Current options for aortic valve replacements are non-viable and thus lack the ability to grow and remodel, which can be problematic for paediatric applications. Toward the development of living valve substitutes that can grow and remodel, porcine aortic valve interstitial cells (VICs) were isolated and encapsulated within proteolytically degradable and cell-adhesive poly(ethylene glycol) (PEG) hydrogels, in an effort to study their phenotypes and functions. The results showed that encapsulated VICs maintained high viability and proliferated within the hydrogels. The VICs actively remodelled the hydrogels via secretion of matrix metalloproteinase-2 (MMP-2) and deposition of new extracellular matrix (ECM) components, including collagens I and III. The soft hydrogels with compressive moduli of ~4.3 kPa quickly reverted VICs from an activated myofibroblastic phenotype to a quiescent, unactivated phenotype, evidenced by the loss of α-smooth muscle actin expression upon encapsulation. In an effort to promote VIC-mediated ECM production, ascorbic acid (AA) was supplemented in the medium to investigate its effects on VIC function and phenotype. AA treatment enhanced VIC spreading and proliferation, and inhibited apoptosis. AA treatment also promoted VIC-mediated ECM remodelling by increasing MMP-2 activity and depositing collagens I and III. AA treatment did not significantly influence the expression of α-smooth muscle actin (myofibroblast activation marker) and alkaline phosphatase (osteogenic differentiation marker). No calcification or nodule formation was observed within the cell-laden hydrogels, with or without AA treatment. These results suggest the potential of this system and the beneficial effect of AA in heart valve tissue engineering. Copyright © 2015 John Wiley & Sons, Ltd.

Advanced tissue engineered heart valves must be constructed from multiple materials to better mimic the heterogeneity found in the native valve. The trilayered structure of aortic valves provides the ability to open and close consistently over a full human lifetime, with each layer performing specific mechanical functions. The middle spongiosa layer consists primarily of proteoglycans and glycosaminoglycans, providing lubrication and dampening functions as the valve leaflet flexes open and closed. In this study, hyaluronan hydrogels were tuned to perform the mechanical functions of the spongiosa layer, provide a biomimetic scaffold in which valve cells were encapsulated in 3D for tissue engineering applications, and gain insight into how valve cells maintain hyaluronan homeostasis within heart valves. Expression of the HAS1 isoform of hyaluronan synthase was significantly higher in hyaluronan hydrogels compared to blank-slate poly(ethylene glycol) diacrylate (PEGDA) hydrogels. Hyaluronidase and matrix metalloproteinase enzyme activity was similar between hyaluronan and PEGDA hydrogels, even though these scaffold materials were each specifically susceptible to degradation by different enzyme types. KIAA1199 was expressed by valve cells and may play a role in the regulation of hyaluronan in heart valves. Cross-linked hyaluronan hydrogels maintained healthy phenotype of valve cells in 3D culture and were tuned to approximate the mechanical properties of the valve spongiosa layer. Therefore, hyaluronan can be used as an appropriate material for the spongiosa layer of a proposed laminate tissue engineered heart valve scaffold.

In this study, a composite scaffold consisting of an electrospun polyurethane and poly(ethylene glycol) hydrogel was investigated for aortic valve tissue engineering. This multilayered approach permitted the fabrication of a scaffold that met the desired mechanical requirements while enabling the 3D culture of cells. The scaffold was tuned to mimic the tensile strength, anisotropy, and extensibility of the natural aortic valve through design of the electrospun polyurethane mesh layer. Valve interstitial cells were encapsulated inside the hydrogel portion of the scaffold around the electrospun mesh, creating a composite scaffold approximately 200 μm thick. The stiffness of the electrospun fibers caused the encapsulated cells to exhibit an activated phenotype that resulted in fibrotic remodeling of the scaffold in a heterogeneous manner. Remodeling was further explored by culturing the scaffolds in both a mechanically constrained state and in a bent state. The constrained scaffolds demonstrated strong fibrotic remodeling with cells aligning in the direction of the mechanical constraint. Bent scaffolds demonstrated that applied mechanical forces could influence cell behavior. Cells seeded on the outside curve of the bend exhibited an activated, fibrotic response, while cells seeded on the inside curve of the bend were a quiescent phenotype, demonstrating potential control over the fibrotic behavior of cells. Overall, these results indicate that this polyurethane/hydrogel scaffold mimics the structural and functional heterogeneity of native valves and warrants further investigation to be used as a model for understanding fibrotic valve disease.

Knowledge of how extracellular matrix (ECM) binding impacts valve interstitial cells (VICs) is critical not only to better understanding the etiology of valvular diseases but also to constructing living valve substitutes that can grow and remodel. Use of ECM-mimicking adhesive peptides with specific affinity to different receptors provides insights into adhesion-mediated cell signaling and downstream outcomes. Expression of adhesion receptors by VICs was assessed by flow cytometry and used to guide the choice of peptides studied. The peptide RGDS with affinity to multiple integrin receptors, and specific receptor-targeting peptides DGEA (integrin α2β1), YIGSR (67kDa laminin/elastin receptor; 67LR), and VAPG (67LR) were incorporated into hydrogels to investigate their effects on VICs. DGEA, YIGSR, and VAPG alone were insufficient to induce stable VIC adhesion. As a result, these peptides were studied in combination with 1 mM RGDS. For VICs cultured on two-dimensional hydrogel surfaces, YIGSR and VAPG down-regulated the expression of smooth muscle α-actin (myofibroblast activation marker); DGEA promoted VIC adhesion and VIC-mediated ECM deposition and inhibited the activity of alkaline phosphatase (osteogenic differentiation marker). Further, YIGSR and DGEA in combination promoted ECM deposition while inhibiting both myofibroblastic and osteogenic differentiation. However, VICs behaved differently to adhesive ligands when cultured within three-dimensional hydrogels, with most VICs assuming a healthy, quiescent phenotype under all peptide conditions tested. DGEA promoted ECM deposition by VICs within hydrogels. Overall, we demonstrate that the presentation of defined peptides targeting specific adhesion receptors can be used to regulate VIC adhesion, phenotype and ECM synthesis.

With an increasing number of patients requiring valve replacements, there is heightened interest in advancing heart valve tissue engineering (HVTE) to provide solutions to the many limitations of current surgical treatments. A variety of materials have been developed as scaffolds for HVTE including natural polymers, synthetic polymers, and decellularized valvular matrices. Among them, biocompatible hydrogels are generating growing interest. Natural hydrogels, such as collagen and fibrin, generally show good bioactivity but poor mechanical durability. Synthetic hydrogels, on the other hand, have tunable mechanical properties; however, appropriate cell-matrix interactions are difficult to obtain. Moreover, hydrogels can be used as cell carriers when the cellular component is seeded into the polymer meshes or decellularized valve scaffolds. In this review, we discuss current research strategies for HVTE with an emphasis on hydrogel applications. The physicochemical properties and fabrication methods of these hydrogels, as well as their mechanical properties and bioactivities are described. Performance of some hydrogels including in vitro evaluation using bioreactors and in vivo tests in different animal models are also discussed. For future HVTE, it will be compelling to examine how hydrogels can be constructed from composite materials to replicate mechanical properties and mimic biological functions of the native heart valve.

The development of advanced scaffolds that recapitulate the anisotropic mechanical behavior and biological functions of the extracellular matrix in leaflets would be transformative for heart valve tissue engineering. In this study, anisotropic mechanical properties were established in poly(ethylene glycol) (PEG) hydrogels by crosslinking stripes of 3.4 kDa PEG diacrylate (PEGDA) within 20 kDa PEGDA base hydrogels using a photolithographic patterning method. Varying the stripe width and spacing resulted in a tensile elastic modulus parallel to the stripes that was 4.1-6.8 times greater than that in the perpendicular direction, comparable to the degree of anisotropy between the circumferential and radial orientations in native valve leaflets. Biomimetic PEG-peptide hydrogels were prepared by tethering the cell-adhesive peptide RGDS and incorporating the collagenase-degradable peptide PQ (GGGPQG↓IWGQGK) into the polymer network. The specific amounts of RGDS and PEG-PQ within the resulting hydrogels influenced the elongation, de novo extracellular matrix deposition and hydrogel degradation behavior of encapsulated valvular interstitial cells (VICs). In addition, the morphology and activation of VICs grown atop PEG hydrogels could be modulated by controlling the concentration or micro-patterning profile of PEG-RGDS. These results are promising for the fabrication of PEG-based hydrogels using anatomically and biologically inspired scaffold design features for heart valve tissue engineering.

There are well completion challenges in Nanpu buried-hill fracture reservoirs due to the co-existence of the risk of loss of circulation and blow out combined with formation temperatures over 150 ℃ . To solve the problems ,the formula for twice-crosslinking gel valves were developed .In the design ,a structural gel was developed to meet operational demands .In the first crosslinking step the gel possesses high strength and quick formation speed .In the second crosslinking step involved insuring proper opera-tions in the isolated borehole section .RM2100 HTHP online gel detector was used to monitor the forma-tion of gel valves .Results showed that viscosity of the gel system could be maintained over 1 000 mPa?s . In the secondary crosslinking step ,gel formation started at 150 min with ultimate viscosity above 12 ,000 mPa?s .Simulated boreholes were used to determine pressurebearing/load capacitywhich could reach over 2 M Pa/100m .In addtion ,the gel at 150 ℃ maintained a satisfactory viscosity for 72 h .Gel-breaking could be done through flushing by using drill bits .Successful application in Well NP23-P2016 indicated effective isolation of hydrocarbons to minimize losses and to provide reliable well-control conditions for deployment of tie-back tailpipes .Research results showed that the high-temperature gel valves could provide satisfacto-ry solutions to challenges in well control encountered during well completion in Nanpu buried-hill reser-voirs .

578 ©2014 Society for Biomaterials

By tuning the polymer-filler interaction, filler size and filler loading, we use a coarse-grained model-based molecular dynamics simulation to study the polymer-filler interfacial structural (the orientations at the bond, segment and chain length scales, chain size and conformation), dynamic and stress-strain properties. Simulated results indicate that the interfacial region is composed of partial segments of different polymer chains, which is consistent with the experimental results presented by Chen et al. (Macromolecules, 2010, 43, 1076). Moreover, it is found that the interfacial region is within one single chain size (R(g)) range, irrespective of the polymer-filler interaction and the filler size, beyond which the bulk behavior appears. In the interfacial region, the orientation and dynamic behaviors are induced by the interfacial enthalpy, while the size and conformation of polymer chains near the filler are controlled by the configurational entropy. In the case of strong polymer-filler interaction (equivalent to the hydrogen bond), the innerest adsorbed polymer segments still undergo adsorption-desorption process, the transport of chain mass center in the interfacial region exhibits away from the glassy behavior, and no plastic-like yielding point appears in the stress-strain curve, which indicates that although the mobility of interfacial polymer chains is restricted, there exist no "polymer glassy layers" surrounding the filler. In addition, it is evidenced that the filler particle prefers selectively adsorbing the long polymer chains for attractive polymer-filler interaction, validating the experimental explanation of the change of the bound rubber (BR). In short, this work provides important information for further experimental and simulation studies of polymer-nanoparticle interfacial behavior.

Modified nano-fumed silica (mn-silica)/poly(glycerol-sebacate-citrate), in which mn-silica loadings varied from 0 to 20 phr, were prepared by in situ polymerization and surface modification. The influence of mn-silica loadings on the structure and properties of the composites was studied. Scanning electron microscope (SEM) and transmission electron microscope (TEM) photos showed that the mn-silica dispersed well as nano-scale network in the matrix, and exhibited good interfacial bonding with the matrix. The mn-silica filled composites exhibited excellent comprehensive properties relative to the unfilled elastomers. Specially, the tensile strength improved from 0.9 MPa to 5.3 MPa. Results of the in vitro degradation test suggested that mn-silica loading could adjust the degradation rate of the composites in simulated body fluid solution. The MTT colorimetry with L929 cells substantiated that the introduction of mn-silica weakened the cytotoxicity of elastomers and made the composites accepted as qualified biomedical materials. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 1612-1620, 2012

Silica/poly(sebacate-glycerol-citrate) nanocomposites, in which nanosilica content varied from 0 phr to 20 phr, were prepared by in-situ polymerization and surface modification. The influence of nanosilica contents on the structure and properties was studied. The composites with different silica contents exhibited excellent comprehensive properties relative to unfilled elastomers. The tensile strength improved from 0.9 MPa to 5.3 MPa. SEM and TEM photos showed that the nanosilica dispersed as nano-scale network in the matrix, and the modified nanosilica exhibited good interfacial bonding with the matrix. XRD measurement was applied to investigate the changes in structure on a short-range molecular level, which suggested that the existence of nanosilica limited the formation of ordered structure. The DSC curves demonstrated that the glass transitions of the composites inclined moving toward a lower temperature as the nanosilica content increased. Results of the in vitro degradation test suggested that nanosilica content could adjust the degradation speed of the composites.

Nano-hydroxyapatite(n-HA)/poly[(1,2-propanediol-sebacate)-citrate] (1,2-PPSC) composites were synthesized by ultrasonic dispersion method and grinding method respectively. The dispersion property of n-HA in n-HA/1,2-PPSC composites and glass transition temperature of two composites were analyzed by differential scanning calorimetry,scanning electron microscopy and transmission electron microscopy. The sol content,mechanical properties and degradation performance of n-HA/1,2-PPSC composites were also tested. The results showed that both the glass transition temperature and sol content of n-HA/1,2-PPSC composites decreased with the increase of mass fractions of n-HA and so did the degradation rate. The grinding method was an effective method to improve the dispersibility of n-HA and the mechanical properties of the composites. With the method,the tensile strength increased by 8.2 times,the modulus increased by 11.4 times,the elongation at break had little change.

This article reviews the degradability of chemically synthesized bioelastomers, mainly designed for soft tissue repair. These bioelastomers involve biodegradable polyurethanes, polyphosphazenes, linear and crosslinked poly(ether/ester)s, poly(ε-caprolactone) copolymers, poly(1,3-trimethylene carbonate) and their copolymers, poly(polyol sebacate)s, poly(diol-citrates) and poly(ester amide)s. The in vitro and in vivo degradation mechanisms and impact factors influencing degradation behaviors are discussed. In addition, the molecular designs, synthesis methods, structure properties, mechanical properties, biocompatibility and potential applications of these bioelastomers were also presented.