Biomaterials (v.27, #30)

Calendar (I).

Biomimetic phosphorylcholine polymer grafting from polydimethylsiloxane surface using photo-induced polymerization by Tatsuro Goda; Tomohiro Konno; Madoka Takai; Toru Moro; Kazuhiko Ishihara (5151-5160).
The biomimetic synthetic phospholipid polymer containing a phosphorylcholine group, 2-methacryloyloxyethyl phosphorylcholine (MPC), has improved the surface property of biomaterials. Both hydrophilic and anti-biofouling surfaces were prepared on polydimethylsiloxane (PDMS) with MPC grafted by surface-initiated photo-induced radical polymerization. Benzophenone was used as the photoinitiator. The quantity of the adsorbed initiator on PDMS was determined by UV absorption and ellipsometry. The poly(MPC)-grafted PDMS surfaces were characterized by XPS, ATR-FTIR and static water contact angle (SCA) measurements. The SCA on PDMS decreased from 115° to 25° after the poly(MPC) grafting. The in vitro single protein adsorption on the poly(MPC)-grafted PDMS decreased 50–75% compared to the unmodified PDMS. The surface friction of the poly(MPC)-grafted PDMS was lower than the unmodified PDMS under wet conditions. The oxygen permeability of the poly(MPC)-grafted PDMS was as high as the unmodified PDMS. The tensile property of PDMS was maintained at about 90% of the ultimate stress and strain after the poly(MPC) grafting. The surface-modified PDMS is expected to be a novel medical elastomer which possesses an excellent surface hydrophilicity, anti-biofouling property, oxygen permeability and tensile property.
Keywords: Polydimethylsiloxane; Phosphorylcholine; Protein adsorption; Wettability; Oxygen permeation; Friction;

Association between UHMWPE particle-induced inflammatory osteoclastogenesis and expression of RANKL, VEGF, and Flt-1 in vivo by Wei Ping Ren; David C. Markel; Renwen Zhang; Xin Peng; Bin Wu; Hawkins Monica; Paul H. Wooley (5161-5169).
Wear debris-induced vascularized granulomatous periprosthetic tissue may augment the progress of prosthetic loosening, a major clinical problem after total joint replacement. The purpose of this study is to investigate the association of ultra-high-molecular-weight polyethylene (UHMWPE) particle-induced inflammatory osteoclastogenesis and expression of RANK/RANKL and VEGF/VEGF receptors (Flt-1 and Flk-1) using a mouse osteolysis model. UHMWPE particles were introduced into established air pouches on BALB/c mice, followed by implantation of calvaria bone from syngeneic littermates. Mice were injected with either recombinant VEGF or VEGF inhibitor (VEGF R2/F c Chimera). Mice without drug treatment, as well as mice injected with saline alone were included. Each group contains 10 mice. Pouch tissues were harvested 2 weeks after bone implantation for histological and molecular analysis. UHMWPE stimulation significantly increased VEGF gene expression, and exerted a lower enhancement effect on the gene expression of Flt-1 and Flk-1. UHMWPE-stimulated VEGF production was markedly reduced by VEGF inhibitor treatment. Immunofluorescent staining indicated that pouch tissue macrophages were the main source of both VEGF and Flt-1 production. A positive association was observed between tissue inflammation and the levels of VEGF and Flt-1 gene transcripts. Both RANK and RANKL gene transcripts were significantly increased by UHMWPE stimulation, which was subsequently reduced by VEGF inhibitor treatment ( p < 0.05 ). VEGF treatment increased TRAP+ cells in pouches either with or without UHMWPE particle stimulation, and VEGF inhibitor treatment caused a significant reduction in the number of TRAP+ cells in UHMWPE-containing pouches. This study suggests that VEGF has a role in the regulation of RANK/RANKL-mediated osteoclastogenesis, and warrant future investigations to elucidate the role of VEGF signaling in the pathogenesis of prosthetic loosening.
Keywords: VEGF; RANKL; Flt-1; Wear debris; Osteoclastogenesis;

The role of titanium surface topography on J774A.1 macrophage inflammatory cytokines and nitric oxide production by Kai Soo Tan; Li Qian; Roy Rosado; Patrick M. Flood; Lyndon F. Cooper (5170-5177).
A role for monocyte/macrophage modulation of wound healing at endosseous implants is proposed. The modification of the endosseous implant surface topography can alter cell adhesion and resultant cell behavior. The aim of this study was to define the effect of increased cpTitanium surface topography on adherent J744A.1 macrophage phenotype in culture. The J744A.1 cells were cultured on 20 mm diameter cpTitanium disks prepared with smooth and grit-blasted/acid rough surface topographies for 24–72 h. Following culture in growth media with or without lipopolysaccharide (LPS), total RNA was isolated and real-time polymerase chain reaction (PCR) was used to measure the steady-state levels of the pro-inflammatory cytokines interleukin 1-beta (IL-1β) and interleukin 6 (IL-6) and the anti-inflammatory cytokine interleukin-10 (IL-10). Additional evidence of pro-inflammatory signaling was sought by measurement of cellular nitric oxide (NO) production. In the absence of LPS, IL-1β levels were increased on grit-blasted/acid rough surfaces during the first 48 h. In contrast, IL-6 levels were reduced on the grit-blasted/acid rough surfaces. When cultures were treated with LPS, high levels of IL-1β and IL-6 expression were measured, irrespective of surface topography. The responses of J744A.1 cells to surface and superimposed LPS stimulation suggest only modest effects of the modeled endosseous implant surface on adherent cell pro-inflammatory cytokine expression and NO signaling.
Keywords: Surface topography; Macrophage; Interleukins; Nitric oxide;

A pH- and thermo-sensitive block copolymer was synthesized by adding pH-sensitive sulfamethazine oligomers (SMOs) to either end of a thermo-sensitive poly(ε-caprolactone-co-lactide)–poly(ethylene glycol)–poly(ε-caprolactone-co-lactide) (PCLA–PEG–PCLA) block copolymer. The resulting pH- and thermo-sensitive SMO–PCLA–PEG–PCLA–SMO block copolymer solution did not form a gel at high pH (pH 8.0) or at increased temperatures (ca. 70 °C), but did form a stable gel under physiological conditions (pH 7.4 and 37 °C). The degradation rate of the pH- and thermo-sensitive block copolymer decreased substantially compared with the control block copolymer of PCLA–PEG–PCLA, due to the buffering effect of the SMO–PCLA–PEG–PCLA–SMO sulfonamide groups on the acidic monomer-induced rapid degradation of PCLA–PEG–PCLA. This suitable sol–gel transition and sustained biodegradability of the pH- and thermo-sensitive SMO–PCLA–PEG–PCLA–SMO block copolymer resolves two of the major drawbacks associated with thermo-sensitive block copolymers, namely premature gelation and rapid degradation. Interestingly, SMO–PCLA–PEG–PCLA–SMO showed no evidence of cytotoxicity in vitro. However, subcutaneous injection of the pH- and thermo-sensitive block copolymer solution (20 wt% in PBS at pH 8.0) into Sprague–Dawley (SD) rats resulted in rapid, stable gel formation, with the injected hydrogel being completely degraded in vivo in just 6 weeks. The injected hydrogel in vivo presented a typical acute inflammation within 2 weeks, although chronic inflammation was not observed during the first 6-week period. As such, the pH- and thermo-sensitive hydrogel of the SMO–PCLA–PEG–PCLA–SMO block copolymer is a suitable candidate for use in drug delivery systems and cell therapy.
Keywords: pH- and thermo-sensitive hydrogel; Sulfonamide-modified block copolymer; Biodegradability; Biocompatibility;

In vivo behavior of calcium phosphate scaffolds with four different pore sizes by Marie-Cécile von Doernberg; Brigitte von Rechenberg; Marc Bohner; Sonja Grünenfelder; G Harry van Lenthe; Ralph Müller; Beat Gasser; Robert Mathys; Gamal Baroud; Jörg Auer (5186-5198).
The goal of the present study was to assess the effect of macropore size on the in vivo behavior of ceramic scaffolds. For that purpose, β-tricalcium phosphate (β-TCP) cylinders with four different macropore sizes (150, 260, 510, and 1220 μm) were implanted into drill hole defects in cancellous bone of sheep and their resorption behavior was followed for 6, 12 and 24 weeks. The scaffolds were evaluated for biocompatibility, and new bone formation was observed macroscopically, histologically and histomorphometrically. Histomorphometrical measurements were performed for the whole defect area and for the area subdivided into three concentric rings (outer, medial, and inner ring). All implants were tolerated very well as evidenced by the low amount of inflammatory cells and the absence of macroscopic signs of inflammation. Resorption proceeded fast since less than 5% ceramic remained at 24-week implantation. Hardly any effect of macropore size was observed on the in vivo response. Samples with an intermediate macropore size (510 μm) were resorbed significantly faster than samples with smaller macropore sizes (150 and 260 μm). However, this fast resorption was associated with a lower bone content and a higher soft tissue content. At 12 and 24 weeks, the latter differences had disappeared. Bone was more abundant in the outer ring than in the rest of the blocks at 6 weeks, and in the outer and medial ring compared to the inner ring at 12 weeks.
Keywords: β-tricalciumphosphate; Pore size; Resorption; Bone; Biocompatibility; Ingrowth;

Differential inflammatory macrophage response to rutile and titanium particles by Gema Vallés; Pablo González-Melendi; José L. González-Carrasco; Laura Saldaña; Elena Sánchez-Sabaté; Luis Munuera; Nuria Vilaboa (5199-5211).
Titanium and its alloys are widely used as implant materials for dental and orthopaedic applications due to their advantageous bulk mechanical properties and biocompatibility, compared to other metallic biomaterials. In order to improve their wear and corrosion resistance, several surface modifications that give rise to an outer ceramic layer of rutile have been developed. The ability of rutile wear debris to stimulate the release of inflammatory cytokines from macrophages has not been addressed to date. We have compared the in vitro biocompatibility of sub-cytotoxic doses of rutile and titanium particles in THP-1 cells driven to the monocyte/macrophage differentiation pathway as well as in primary cultures of human macrophages. Confocal microscopy experiments indicated that differentiated THP-1 cells and primary macrophages efficiently internalised rutile and titanium particles. Treatment of THP-1 cells with rutile particles stimulated the release of TNF-α, IL-6 and IL-1β to a lesser extent than titanium. The influence of osteoblasts on the particle-induced stimulation of TNF-α and IL-1β was analysed by co-culturing differentiated THP-1 cells with human primary osteoblasts. Under these conditions, secretion levels of both cytokines after treatment of THP-1 cells with rutile particles were lower than after exposure to titanium. Finally, we observed that primary macrophages released higher amounts of TNF-α, IL-6 and IL-1β after incubation with titanium particles than with rutile. Taken together, these data indicate that rutile particles are less bioreactive than titanium particles and, therefore, a higher biocompatibility of titanium-based implants modified with an outer surface layer of rutile is expected.
Keywords: Osteolysis; Wear debris; Macrophage; Cytokine; Titanium; Titanium oxide;

The correlation of RANK, RANKL and TNFα expression with bone loss volume and polyethylene wear debris around hip implants by Christopher A. Holding; David M. Findlay; Roumen Stamenkov; Susan D. Neale; Helen Lucas; A.S.S.K. Dharmapatni; Stuart A. Callary; Kush R. Shrestha; Gerald J. Atkins; Donald W. Howie; David R. Haynes (5212-5219).
This study investigates receptor activator NF-κB (RANK), RANK ligand (RANKL) and tumour necrosis factor (TNFα), key factors regulating bone turnover, present in the tissues near peri-prosthetic osteolysis. Tissue was obtained from zones of peri-prosthetic osteolysis from 11 patients undergoing revision of total hip prostheses, analysed preoperatively by high-resolution spiral multislice CT using a metal artefact suppression protocol. Synovial tissue from 10 patients with osteoarthritis undergoing primary hip replacement was used as control tissue. Immunohistochemical analysis of formalin fixed tissue sections demonstrated that RANK, RANKL and TNFα were strongly expressed by large multinucleated cells containing polyethylene wear debris in revision tissues. Control tissue stained weakly for RANK, RANKL and TNFα. A strong statistical correlation ( p < 0.02 ) was found between the five parameters, volume of bone loss, polyethylene wear debris, RANK, RANKL and TNFα expression. Importantly, in vitro studies revealed that RANKL and TNFα synergise to increase the volume of bone resorbed, by more than seven fold, when compared to the effect of either cytokine treatment alone. This suggests that the interaction of TNFα and RANKL promotes osteoclast activity associated with polyethylene wear and therapies targeting TNF activity may be useful to treat peri-implant osteolysis.
Keywords: Osteolysis; Osteoclast; Wear debris; Polyethylene; Image analysis;

The aim of this study was to investigate the potential of using PDLLA/45S5 (PDLLA—poly(d,l-lactide)) Bioglass® composite films for the culture of annulus fibrosus (AF) cells in vitro with a view to a tissue engineering application. PDLLA films incorporated with different percentages (0, 5 and 30 (wt%)) of Bioglass® particles were prepared by solvent casting and characterized by scanning electron microscopy (SEM), water contact angle and white-light interferometry. Bovine AF cell morphology and attachment were analysed using SEM. Cytoskeletal organization was determined by actin labelling with FITC-phalloidin using fluorescence microscopy. The amount of sulphated glycosaminoglycan (sGAG) and collagen produced by AF cells were quantified using the 1,9-dimethylmethylene blue (DMMB) and Sircol™ assays after 4 weeks in culture. Composite films of PDLLA filled with Bioglass® are an appropriate substrate for annulus cells and these films promote the production of an extracellular matrix (ECM) containing abundant sGAGs and collagen. These findings provide a basis for the understanding of the production of ECM molecules by cells cultured on 2D PDLLA/45S5 Bioglass® composite films. The results will provide new insights into the design and development of composites containing Bioglass® and resorbable polymers as scaffolds for intervertebral disc tissue repair.
Keywords: Intervertebral disc; Tissue engineering; Annulus fibrosus cells; Bioactive glasses; Polylactic acid;

The effect of topographic characteristics on cell migration velocity by Jean-Pierre Kaiser; Andreas Reinmann; Arie Bruinink (5230-5241).
The migration of cells on structured surfaces is known to be affected by its surface topography. Although the effects of topography have been extensively investigated the crucial parameters determining the cell–surface reaction are largely unknown. The present study was performed to describe and to define the role of groove/elevation (ridge) dimensions at the micrometre scale on fibroblast cell migration by correlating cell shape, migration angle α, cell orientation β and velocity with these dimensions. For this a quantitative method was developed. We could show that the surface structures significantly influenced migration direction α, cell orientation β and mean velocity, as well as migration speed in the directions parallel and perpendicular to the grooves/elevations in a surface structure dependant way. Cell migration velocity parallel, respectively, perpendicular to the structures was significantly affected by the geometries and dimensions of the substratum. Surface structures were not able to significantly affect distribution patterns of cell shapes. Overall, it could be shown that differently structured surfaces influenced the cells but no crucial feature could be clearly identified, suggesting that the reaction of the surface structure might be far more complex than generally is assumed.
Keywords: Cell migration; Cell orientation; Cell shape; Image analysis; Fibroblast;

Heparin-regulated release of growth factors in vitro and angiogenic response in vivo to implanted hyaluronan hydrogels containing VEGF and bFGF by Daniel B. Pike; Shenshen Cai; Kyle R. Pomraning; Matthew A. Firpo; Robert J. Fisher; Xiao Zheng Shu; Glenn D. Prestwich; Robert A. Peattie (5242-5251).
Controlled release of human vascular endothelial growth factor (VEGF) or basic fibroblast growth factor (bFGF) from hydrogels composed of chemically modified hyaluronan (HA) and gelatin (Gtn) was evaluated both in vitro and in vivo. We hypothesized that inclusion of small quantities of heparin (Hp) in these gels would regulate growth factor (GF) release over an extended period, while still maintaining the in vivo bioactivity of released GFs. To test this hypothesis, HA, Gtn, and Hp (15 kDa) were modified with thiol groups, then co-crosslinked with poly (ethylene glycol) diacrylate (PEGDA). Either VEGF or bFGF was incorporated into the gels before crosslinking with PEGDA. Release of these GFs in vitro could be sustained over 42 days by less than 1% Hp content, and was found to decrease monotonically with increasing Hp concentration. As little as 0.03% Hp in the gels reduced the released VEGF fraction from 30% to 21%, while 3% Hp reduced it to 19%. Since the minimum Hp concentration capable of effective controlled GF release in vitro was found to be 0.3% (w/w), this concentration was selected for subsequent in vivo experiments. To evaluate the bioactivity of released GFs in vivo, gel samples were implanted into the ear pinnas of Balb/c mice and the resulting neovascularization response measured. In the presence of Hp, vascularization was sustained over 28 days. GF release was more rapid in vitro from gels containing Gtn than from gels lacking Gtn, though unexpectedly, the in vivo neovascularization response to Gtn-containing gels was decreased. Nevertheless significant numbers of neovessels were generated. The ability to stimulate localized microvessel growth at controlled rates for extended times through the release of GFs from covalently linked, Hp-supplemented hydrogels will ultimately provide a powerful therapeutic tool.
Keywords: Angiogenesis; Controlled drug release; Glycosaminoglycans; Growth factors; Cytokine;

Many cells in the body reside in a complex three-dimensional (3D) environment stimulated by mechanical force. In vitro bioreactor systems have greatly improved our understanding of the mechanisms behind cell mechanotransduction. Current systems to impose strain in vitro are limited either by the lack of uniform strain profile or inability to strain 3D engineered tissues. In this study, we present a system capable of generating cyclic equibiaxial strain to an engineered vascular wall model. Type I collagen hydrogels populated with rat aortic smooth muscle cells (RASMCs) were created either as a compacting disk or constrained hemisphere. Both models were adhered to silicone membranes precoated with collagen I, fibronectin, or Cell-Tak and assayed for adhesion characteristics. The best performing model was then exposed to 48 h of 10% strain at 1 Hz to simulate wall strain profiles found in vascular aneurysms, with static cultures serving as controls. The finite strain profile at the level of the membrane and the free surface of the construct was quantified using microbeads. The results indicate that the hemisphere model adhered with Cell-Tak had the most stable adhesion, followed by fibronectin and collagen I. Disk models did not adhere well under any coating condition. Uniform strain propagation was possible up to a maximum area strain of 20% with this system. RASMC responded to 10% equibiaxial strain by becoming less elongated, and immunohistochemistry suggested that stretched RASMC shifted to a more synthetic phenotype in comparison to static controls. These results suggest that equibiaxial strain may induce smooth muscle cell differentiation. We conclude that this system is effective in stimulating cells with cyclic equibiaxial strain in 3D cultures, and can be applied to a variety of biomaterial and tissue engineering applications.
Keywords: Soft tissue biomechanics; Adhesion; Tissue engineering; Bioreactor;

Micromolding of photocrosslinkable chitosan hydrogel for spheroid microarray and co-cultures by Junji Fukuda; Ali Khademhosseini; Yoon Yeo; Xiaoyu Yang; Judy Yeh; George Eng; James Blumling; Chi-Fong Wang; Daniel S. Kohane; Robert Langer (5259-5267).
Bioengineering approaches, such as co-cultures of multiple cell types, that aim to mimic the physiological microenvironment may be beneficial for optimizing cell function and for engineering tissues in vitro. This study describes a novel method for preparing a spheroid microarray on microfabricated hydrogels, alone or in co-cultures. Photocrosslinkable chitosan was synthesized and utilized for fabricating hydrogel microstructures through a micromolding process. The chitosan surface was initially cell repellent but became increasingly cell adhesive over time. By using this unique property of chitosan hydrogels, it was possible to generate patterned co-cultures of spheroids and support cells. In this scheme, cells were initially microarrayed within low shear stress regions of microwells. Human hepatoblastoma cells, Hep G2, seeded in these wells formed spheroids with controlled sizes and shapes and stably secreted albumin during the culture period. The change of cell adhesive properties in the chitosan surface facilitated the adhesion and growth of a second cell type, NIH-3T3 fibroblast, and therefore enabled co-cultures of hepatocyte spheroids and fibroblast monolayers. This co-culture system could be a useful platform for studying heterotypic cell–cell interactions, for drug screening, and for developing implantable bioartificial organs.
Keywords: Spheroid; Chitosan; Hydrogel; Co-culture; Hepatocyte; Fibroblast;

Collagen mimetic peptide-conjugated photopolymerizable PEG hydrogel by H. Janice Lee; Jin-Soo Lee; Thanissara Chansakul; Christopher Yu; Jennifer H. Elisseeff; Seungju M. Yu (5268-5276).
Collagen mimetic peptide (CMP) with a specific amino acid sequence, –(Pro–Hyp–Gly) x –, forms a triple helix conformation that resembles the native protein structure of natural collagens. CMP previously has been shown to associate with type I collagen molecules and fibers via a strand invasion process. We hypothesized that when poly(ethylene glycol) (PEG) hydrogel, a non-adhesive tissue engineering scaffold, is conjugated with CMP, it may retain cell-secreted collagens and also form physical crosslinks that can be manipulated by cells. A photopolymerizable CMP derivative was synthesized and copolymerized with poly(ethylene oxide) diacrylate to create a novel PEG hydrogel. In a model retention experiment, diffusional loss of type I collagen that was added to the hydrogel was limited. Chondrocytes were encapsulated in the hydrogel to examine its use as a tissue engineering scaffold. After 2 weeks, the biochemical analysis of the CMP-conjugated PEG gel revealed an 87% increase in glycosaminoglycan content and a 103% increase in collagen content compared to that of control PEG hydrogels. The histology and immunohistochemistry analyses also showed increased staining of extracellular matrix. These results indicate that the CMP enhances the tissue production of cells encapsulated in the PEG hydrogel by providing cell-manipulated crosslinks and collagen binding sites that simulate natural extracellular matrix.
Keywords: Collagen; Polyethylene oxide; Hydrogel; Chondrocyte; Cell encapsulation; Scaffold;

Synthesis of cell-adhesive dextran hydrogels and macroporous scaffolds by Stéphane G. Lévesque; Molly S. Shoichet (5277-5285).
Dextran hydrogels have been previously investigated as drug delivery vehicles and more recently as macroporous scaffolds; however, the non-cell-adhesive nature of dextran has limited its utility for tissue engineering. To overcome this limitation, macroporous scaffolds of methacrylated dextran (Dex-MA) copolymerized with aminoethyl methacrylate (AEMA) were synthesized, thereby introducing primary amine groups for covalent immobilization of extracellular-matrix-derived peptides. The amino group density for hydrogels copolymerized with 0.5 wt% AEMA was found to be 36.1±0.4 μmol/cm3 by elemental analysis. To further enhance cellular interaction, poly(Dex-MA-co-AEMA) hydrogels were modified with either CRGDS or a mixture of CDPGYIGSR and CQAASIKVAV (1:1, v/v) using sulfo-(N-maleimidomethyl)cyclohexane-1-carboxylate (sulfo-SMCC). The immobilized peptide concentration was determined using amino acid analysis at: 2.6±0.9 μmol/cm3 for CRGDS-derived hydrogels and 2.2±0.3 μmol/cm3 plus 1.9±0.2 μmol/cm3 for CDPGYIGSR plus CQAASIKVAV-derived hydrogels, respectively. Cellular interactions of primary embryonic chick dorsal root ganglia (DRGs) were compared on the hydrogels. Cell adhesion and neurite outgrowth on poly(Dex-MA) increased with copolymerization of AEMA and further improved with peptide modification and significantly for CDPGYIGSR/CQAASIKVAV-derived poly(Dex-MA-co-AEMA) hydrogels. Moreover, DRGs penetrated within the first 600 μm of the scaffolds, thereby demonstrating the potential of this scaffold for guided cell and axonal regeneration in vivo.
Keywords: Dextran; Cell adhesion; Macroporous structure; Nerve tissue engineering;

The in vitro adsorption of cytokines by polymer-pyrolysed carbon by Carol A. Howell; Susan R. Sandeman; Gary J. Phillips; Andrew W. Lloyd; J. Graham Davies; Sergey V. Mikhalovsky; Steve R. Tennison; Anthony P. Rawlinson; Oleksandr P. Kozynchenko; Hannah L.H. Owen; John D.S. Gaylor; Jennifer J. Rouse; James M. Courtney (5286-5291).
This study investigated a range of phenol–formaldehyde–aniline-based pyrolysed carbon matrices and their component materials, for their ability to adsorb a range of inflammatory cytokines crucial to the progression of sepsis. The efficiency of adsorption of the target molecules from human plasma was assessed and compared to that of Adsorba® 300C, a commercially available cellulose-coated activated charcoal. Results indicate that a number of the primary carbon/resin materials demonstrate efficient adsorption of the cytokines studied here (TNF, IL-6 and IL-8), comparable to other adsorbents under clinical investigation. Our findings also illustrate that these adsorbent capabilities are retained when the primary particles are combined to form a pyrolysed carbon matrix. This capability will enable the engineering of the carbon matrix porosity allowing a blend of carbonised particle combinations to be tailored for maximum adsorption of inflammatory cytokines. The present findings support further investigation of this carbon material as a combined carbon-based filtration/adsorbent device for direct blood purification.
Keywords: Adsorption; Carbon; Cytokine; Extracorporeal circulation; Sepsis;

A glutathione-sensitive cross-linked polyethylenimine gene vector CLPEI50% was specially designed via the cross-linking reaction between the low molecular weight polyethylenimine (PEI1800) and dimethyl 3.3′-dithiopropionimidate dihydrochloride (DTBP). The acid–base titration test indicated that CLPEI50% still possessed efficient proton sponge effect. The property of CLPEI50%–DNA complexes were investigated by atomic force microscopy (AFM) and dynamic light scattering (DLS). CLPEI50% induced DNA condensation and formed spherical nanoparticles. The diameter of polyplexes prepared at pH value of 6.0 and 7.4 was about 150 and 260 nm, respectively. It was interesting to find the polyplexes were sensitive to the reductive glutathione (GSH). The CLPEI50%–DNA polyplexes prepared at N/P ratio of 10 were unpacked at GSH concentration of 3 mm, which was comparable to the intracellular environment. The in vitro cytotoxicity of CLPEI50% was also significantly reduced comparing with PEI25k. The biomimetic CLPEI50%–DNA polyplexes with the low cytotoxicity and GSH-sensitive property could be a good candidate for gene delivery.
Keywords: Gene delivery; Biomimetic; Glutathione-sensitive; Low cytotoxicity;

Polymer microarrays: Identification of substrates for phagocytosis assays by Alexandra Mant; Guilhem Tourniaire; Juan J. Diaz-Mochon; Tim J. Elliott; Anthony P. Williams; Mark Bradley (5299-5306).
A polymer microarray of 120 polyurethanes was used to identify polymers that promoted the adhesion of bone marrow dendritic cells (BMDC). Identified polymers were coated onto glass cover slips and shown to be efficient substrates for the immobilisation of these primary cells, which underwent efficient phagocytosis while still presumably maintaining their immature state.
Keywords: Dendritic cells; Microarrays; Polyurethane; Cell adhesion; Phagocytosis;

The specific adhesive interaction between a non-spherical particle and a cell layer under a linear shear flow is analyzed. The effect of the characteristic particle size, expressed in terms of the volume V , and shape, expressed in terms of the aspect ratio γ , on the adhesive strength is investigated. It is shown that for a fixed shape, there exists an optimal volume V opt for which the adhesive strength has a maximum. A surprisingly accurate relationship has been derived between the optimal volume V opt and the ratio μ S / m r (wall shear stress to the receptors surface density) having the form V opt = α ( m r / μ S ) β . Also, oblate particles have been shown to adhere more effectively to the biological substrate than classical spherical particles for the same volume V . As a consequence, non-spherical particles can carry a larger amount of drugs and contrast agents than classical spherical particles with the same adhesive strength, improving the therapeutic and imaging efficacy. The formulae and the procedures described in the present work can guide the optimal design of intravascularly injectable micro/nano carriers.
Keywords: Adhesion; Micro/nano particle; Endothelium; Biomimetic material; Drug delivery;

A constitutive model for protein-based materials by Xiaoyi Wu; Marc E. Levenston; Elliot L. Chaikof (5315-5325).
Protein-based materials are critical to the construction of tissue substitutes that exhibit precisely defined mechanical properties. Under physiologically relevant conditions, materials derived from natural or synthetic structural proteins are characterized by nonlinear elastic responses at medium and large deformations, time-dependent or viscoelastic behavior, and display the effects of strain-induced structural changes. Although a constitutive model that accurately describes mechanical behavior is essential for the rational design of tissue constructs, few models account for all of these characteristics. In this report, we present a new constitutive model for protein based materials, in which nonlinear elasticity is captured by the Arruda–Boyce eight-chain model, time dependant viscoelasticity is described by a generalized Maxwell model, and the effect of strain-induced structural change is incorporated using a network alteration theory originally proposed by Tobolsky. The model was applied to a number of protein-based materials and cell containing constructs, including recombinant elastin-mimetic protein polymers and fibroblast populated collagen gel matrices. Significantly, numerical implementation of this model is straightforward and mechanical behavior accurately described under a variety of loading conditions. Moreover, when calibrated using stress relaxation data alone, the model accurately predicted cyclic loading responses. Although limitations exist, this model provides a convenient tool to correlate viscoelastic data obtained by different testing modes and may assist in reducing the number of experimental tests required to fully capture the range of viscoelastic responses of protein-based materials.
Keywords: Constitutive model; Protein polymer; Protein-based material;

Micro-finite element models of bone tissue-engineering scaffolds by Damien Lacroix; Arnaud Chateau; Maria-Pau Ginebra; Josep A. Planell (5326-5334).
Tissue engineering is an emerging area in bioengineering at the frontiers between biomaterials, biology and biomechanics. The basic knowledge of the interactions between mechanical stimuli, cells and biomaterials is growing but the quantitative effect of mechanical stimuli on cells attached to biomaterials is still unknown. The objective of this study was to develop finite element models of various bone scaffolds based on calcium phosphate in order to calculate the load transfer from the biomaterial structure to the biological entities. Samples of porous calcium phosphate bone cement and biodegradable glass were scanned using micro-CT to determine the overall macroporosity, architecture and to develop finite element models of such materials. Compressive loads were applied on the models to simulate the in vitro environment of a bioreactor and stress and strain distributions were calculated. It was found that the effective Young's modulus was linearly related to the sample macroporosity. Results suggest that a 0.5% overall compressive strain can produce internal strain of the same order of magnitude as found in previous in vitro mechanically cell-strained studies or in mechanoregulation studies. Stress and strain concentrations due to the porous structures are possible candidate for favouring cell differentiation. Although strain distributions were similar between bone cement and porous glass, the stress distribution is clearly different. Future in vitro results could correlate the results obtained with such finite element study to explain the influence of mechanical stimuli on cell behaviour.
Keywords: Finite element analysis; Tissue engineering; Scaffold; Porosity; Ceramic;