shown potential for tissue engineering applications, but require biocompatibility testing for in vivo use. Enzymatically crosslinked protein polymer-based hydrogels were tested in vitro and in vivo to evaluate their biocompatibility. Endotoxins present in the hydrogel were removed by Trition X-114 phase separation. The reduction of endotoxins decreased TNF-α production by a macrophage cell line in vitro; however, significant inflammatory response was still present compared to collagen control gels. A branched PEG molecule and dexamethasone were added to the hydrogel to reduce the response. In vitro testing showed a decrease in the TNF-α levels with the addition of dexamethasone. In vivo implantations into the epididymal fat pad of C57/BL6 mice, however, indicated a decreased inflammatory mediated immune response with a hydrogel treated with both PEGylation and endotoxin reduction. This study demonstrates the importance of endotoxin testing and removal in determining the biocompatibility of biomaterials.

Mass transfer trends occurring in engineered ex vivo tissue scaffolds

In vivo the vasculature provides an effective delivery system for cellular nutrients; however, artificial scaffolds have no such mechanism, and the ensuing limitations in mass transfer result in limited regeneration. In these investigations, the regional mass transfer properties that occur through a model scaffold derived from the human umbilical vein (HUV) were assessed. Our aim was to define the heterogeneous behavior associated with these regional variations, and to establish if different decellularization technologies can modulate transport conditions to improve microenvironmental conditions that enhance cell integration. The effect of three decellularization methods [Triton X-100 (TX100), sodium dodecyl sulfate (SDS), and acetone/ethanol (ACE/EtOH)] on mass transfer, cellular migration, proliferation, and metabolic activity were assessed. Results show that regional variation in tissue structure and composition significantly affects both mass transfer and cell function. ACE/EtOH decellularization was shown to increase albumin mass flux through the intima and proximate-medial region (0-250 μm) when compared with sections decellularized with TX100 or SDS; although, mass flux remained constant over all regions of the full tissue thickness when using TX100. Scaffolds decellularized with TX100 were shown to promote cell migration up to 146% further relative to SDS decellularized samples. These results show that depending on scaffold derivation and expectations for cellular integration, specificities of the decellularization chemistry affect the scaffold molecular architecture resulting in variable effects on mass transfer and cellular response.

A fiber-optic-based imaging system for nondestructive assessment of cell-seeded tissue-engineered scaffolds

A major limitation in tissue engineering is the lack of nondestructive methods that assess the development of tissue scaffolds undergoing preconditioning in bioreactors. Due to significant optical scattering in most scaffolding materials, current microscope-based imaging methods cannot "see" through thick and optically opaque tissue constructs. To address this deficiency, we developed a fiber-optic-based imaging method that is capable of nondestructive imaging of fluorescently labeled cells through a thick and optically opaque scaffold, contained in a bioreactor. This imaging modality is based on the local excitation of fluorescent cells, the acquisition of fluorescence through the scaffold, and fluorescence mapping based on the position of the excitation light. To evaluate the capability and accuracy of the imaging system, human endothelial cells (ECs), stably expressing green fluorescent protein (GFP), were imaged through a fibrous scaffold. Without sacrificing the scaffolds, we nondestructively visualized the distribution of GFP-labeled cells through a ~500 μm thick scaffold with cell-level resolution and distinct localization. These results were similar to control images obtained using an optical microscope with direct line-of-sight access. Through a detailed quantitative analysis, we demonstrated that this method achieved a resolution on the order of 20-30 μm, with 10% or less deviation from standard optical microscopy. Furthermore, we demonstrated that the penetration depth of the imaging method exceeded that of confocal laser scanning microscopy by more than a factor of 2. Our imaging method also possesses a working distance (up to 8 cm) much longer than that of a standard confocal microscopy system, which can significantly facilitate bioreactor integration. This method will enable the nondestructive monitoring of ECs seeded on the lumen of a tissue-engineered vascular graft during preconditioning in vitro, as well as for other tissue-engineered constructs in the future.

Systematic selection of solvents for the fabrication of 3D combined macro- and microporous polymeric scaffolds for soft tissue engineering

In this study, we investigate the fabrication of 3D porous poly(lactic-co-glycolic acid) (PLGA) scaffolds using the thermally-induced phase separation technique. The current study focuses on the selection of alternative solvents for this process using a number of criteria, including predicted solubility, toxicity, removability and processability. Solvents were removed via either vacuum freeze-drying or leaching, depending on their physical properties. The residual solvent was tested using gas chromatography-mass spectrometry. A large range of porous, highly interconnected scaffold architectures with tunable pore size and alignment was obtained, including combined macro- and microporous structures and an entirely novel 'porous-fibre' structure. The morphological features of the most promising poly(lactic-co-glycolic acid) scaffolds were analysed via scanning electron microscopy and X-ray micro-computed tomography in both two and three dimensions. The Young's moduli of the scaffolds under conditions of temperature, pH and ionic strength similar to those found in the body were tested and were found to be highly dependent on the architectures.

The effects of substrate stiffness on the in vitro activation of macrophages and in vivo host response to poly(ethylene glycol)-based hydrogels

Poly(ethylene glycol) (PEG) hydrogels, modified with RGD, are promising platforms for cell encapsulation and tissue engineering. While these hydrogels offer tunable mechanical properties, the extent of the host response may limit their in vivo applicability. The overall objective was to characterize the effects of hydrogel stiffness on the in vitro macrophage response and in vivo host response. We hypothesized that stiffer substrates induce better attachment, adhesion, and increased cell spreading, which elevates the macrophage classically activated phenotype and leads to a more severe foreign body reaction (FBR). PEG-RGD hydrogels were fabricated with compressive moduli of 130, 240, and 840 kPa, and the same RGD concentration. Hydrogel stiffness did not impact macrophage attachment, but elicited differences in cell morphology. Cells retained a round morphology on 130 kPa substrates, with localized and dense F-actin and localized α(V) integrin stainings. Contrarily, cells on stiffer substrates were more spread, with filopodia protruding from the cell, a more defined F-actin, and greater α(V) integrin staining. When stimulated with lipopolysaccharide, macrophages had a classical activation phenotype, with increased expression of TNF-α, IL-1β, and IL-6, however the degree of activation was significantly reduced with the softest hydrogels. A FBR ensued in response to all hydrogels when implanted subcutaneously in mice, but 28 days postimplantation the layer of macrophages at the implant surface was significantly lower in the softest hydrogels. In conclusion, hydrogels with lower stiffness led to reduced macrophage activation and a less severe and more typical FBR, and therefore are more suited for in vivo tissue engineering applications.

Using polymeric materials to control stem cell behavior for tissue regeneration

Patients with organ failure often suffer from increased morbidity and decreased quality of life. Current strategies of treating organ failure have limitations, including shortage of donor organs, low efficiency of grafts, and immunological problems. Tissue engineering emerged about two decades ago as a strategy to restore organ function with a living, functional engineered substitute. However, the ability to engineer a functional organ is limited by a limited understanding of the interactions between materials and cells that are required to yield functional tissue equivalents. Polymeric materials are one of the most promising classes of materials for use in tissue engineering, due to their biodegradability, flexibility in processing and property design, and the potential to use polymer properties to control cell function. Stem cells offer potential in tissue engineering because of their unique capacity to self-renew and differentiate into neurogenic, osteogenic, chondrogenic, and myogenic lineages under appropriate stimuli from extracellular components. This review examines recent advances in stem cell-polymer interactions for tissue regeneration, specifically highlighting control of polymer properties to direct adhesion, proliferation, and differentiation of stem cells, and how biomaterials can be designed to provide some of the stimuli to cells that the natural extracellular matrix does.

Degradation studies of hydrophilic, partially degradable and bioactive cements (HDBCs) incorporating chemically modified starch

The degradation rate in Hydrophilic, Degradable and Bioactive Cements (HDBCs) containing starch/cellulose acetate blends (SCA) is still low. In order to increase degradation, higher amounts of starch are required to exceed the percolation threshold. In this work, gelatinization, acetylation and methacrylation of corn starch were performed and assessed as candidates to replace SCA in HDBCs. Formulations containing methacrylated starch were prepared with different molar ratios of 2-hydroxyethyl methacrylate and methyl methacrylate in the liquid component and the amount of residual monomer released into water was evaluated. The concentration of reducing sugars, percentage of weight loss and morphologic analyses after degradation all confirmed increased degradation of HDBC with α-amylase, with the appearance of pores and voids from enzymatic action. Methacrylated starch therefore is a better alternative to be used as the solid component of HDBC then SCA, since it leads to the formation of cements with a lower release of toxic monomers and more prone to hydrolytic degradation while keeping the other advantages of HDBCs.

Development of hydrogels and biomimetic regulators as tissue engineering scaffolds

This paper reviews major research and development issues relating to hydrogels as scaffolds for tissue engineering, the article starts with a brief introduction of tissue engineering and hydrogels as extracellular matrix mimics, followed by a description of the various types of hydrogels and preparation methods, before a discussion of the physical and chemical properties that are important to their application. There follows a short comment on the trends of future research and development. Throughout the discussion there is an emphasis on the genetic understanding of bone tissue engineering application.

Intestinal tissue engineering: current concepts and future vision of regenerative medicine in the gut

Functional tissue engineering of the gastrointestinal (GI) tract is a complex process aiming to aid the regeneration of structural layers of smooth muscle, intrinsic enteric neuronal plexuses, specialized mucosa, and epithelial cells as well as interstitial cells. The final tissue-engineered construct is intended to mimic the native GI tract anatomically and physiologically. Physiological functionality of tissue-engineered constructs is of utmost importance while considering clinical translation. The construct comprises of cellular components as well as biomaterial scaffolding components. Together, these determine the immune response a tissue-engineered construct would elicit from a host upon implantation. Over the last decade, significant advances have been made to mitigate adverse host reactions. These include a quest for identifying autologous cell sources like embryonic and adult stem cells, bone marrow-derived cells, neural crest-derived cells, and muscle derived-stem cells. Scaffolding biomaterials have been fabricated with increasing biocompatibility and biodegradability. Manufacturing processes have advanced to allow for precise spatial architecture of scaffolds to mimic in vivo milieu closely and achieve neovascularization. This review will focus on the current concepts and the future vision of functional tissue engineering of the diverse neuromuscular structures of the GI tract from the esophagus to the internal anal sphincter.

Cytotoxicity of free versus multilayered polyelectrolytes

The cytotoxicity of polyelectrolytes commonly employed for layer-by-layer deposition of polyelectrolyte multilayers (PEMUs) was assessed using rat smooth muscle A7r5 and human osteosarcoma U-2 OS cells. Cell growth, viability, and metabolic assays were used to compare the responses of both cell lines to poly(acrylic acid), PAA, and poly(allylamine hydrochloride), PAH, in solution at concentrations up to 10 mM and to varying thicknesses of (PAA/PAH) PEMUs. Cytotoxicity correlated with increasing concentration of solution polyelectrolytes for both cell types and was greater for the positively charged PAH than for the negatively charged PAA. While metabolism and proliferation of both cell types was slower on PEMUs than on tissue culture plastic, little evidence for direct toxicity on cells was observed. In fact, evidence for more extensive adhesion and cytoskeletal organization was observed with PAH-terminated PEMUs. Differences in cell activity and viability on different thickness PEMU surfaces resulted primarily from differences in attachment for these adhesion-dependent cell lines.

Quantitative multispectral imaging of Herovici's polychrome for the assessment of collagen content and tissue remodelling

Bioprosthetic devices, constructed from a variety of materials, are routinely implanted in a variety of anatomical locations. Essential to their success is the formation of a non-destructive interface with the host tissue and appropriate tissue remodelling. Traditionally, the main method of assessing the host-material interface has been qualitative histological evaluation, using pattern recognition and comparative assessment to identify changes in the normal tissue architecture that are characteristic of scar tissue. In the present study, the recently developed technique of multispectral imaging was used to revisit a little-described histological stain, Herovici's polychrome, which is capable of distinguishing between types I and III collagen. Combined, these techniques allowed quantification of collagen content and distribution of collagen types within a tissue sample. Samples of rat tail and human scar tissue were used to optimize the staining, while comparison with immunolabelled samples was used to develop a reproducible quantification system, based on the specific colour profiles for types I and III collagen. Finally the remodelling of rat abdominal wall defects repaired with crosslinked or non-crosslinked extracellular matrix scaffolds derived from porcine urinary bladder was assessed with this technique. Compared to standard histological assessment, the combination of multispectral imaging and Herovici's polychrome staining presents a quick, simple, reliable technique that can provide accurate quantification of tissue remodelling and specifically identify the expression and distribution of types I and III collagen.

Osteoinduction of human mesenchymal stem cells by bioactive composite scaffolds without supplemental osteogenic growth factors

The development of a new family of implantable bioinspired materials is a focal point of bone tissue engineering. Implant surfaces that better mimic the natural bone extracellular matrix, a naturally nano-composite tissue, can stimulate stem cell differentiation towards osteogenic lineages in the absence of specific chemical treatments. Herein we describe a bioactive composite nanofibrous scaffold, composed of poly-caprolactone (PCL) and nano-sized hydroxyapatite (HA) or beta-tricalcium phosphate (TCP), which was able to support the growth of human bone marrow mesenchymal stem cells (hMSCs) and guide their osteogenic differentiation at the same time. Morphological and physical/chemical investigations were carried out by scanning, transmission electron microscopy, Fourier-transform infrared (FTIR) spectroscopy, mechanical and wettability analysis. Upon culturing hMSCs on composite nanofibers, we found that the incorporation of either HA or TCP into the PCL nanofibers did not affect cell viability, meanwhile the presence of the mineral phase increases the activity of alkaline phosphatase (ALP), an early marker of bone formation, and mRNA expression levels of osteoblast-related genes, such as the Runt-related transcription factor 2 (Runx-2) and bone sialoprotein (BSP), in total absence of osteogenic supplements. These results suggest that both the nanofibrous structure and the chemical composition of the scaffolds play a role in regulating the osteogenic differentiation of hMSCs.

Encapsulation of adipose-derived stem cells and transforming growth factor-β1 in carrageenan-based hydrogels for cartilage tissue engineering

Tissue engineering (TE) is an emerging field for the regeneration of damaged tissues. The combination of hydrogels with stem cells and growth factors (GFs) has become a promising approach to promote cartilage regeneration. In this study, carrageenan-based hydrogels were used to encapsulate both cells and transforming growth factor-β1 (TGF-β1). The ATDC5 cell line was encapsulated to determine the cytotoxicity and the influence of polymer concentration on cell viability and proliferation. Human adipose-derived stem cells (hASCs) were encapsulated with TGF-β1 in the hydrogel networks to enhance the chondrogenic differentiation of hASCs. Specific cartilage extracellular matrix molecules expression by hASCs were observed after 14 days of cultures of the constructs under different conditions. The κ-carrageenan was found to be a suitable biomaterial for cell and GF encapsulation. The incorporation of TGF-β1 within the carrageenan-based hydrogel enhanced the cartilage differentiation of hASCs. These findings indicate that this new system for cartilage TE is very promising for injectable thermoresponsive formulation applications.

Carboxybetaine methacrylate polymers offer robust, long-term protection against cell adhesion

Films of poly(carboxybetaine methacrylate), poly(CBMA), grafted onto microetched gold slides are effective in preventing nonspecific adhesion of cells of different types. The degree of adhesion resistance is comparable to that achieved with the self-assembled monolayers, SAMs, of oligo(ethylene glycol) alkanethiolates. In sharp contrast to the SAMs, however, substrates protected with poly(CBMA) can be stored in dry state without losing their protective properties for periods up to 2 weeks.

Development and optimization of a dual-photoinitiator, emulsion-based technique for rapid generation of cell-laden hydrogel microspheres

A growing number of clinical trials explore the use of cell-based therapies for the treatment of disease and restoration of damaged tissue; however, limited cell survival and engraftment remains a significant challenge. As the field continues to progress, microencapsulation strategies are proving to be a valuable tool for protecting and supporting these cell therapies while preserving minimally invasive delivery. This work presents a novel, dual-photoinitiator technique for encapsulation of cells within hydrogel microspheres. A desktop vortexer was used to generate an emulsion of poly(ethylene glycol) diacrylate (PEGDA) or PEGDA-based precursor solution in mineral oil. Through an optimized combination of photoinitiators added to both the aqueous and the oil phase, rapid gelation of the suspended polymer droplets was achieved. The photoinitiator combination provided superior cross-linking consistency and greater particle yield, and required lower overall initiator concentrations compared with a single initiator system. When cells were combined with the precursor solution, these benefits translated to excellent microencapsulation yield with 60-80% viability for the tested cell types. It was further shown that the scaffold material could be modified with cell-adhesive peptides to be used as surface-seeded microcarriers, or additionally with enzymatically degradable sequences to support three-dimensional spreading, migration and long-term culture of encapsulated cells. Three cell lines relevant to neural stem cell therapies are demonstrated here, but this technology is adaptable, scalable and easy to implement with standard laboratory equipment, making it a useful tool for advancing the next generation of cell-based therapeutics.

The Effect of Polymer Molecular Weight and UV Radiation on Physical Properties and Bioactivities of PCL Films

Biodegradable polymers, such as polycaprolactone (PCL), have been well studied in the application of tissue engineering and drug delivery systems due to their favourable biocompatibility profiles. However, little research has discussed the influence of the molecular weight of these polymers on cell behaviours. Here, PCL films were synthesized using polymers with various molecular weight and the physical characteristics of these films and their effect on cell growth were evaluated. Results showed that the tensile strength and elongation rate of PCL films increased from 1.82 MPa and 95.3% to 2.01 MPa and 741.6% when the molecular weight (Mn) of polymers increased from 40,000 to 80,000. A significant difference in polymer crystallinity was also observed between these formulations. Moreover, we found that prolonged exposure to UV radiation of these films could lead to the breakdown of polymer chains and consequently increased the crystallinity of PCL films with a corresponding reduction in the tensile strength. Using Hs68 human foreskin fibroblasts, PCL films prepared using high molecular weight polymers (CAPA 6800) displayed a higher cell proliferation rate compared to ones prepared using low molecular weight polymers (CAPA 6400 and 6500). In contrast, a higher cell proliferation rate was observed for PCL films made of low molecular weight polymers, especially CAPA 6500, when H9c2 rat heart myoblasts were used. In conclusion, polymer molecular weight can alter the

Adhesion of osteoblasts to a nanorough titanium implant surface

This work considers the adhesion of cells to a nanorough titanium implant surface with sharp edges. The basic assumption was that the attraction between the negatively charged titanium surface and a negatively charged osteoblast is mediated by charged proteins with a distinctive quadrupolar internal charge distribution. Similarly, cation-mediated attraction between fibronectin molecules and the titanium surface is expected to be more efficient for a high surface charge density, resulting in facilitated integrin mediated osteoblast adhesion. We suggest that osteoblasts are most strongly bound along the sharp convex edges or spikes of nanorough titanium surfaces where the magnitude of the negative surface charge density is the highest. It is therefore plausible that nanorough regions of titanium surfaces with sharp edges and spikes promote the adhesion of osteoblasts.

1,4-Dioxane enhances properties and biocompatibility of polyanionic collagen for tissue engineering applications

Polyanionic collagen obtained from bovine pericardial tissue submitted to alkaline hydrolysis is an acellular matrix with strong potential in tissue engineering. However, increasing the carboxyl content reduces fibril formation and thermal stability compared to the native tissues. In the present work, we propose a chemical protocol based on the association of alkaline hydrolysis with 1,4-dioxane treatment to either attenuate or revert the drastic structural modifications promoted by alkaline treatments. For the characterization of the polyanionic membranes treated with 1,4-dioxane, we found that (1) scanning electron microscopy (SEM) shows a stronger reorientation and aggregation of collagen microfibrils; (2) histological evaluation reveals recovering of the alignment of collagen fibers and reassociation with elastic fibers; (3) differential scanning calorimetry (DSC) shows an increase in thermal stability; and (4) in biocompatibility assays there is a normal attachment, morphology and proliferation associated with high survival of the mouse fibroblast cell line NIH3T3 in reconstituted membranes, which behave as native membranes. Our conclusions reinforce the ability of 1,4-dioxane to enhance the properties of negatively charged polyanionic collagen associated with its potential use as biomaterials for grafting, cationic drug- or cell-delivery systems and for the coating of cardiovascular devices.

Immunoblot analysis of proteins associated with self-assembled monolayer surfaces of defined chemistries

Intact and fragmented proteins, eluted from self-assembled monolayer (SAM) surfaces of alkanethiols of different chemistries (-CH₃, -OH, -COOH, -NH₂), following exposure to human plasma (HP) or human serum (HS), were examined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting techniques. The SAM surfaces were incubated for 1 h with 10% (v/v) sterile-filtered, heat-inactivated (h.i.) HS or 1% (v/v) sterile-filtered h.i. HP preparations [both in phosphate buffered saline (PBS)]. Adsorbed proteins were eluted using 10% SDS/2.3% dithioerythritol for characterization of protein profiles. The type of incubating medium may be an important determinant of adsorbed protein profiles, since some variations were observed in eluates from filtered versus control unfiltered h.i. 10% HS or 1% HP. Albumin and apolipoprotein A1 were consistently detected in both filtered h.i 10% HS and 1% HP eluates from all SAM surfaces and from control tissue culture-treated polystyrene (TCPS). Interestingly, Factor H and Factor I, antithrombin, prothrombin, high molecular weight kininogen (HMWK), and IgG were present in eluates from OH, COOH, and NH₂ SAM surfaces and in eluates from TCPS but not in eluates from CH₃ SAM surfaces, following exposure to filtered h.i. 10% HS. These results suggest that CH₃ SAM surfaces were the least proinflammatory of all SAM surfaces. Overall, similar trends were observed in the profiles of proteins eluted from surfaces exposed to filtered 10% HS or 1% HP. However, the unique profiles of adsorbed proteins on different SAM surface chemistries may be related to their differential interactions with cells, including immune/inflammatory cells.

The physics of cancer: the role of physical interactions and mechanical forces in metastasis

Metastasis is a complex, multistep process responsible for >90% of cancer-related deaths. In addition to genetic and external environmental factors, the physical interactions of cancer cells with their microenvironment, as well as their modulation by mechanical forces, are key determinants of the metastatic process. We reconstruct the metastatic process and describe the importance of key physical and mechanical processes at each step of the cascade. The emerging insight into these physical interactions may help to solve some long-standing questions in disease progression and may lead to new approaches to developing cancer diagnostics and therapies.

Nanomaterials: amyloids reflect their brighter side

Amyloid fibrils belong to the group of ordered nanostructures that are self-assembled from a wide range of polypeptides/proteins. Amyloids are highly rigid structures possessing a high mechanical strength. Although amyloids have been implicated in the pathogenesis of several human diseases, growing evidence indicates that amyloids may also perform native functions in host organisms. Discovery of such amyloids, referred to as functional amyloids, highlight their possible use in designing novel nanostructure materials. This review summarizes recent advances in the application of amyloids for the development of nanomaterials and prospective applications of such materials in nanotechnology and biomedicine.

Biomaterials that regulate growth factor activity via bioinspired interactions

Growth factor activity is localized within the natural extracellular matrix (ECM) by specific non-covalent interactions with core ECM biomolecules, such as proteins and proteoglycans. Recently, these interactions have inspired us and others to develop synthetic biomaterials that can non-covalently regulate growth factor activity for tissue engineering applications. For example, biomaterials covalently or non-covalently modified with heparin glycosaminoglycans can augment growth factor release strategies. In addition, recent studies demonstrate that biomaterials modified with heparin-binding peptides can sequester cell-secreted heparin proteoglycans and, in turn, sequester growth factors and regulate stem cell behavior. Another set of studies show that modular versions of growth factor molecules can be designed to interact with specific components of natural and synthetic ECMs, including collagen and hydroxyapatite. In addition, layer-by-layer assemblies of GAGs and other natural polyelectrolytes retain growth factors at a cell-material interface via specific non-covalent interactions. This review will detail the various bioinspired strategies being used to non-covalently localize growth factor activity within biomaterials, and will highlight in vivo examples of the efficacy of these materials to promote tissue regeneration.

Micro-/nano-engineered cellular responses for soft tissue engineering and biomedical applications

The development of biomedical devices and reconstruction of functional ex vivo tissues often requires the need to fabricate biomimetic surfaces with features of sub-micrometer precision. This can be achieved with the advancements in micro-/nano-engineering techniques, allowing researchers to manipulate a plethora of cellular behaviors at the cell-biomaterial interface. Systematic studies conducted on these 2D engineered surfaces have unraveled numerous novel findings that can potentially be integrated as part of the design consideration for future 2D and 3D biomaterials and will no doubt greatly benefit tissue engineering. In this review, recent developments detailing the use of micro-/nano-engineering techniques to direct cellular orientation and function pertinent to soft tissue engineering will be highlighted. Particularly, this article aims to provide valuable insights into distinctive cell interactions and reactions to controlled surfaces, which can be exploited to understand the mechanisms of cell growth on micro-/nano-engineered interfaces, and to harness this knowledge to optimize the performance of 3D artificial soft tissue grafts and biomedical applications.

Analysis of the roles of microporosity and BMP-2 on multiple measures of bone regeneration and healing in calcium phosphate scaffolds

Osteoinductive agents, such as BMP-2, are known to improve bone formation when combined with scaffolds. Microporosity (<20 μm) has also been shown to influence bone regeneration in calcium phosphate (CaP) scaffolds. However, many studies use only the term "osteoconductive" to describe the effects of BMP-2 and microporosity on bone formation, and do not assess the degree of healing that occurred. The objective of this study was to quantify the influence of BMP-2 and microporosity on bone regeneration and healing in biphasic calcium phosphate scaffolds using multiple measures including bone volume fraction, radial distribution, and specific surface area. These measures were quantitatively compared by analyzing microcomputed tomography data and used to formally define and assess healing. A custom image segmentation program was used to segment >100 samples, with 900 images each, that were implanted in porcine mandibular defects for 3, 6, 12 and 24 weeks. The assessment of healing presented in this work demonstrates the level of detail possible in evaluating scaffold-guided bone regeneration. The analysis shows that BMP-2 and microporosity accelerate healing up to 4-fold. BMP-2 and microporosity were shown to have different and complementary roles in bone formation that effect the time needed for a defect to heal.