Hyaluronic acid-silica nanohybrid gels.

Excessive water sorption and low mechanical properties are a severe drawback in some biomedical applications of hyaluronic acid (HA). A way to improve these properties is here explored through the novel concept of nanohybrid hydrogels consisting of a HA matrix including different amounts of silica-derived species. This inorganic filler phase controls the mechanical and swelling properties of HA cross-linked matrices. Below a 2 wt % of silica in the systems, nanoparticle aggregates of tens of nanometers and silica oligomers are distributed more or less homogeneously throughout the organic matrix, without percolating. This morphology of the silica phase is accompanied by an increased swelling degree of the composite when compared with pure HA. For higher silica mass ratios in the composites the inorganic counterpart coalesces, leading to a continuous inorganic silica network interpenetrated with the organic HA network, which coexists with a dispersed phase of silica-nanoparticle aggregates. Silica oligomers originating in the exposition of the nanoparticles to reactives during the composite preparation procedure contribute to the continuity of the silica network. For these compositions, swelling is reduced three times when compared with pure HA, and a significant improvement of the mechanical properties occurs. Water-containing samples of these materials exhibited a glass transition, which pure dry HA does not. None of the compositions studied showed any cytotoxicity. Thus, the materials could be of use in tissue engineering applications where these properties of HA need to be modulated.

Regenerative and Resorbable PLA/HA Hybrid Construct for Tendon/Ligament Tissue Engineering.

Tendon and ligament shows extremely limited endogenous regenerative capacity. Current treatments are based on the replacement and or augmentation of the injured tissue but the repaired tissue rarely achieve functionality equal to that of the preinjured tissue. To address this challenge, tissue engineering has emerged as a promising strategy. This study develops a regenerative and resorbable hybrid construct for tendon and ligament engineering. The construct is made up by a hollow poly-lactic acid braid with embedded microspheres carrying cells and an anti-adherent coating, with all the parts being made of biodegradable materials. This assembly intends to regenerate the tissue starting from the interior of the construct towards outside while it degrades. Fibroblasts cultured on poly lactic acid and hyaluronic acid microspheres for 6 h were injected into the hollow braid and the construct was cultured for 14 days. The cells thus transported into the lumen of the construct were able to migrate and adhere to the braid fibers naturally, leading to a homogeneous proliferation inside the braid. Moreover, no cells were found on the outer surface of the coating. Altogether, this study demonstrated that PLA/HA hybrid construct could be a promising material for tendon and ligament repair.

Positioning of the cross-stitch on the modified Kessler core tendon suture.

Cryopreserved human tendons were sutured with different variations of a modified Kessler-type grasping suture in a series of different designs in order to assess the influence of the distance between the cross-stitch on the core suture (5 and 10 mm from the cut tendon edge) on the peripheral suture. An original mathematical model was employed to explain the mechanical behavior (strength, deformation, and distribution of load) of the different suture designs. The effect of the peripheral epitendinous suture, combined with the distance of the core suture, was evaluated. The variation of core suture distance had no relevant consequences on the overall resilience of the design. However, increasing the distance between the cross-stitches of the core suture reduces the deformation that is absorbed not only by the core suture itself but also by the peripheral suture. Adding a peripheral epitendinous suture to a 10-mm design almost doubles the breaking load in absolute values. The mathematical model predicts that the peripheral suture will support a greater load when the distance of the core suture cross-stitches is increased. The evidence level is II.

Influence of the substrate's hydrophilicity on the in vitro Schwann cells viability.

A series of polymeric biomaterials including poly (methyl acrylate) (PMA), chitosan (CHT), poly(ethyl acrylate) (PEA), poly(hydroxyethyl acrylate) (PHEA), and a series of random copolymers containing ethyl acrylate and hydroxyethyl acrylate monomeric units were tested in vitro as culture substrates and compared for their impact on the proliferation and expansion of Schwann cells (SCs). Immunocytochemical staining assay and scanning electron microscopy techniques were applied to perform a quantitative analysis to determine the correct maintenance of the cultured glial cells on the different biomaterials. The results strongly suggest that cell attachment and proliferation is influenced by the substrate's surface chemistry, and that hydrophobic biomaterials based on PMA, PEA, and the copolymers PEA and PHEA in a narrow composition window are suitable substrates to promote cell attachment and proliferation of SCs in vitro.

Scaffolds of Hyaluronic Acid-Poly(Ethyl Acrylate) Interpenetrating Networks: Characterization and In Vitro Studies.

Hyaluronic acid (HA) provides many advantages to regenerative implants through its bioactive properties, but it also has many limitations as a biomaterial if it is not chemically modified. In order to overcome some of these limitations, HA has been combined with poly(ethyl acrylate) in the form of interpenetrating polymeric networks (IPNs), in which the HA network is crosslinked with divinyl sulfone. Scaffolds of this IPN have been produced through a template-leaching methodology, and their properties have been compared with those of single-network scaffolds made of either PEA or crosslinked HA. A fibroblast cell line has been used to assess the in vitro performance of the scaffolds, revealing good cell response and a differentiated behavior on the IPN surface when compared to the individual polymers. Altogether, the results confirm that this type of material offers an interesting microenvironment for cells, which can be further improved toward its potential use in medical implants.

Topologically controlled hyaluronan-based gel coatings of hydrophobic grid-like scaffolds to modulate drug delivery.

Scaffolds based on poly(ethyl acrylate) having interwoven channels were coated with a hyaluronan (HA) hydrogel to be used in tissue engineering applications. Controlled typologies of coatings evolving from isolated aggregates to continuous layers, which eventually clog the channels, were obtained by using hyaluronan solutions of different concentrations. The efficiency of the HA loading was determined using gravimetric and thermogravimetric methods, and the hydrogel loss during the subsequent crosslinking process was quantified, seeming to depend on the mass fraction of hyaluronan initially incorporated to the pores. The effect of the topologically different coatings on the scaffolds, in terms of mechanical properties and swelling at equilibrium under different conditions was evaluated and correlated with the hyaluronan mass fraction. The potential of these hydrogel coatings as vehicle for controlled drug release from the scaffolds was validated using a protein model.

Hydrophilic surface modification of acrylate-based biomaterials.

Acrylic polymers have proved to be excellent with regard to cell adhesion, colonization and survival, in vitro and in vivo. Highly ordered and regular pore structures thereof can be produced with the help of polyamide templates, which are removed with nitric acid. This treatment converts a fraction of the ethyl acrylate side groups into acrylic acid, turning poly(ethyl acrylate) scaffolds into a more hydrophilic and pH-sensitive substrate, while its good biological performance remains intact. To quantify the extent of such a modification, and be able to characterize the degree of hydrophilicity of poly(ethyl acrylate), poly(ethyl acrylate) was treated with acid for different times (four, nine and 17 days), and compared with poly(acrylic acid) and a 90/10%wt. EA/AAc copolymer (P(EA-co-AAc)). The biological performance was also assessed for samples immersed in acid up to four days and the copolymer, and it was found that the incorporation of acidic units on the material surface was not prejudicial for cells. This surface modification of 3D porous hydrophobic scaffolds makes easier the wetting with culture medium and aqueous solutions in general, and thus represents an advantage in the manageability of the scaffolds.

Scaffolds based on hyaluronan and carbon nanotubes gels.

Physico-chemical and mechanical properties of hyaluronic acid/carbon nanotubes nanohybrids have been correlated with the proportion of inorganic nanophase and the preparation procedure. The mass fraction of -COOH functionalized carbon nanotubes was varied from 0 to 0.05. Hyaluronic acid was crosslinked with divinyl sulfone to improve its stability in aqueous media and allow its handling as a hydrogel. A series of samples was dried by lyophilization to obtain porous scaffolds whereas another was room-dried allowing the collapse of the hybrid structures. The porosity of the former, together with the tighter packing of hyaluronic acid chains, results in a lower water absorption and lower mechanical properties in the swollen state, because of the easier water diffusion. The presence of even a small amount of carbon nanotubes (mass fraction of 0.05) limits even more the swelling of the matrix, owing probably to hybrid interactions. These nanohybrids do not seem to degrade significantly during 14 days in water or enzymatic medium.

Schwann-cell cylinders grown inside hyaluronic-acid tubular scaffolds with gradient porosity.

Cell transplantation therapies in the nervous system are frequently hampered by glial scarring and cell drain from the damaged site, among others. To improve this situation, new biomaterials may be of help. Here, novel single-channel tubular conduits based on hyaluronic acid (HA) with and without poly-l-lactide acid fibers in their lumen were fabricated. Rat Schwann cells were seeded within the conduits and cultured for 10days. The conduits possessed a three-layered porous structure that impeded the leakage of the cells seeded in their interior and made them impervious to cell invasion from the exterior, while allowing free transport of nutrients and other molecules needed for cell survival. The channel's surface acted as a template for the formation of a cylindrical sheath-like tapestry of Schwann cells continuously spanning the whole length of the lumen. Schwann-cell tubes having a diameter of around 0.5mm and variable lengths can thus be generated. This structure is not found in nature and represents a truly engineered tissue, the outcome of the specific cell-material interactions. The conduits might be useful to sustain and protect cells for transplantation, and the biohybrids here described, together with neuronal precursors, might be of help in building bridges across significant distances in the central and peripheral nervous system. STATEMENT OF SIGNIFICANCE: The paper entitled "Schwann-cell cylinders grown inside hyaluronic-acid tubular scaffolds with gradient porosity" reports on the development of a novel tubular scaffold and on how this scaffold acts on Schwann cells seeded in its interior as a template to produce macroscopic hollow continuous cylinders of tightly joined Schwann cells. This cellular structure is not found in nature and represents a truly engineered novel tissue, which obtains as a consequence of the specific cell-material interactions within the scaffold.

New semi-biodegradable materials from semi-interpenetrated networks of poly(ϵ-caprolactone) and poly(ethyl acrylate).

Semi-degradable materials may have many applications. Here poly(ethyl acrylate) and poly(ϵ-caprolactone) were combined as semi-interpenetrated networks, and thoroughly characterized in terms of final composition, interactions between components, wettability, and mechanical properties. PCL modulates the mechanical properties of the PEA elastomeric network. Cultures of fibroblasts and adipose-tissue derived stem cells showed excellent biological performance of the materials. The results are relevant for applications seeking materials leaving a permanent supporting skeleton after the partial degradation, as in patches for cardiac regeneration or in abdominal wall meshes.

Nonlinear viscoelastic behaviour of the flexor tendon of the human hand.

The mechanical behaviour of the flexor tendon of the human hand is here investigated from the point of view of its nonlinear viscoelasticity. The samples are subjected to several single and multiple step loading histories. A quasilinear viscoelastic constitutive relationship between strain and stress history is assumed. Its characteristic material functions are determined with the aid of simple creep results, and model predictions are compared with the experimental results of complex loading histories. The validity of the quasilinear approach to tendon behaviour is discussed in connection with the deformation mechanism suggested by it.

Electrospun adherent-antiadherent bilayered membranes based on cross-linked hyaluronic acid for advanced tissue engineering applications.

A procedure to obtain electrospun mats of hyaluronic acid (HA) stable in aqueous media in one single step has been developed. It consists in combining an HA solution with a divinyl sulfone one as cross-linker in a three-way valve to immediately electroblow their mixture. Membranes obtained with this method, after sterilization and conditioning, are ready to use in cell culture without need of any additional post-treatment. HA nanofibers are deposited onto previously electrospun poly(l-lactic acid) (PLLA) mats in order to obtain stably joined bilayered membranes with an adherent face and the opposite face non-adherent, despite their different hydrophilicity and mechanical properties. These bilayered HA/PLLA membranes may be of use, for example, in applications seeking to transplant cells on a tissue surface and keep them protected from the environment: the PLLA nanofiber face is cell friendly and promotes cell attachment and spreading and can thus be used as a cell supply vehicle, while the HA face hinders cell adhesion and thus may prevent post-surgical adherences, a major issue in many surgeries.

Grid polymeric scaffolds with polypeptide gel filling as patches for infarcted tissue regeneration.

Scaffolds of poly(ethyl acrylate) (PEA) with interconnected cylindrical orthogonal pores filled with a self-assembling peptide (SAP) gel are here proposed as patches for infarcted tissue regeneration. These combined systems aim to support cell therapy and meet further requirements posed by the application: the three-dimensional architecture of the elastomeric scaffold is expected to lodge the cells of interest in the damaged zone avoiding their death or migration, and at the same time conduct cell behavior and give mechanical support if necessary; the ECM-like polypeptide gel provides a cell-friendly aqueous microenvironment, facilitates diffusion of nutrients and cell wastes and is expected to improve the distribution and viability of the seeded cells within the pores and stimulate angiogenesis.

Combining self-assembling peptide gels with three-dimensional elastomer scaffolds.

Some of the problems raised by the combination of porous scaffolds and self-assembling peptide (SAP) gels as constructs for tissue engineering applications are addressed for the first time. Scaffolds of poly(ethyl acrylate) and the SAP gel RAD16-I were employed. The in situ gelation of the SAP gel inside the pores of the scaffolds was studied. The scaffold-cum-gel constructs were characterized morphologically, physicochemically and mechanically. The possibility of incorporating an active molecule (bovine serum albumin, taken here as a model molecule for others) in the gel within the scaffold's pores was assessed, and the kinetics of its release in phosphate-buffered saline was followed. Cell seeding and colonization of these constructs were preliminarily studied with L929 fibroblasts and subsequently checked with sheep adipose-tissue-derived stem cells intended for further preclinical studies. Static (conventional) and dynamically assisted seedings were compared for bare scaffolds and the scaffold-cum-gel constructs. The SAP gel inside the pores of the scaffold significantly improved the uniformity and density of cell colonization of the three-dimensional (3-D) structure. These constructs could be of use in different advanced tissue engineering applications, where, apart from a cell-friendly extracellular matrix -like aqueous environment, a larger-scale 3-D structure able to keep the cells in a specific place, give mechanical support and/or conduct spatially the tissue growth could be required.

Effect of poly(L-lactide) surface topography on the morphology of in vitro cultured human articular chondrocytes.

Human articular chondrocytes were cultured in vitro on poly(L-lactic) acid, PLLA, substrates. Influence of the surface topography on cell morphology was found. Different surface microtopographies were obtained on PLLA by crystallizing at 120 degrees C after nucleation treatments that include isothermal stages at temperatures just below (55 degrees C) and just above (75 degrees C) the glass transition temperature (T(g) = 65 degrees C). Isothermal crystallization from the melt gave rise to big spherulites (approx. 50 microm diameter) with approx. 1 microm depth. Crystallization after nucleation treatments results in smaller (approx. 5 microm)-difficult to distinguish-spherulites. Cell viability was excellent and not affected by the surface roughness. Cell population on the nucleated samples resembles the result of culture on the reference tissue culture polystyrene (TCPS). However, cells cultured on big spherulites (PLLA isothermally crystallized without nucleation treatment) show a peculiar morphology, with a more isolated disposition and growth oriented in a characteristic direction.

The kinetics of the structural relaxation process in PHEMA-silica nanocomposites based on an equation for the configurational entropy.

The enthalpy relaxation of polymer-silica nanocomposites prepared by simultaneous polymerization of poly(2-hydroxyethyl methacrylate) (PHEMA) and tetraethyloxysilane, TEOS, a silica precursor, is investigated. Both the glass transition temperature, Tg, and the temperature interval of the glass transition, DeltaTg , increase as the silica content in the sample does. Structural relaxation experiments show that the temperature interval in which conformational motions take place broadens as the silica content in the hybrid increases. A phenomenological model based on the evolution of the configurational entropy during the structural relaxation process, the SC model, has been used for determining the temperature dependence of the relaxation times during the process. The results show an increase of the fragility of the polymer as the silica content increases, a feature that can be related to the broadening of the distribution of relaxation times characterized by the beta parameter of the stretched exponential distribution. On another hand the silica content increase produces a significant change of the relaxation times in the glassy state.