Evaluation of the short-term host response and biomechanics of an absorbable poly-4-hydroxybutyrate scaffold in a sheep model following vaginal implantation.

OBJECTIVE: To evaluate the host- and biomechanical response to a fully absorbable poly-4-hydroxybutyrate (P4HB) scaffold in comparison with the response to polypropylene (PP) mesh. DESIGN: In vivo animal experiment. SETTING: KU Leuven Center for Surgical Technologies. POPULATION: Fourteen parous female Mule sheep. METHODS: P4HB scaffolds were surgically implanted in the posterior vaginal wall of sheep. The comparative PP mesh data were obtained from an identical study protocol performed previously. MAIN OUTCOME MEASURES: Gross necropsy, host response and biomechanical evaluation of explants, and the in vivo P4HB scaffold degradation were evaluated at 60- and 180-days post-implantation. Data are reported as mean ± standard deviation (SD) or standard error of the mean (SEM). RESULTS: Gross necropsy revealed no implant-related adverse events using P4HB scaffolds. The tensile stiffness of the P4HB explants increased at 180-days (12.498 ± 2.66 N/mm SEM [p =0.019]) as compared to 60-days (4.585 ± 1.57 N/mm) post-implantation, while P4HB degraded gradually. P4HB scaffolds exhibited excellent tissue integration with dense connective tissue and a moderate initial host response. P4HB scaffolds induced a significantly higher M2/M1 ratio (1.70 ± 0.67 SD, score 0-4), as compared to PP mesh(0.99 ± 0.78 SD, score 0-4) at 180-days. CONCLUSIONS: P4HB scaffold facilitated a gradual load transfer to vaginal tissue over time. The fully absorbable P4HB scaffold, in comparison to PP mesh, has a favorable host response with comparable load-bearing capacity. If these results are also observed at longer follow-up in-vivo, a clinical study using P4HB for vaginal POP surgery may be warranted to demonstrate efficacy. TWEETABLE ABSTRACT: Degradable vaginal P4HB implant might be a solution for treatment of POP.

Assessment of Electrospun and Ultra-lightweight Polypropylene Meshes in the Sheep Model for Vaginal Surgery.

BACKGROUND: There is an urgent need to develop better materials to provide anatomical support to the pelvic floor without compromising its function. OBJECTIVE: Our aim was to assess outcomes after simulated vaginal prolapse repair in a sheep model using three different materials: (1) ultra-lightweight polypropylene (PP) non-degradable textile (Restorelle) mesh, (2) electrospun biodegradable ureidopyrimidinone-polycarbonate (UPy-PC), and (3) electrospun non-degradable polyurethane (PU) mesh in comparison with simulated native tissue repair (NTR). These implants may reduce implant-related complications and avoid vaginal function loss. DESIGN, SETTING, AND PARTICIPANTS: A controlled trial was performed involving 48 ewes that underwent NTR or mesh repair with PP, UPy-PC, or PU meshes (n=12/group). Explants were examined 60 and 180 d (six per group) post-implantation. INTERVENTION: Posterior rectovaginal dissection, NTR, or mesh repair. OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS: Implant-related complications, vaginal contractility, compliance, and host response were assessed. Power calculation and analysis of variance testing were used to enable comparison between the four groups. RESULTS: There were no visible implant-related complications. None of the implants compromised vaginal wall contractility, and passive biomechanical properties were similar to those after NTR. Shrinkage over the surgery area was around 35% for NTR and all mesh-augmented repairs. All materials were integrated well with similar connective tissue composition, vascularization, and innervation. The inflammatory response was mild with electrospun implants, inducing both more macrophages yet with relatively more type 2 macrophages present at an early stage than the PP mesh. CONCLUSIONS: Three very different materials were all well tolerated in the sheep vagina. Biomechanical findings were similar for all mesh-augmented repair and NTR. Constructs induced slightly different mid-term inflammatory profiles. PATIENT SUMMARY: Product innovation is needed to reduce implant-related complications. We tested two novel implants, electrospun and an ultra-lightweight polypropylene textile mesh, in a physiologically relevant model for vaginal surgery. All gave encouraging outcomes.

Publisher Correction: Tear resistance of soft collagenous tissues.

The original version of this Article contained errors in the 'Computational material models' section of the Methods in which some values were displayed with incorrect units. As a result of this, a number of changes have been made to both the PDF and the HTML versions of the Article. A full description of these changes is available online and can be accessed via a link at the top of the Article.

Tear resistance of soft collagenous tissues.

Fracture toughness characterizes the ability of a material to maintain a certain level of strength despite the presence of a macroscopic crack. Understanding this tolerance for defects in soft collagenous tissues (SCT) has high relevance for assessing the risks of fracture after cutting, perforation or suturing. Here we investigate the peculiar toughening mechanisms of SCT through dedicated experiments and multi-scale simulations, showing that classical concepts of fracture mechanics are inadequate to quantify and explain the high defect tolerance of these materials. Our results demonstrate that SCT strength is only modestly reduced by defects as large as several millimeters. This defect tolerance is achieved despite a very narrow process zone at the crack tip and even for a network of brittle fibrils. The fracture mechanics concept of tearing energy fails in predicting failure at such defects, and its magnitude is shown to depend on the chemical potential of the liquid environment.

In Vitro Endothelialization of Surface-Integrated Nanofiber Networks for Stretchable Blood Interfaces.

Despite major technological advances within the field of cardiovascular engineering, the risk of thromboembolic events on artificial surfaces in contact with blood remains a major challenge and limits the functionality of ventricular assist devices (VADs) during mid- or long-term therapy. Here, a biomimetic blood-material interface is created via a nanofiber-based approach that promotes the endothelialization capability of elastic silicone surfaces for next-generation VADs under elevated hemodynamic loads. A blend fiber membrane made of elastic polyurethane and low-thrombogenic poly(vinylidene fluoride- co-hexafluoropropylene) was partially embedded into the surface of silicone films. These blend membranes resist fundamental irreversible deformation of the internal structure and are stably attached to the surface, while also exhibiting enhanced antithrombotic properties when compared to bare silicone. The composite material supports the formation of a stable monolayer of endothelial cells within a pulsatile flow bioreactor, resembling the physiological in vivo situation in a VAD. The nanofiber surface modification concept thus presents a promising approach for the future design of advanced elastic composite materials that are particularly interesting for applications in contact with blood.

The multiscale stiffness of electrospun substrates and aspects of their mechanical biocompatibility.

In contrast to homogeneous materials, the mechanical properties of fibrous substrates depend on the probing lengthscale. This suggests that cells feel very different mechanical cues than expected from the macroscale characterisation of the substrate materials. By means of multiscale computational analyses we study here the mechanical environment of cells adhering to typical electrospun networks used in biomedical applications, with comparable macroscopic stiffness but different fibre diameters. The stiffness evaluated at the level of focal adhesions varies significantly, and the overall magnitude is strongly affected by the fibre diameter. The microscopic stiffness evaluated at cell scale depends substantially on the network topology and is about one order of magnitude lower than the macroscopic stiffness of the substrate, and two to three orders of magnitude below the fibres' elastic modulus. Moreover, the translation of stiffness over the scales is modulated by global deformations of the scaffold. In particular, uniaxial or biaxial stretching of the substrate induces nonlinear microscopic stiffening. Finally, although electrospun networks allow long-range transmission of cell-induced deformations, the comparison between the range of forces measured in cell traction force microscopy and those required to markedly deform typical electrospun networks reveals an order of magnitude difference, suggesting that these scaffolds provide a rather rigid environment for cells. All these results underline that the achievement of mechanical biocompatibility at all relevant lengthscales, and over the whole range of physiological loading states is extremely challenging. At the same time, the study shows that the diameter, length and curvature of fibre segments might be tunable towards achieving this goal. STATEMENT OF SIGNIFICANCE: Electrospun fabrics have growing use as substrates and scaffolds in tissue engineering and other biomedical applications. Based on multiscale computational analyses, this study shows that substrates of comparable macroscopic stiffness can provide tremendously different mechanical micro-environments, and that cells adhering to fibrous substrates may thus experience by orders of magnitude different mechanical cues than it would be expected from macroscale material characterisation. The simulations further reveal that the transfer of stiffness over the length scales changes with macroscopic deformation, and identify some key parameters that govern the transfer ratio. We believe that such refined understanding of the multiscale aspects of mechanical biocompatibility is key to the development of successful scaffold materials.

Inverse poroelasticity as a fundamental mechanism in biomechanics and mechanobiology.

Understanding the mechanisms of deformation of biological materials is important for improved diagnosis and therapy, fundamental investigations in mechanobiology, and applications in tissue engineering. Here we demonstrate the essential role of interstitial fluid mobility in determining the mechanical properties of soft tissues. Opposite to the behavior expected for a poroelastic material, the tissue volume of different collagenous membranes is observed to strongly decrease with tensile loading. Inverse poroelasticity governs monotonic and cyclic responses of soft biomembranes, and induces chemo-mechanical coupling, such that tensile forces are modulated by the chemical potential of the interstitial fluid. Correspondingly, the osmotic pressure varies with mechanical loads, thus providing an effective mechanism for mechanotransduction. Water mobility determines the tissue's ability to adapt to deformation through compaction and dilation of the collagen fiber network. In the near field of defects this mechanism activates the reversible formation of reinforcing collagen structures which effectively avoid propagation of cracks.How soft tissues respond to mechanical load is essential to their biological function. Here, the authors discover that - contrary to predictions of poroelasticity - fluid mobility in collagenous tissues induces drastic volume decrease with tensile loading and pronounced chemo-mechanical coupling.

A 2.5D approach to the mechanics of electrospun fibre mats.

In this paper, a discrete random network modelling approach specific to electrospun networks is presented. Owing to the manufacturing process, fibres in these materials systems have an enormous length, as compared to their diameters, and form sparse networks since fibre contact over thickness is limited to a narrow range. Representative volume elements are generated, in which fibres span the entire domain, and a technique is developed to apply computationally favourable periodic boundary conditions despite the initial non-periodicity of the networks. To capture sparsity, a physically motivated method is proposed to distinguish true fibre cross-links, in which mechanical interaction takes place, from mere fibre intersections. The model is exclusively informed by experimentally accessible parameters, demonstrates excellent agreement with the mechanical response of electrospun fibre mats, captures typical microscopic deformation patterns, and provides information on the kinematics of fibres and pores. This ability to address relevant mechanisms of deformation at both micro- and macroscopic length scales, together with the moderate computational cost, render the proposed modelling approach a highly qualified tool for the computer-based design and optimization of electrospun networks.

Physiologic musculofascial compliance following reinforcement with electrospun polycaprolactone-ureidopyrimidinone mesh in a rat model.

PURPOSE: Electrospun meshes may be considered as substitutes to textile polypropylene implants. We compared the host response and biomechanical properties of the rat abdominal wall following reinforcement with either polycaprolactone (PCL) modified with ureidopyrimidinone-motifs (UPy) or polypropylene mesh. METHODS: First we measured the response to cyclic uniaxial load within the physiological range both dry (room temperature) and wet (body temperature). 36 rats underwent primary repair of a full-thickness abdominal wall defect with a polypropylene suture (native tissue repair), or reinforced with either UPy-PCL or ultra-light weight polypropylene mesh (n = 12/group). Sacrifice was at 7 and 42 days. Outcomes were compliance of explants, mesh dimensions, graft related complications and semi-quantitative assessment of inflammatory cell (sub) types, neovascularization and remodeling. RESULTS: Dry UPy-PCL implants are less stiff than polypropylene, both are more compliant in wet conditions. Polypropylene loses stiffness on cyclic loading. Both implant types were well incorporated without clinically obvious degradation or herniation. Exposure rates were similar (n = 2/12) as well as mesh contraction. There was no reinforcement at low loads, while, at higher tension, polypropylene explants were much stiffer than UPy-PCL. The latter was initially weaker yet by 42 days it had a compliance similar to native abdominal wall. There were eventually more foreign body giant cells around UPy-PCL fibers yet the amount of M1 subtype macrophages was higher than in polypropylene explants. There were less neovascularization and collagen deposition. CONCLUSION: Abdominal wall reconstruction with electrospun UPy-PCL mesh does not compromise physiologic tissue biomechanical properties, yet provokes a vivid inflammatory reaction.

Factors influencing the determination of cell traction forces.

Methods summarized by the term Traction Force Microscopy are widely used to quantify cellular forces in mechanobiological studies. These methods are inverse, in the sense that forces must be determined such that they comply with a measured displacement field. This study investigates how several experimental and analytical factors, originating in the realization of the experiments and the procedures for the analysis, affect the determined traction forces. The present results demonstrate that even for very high resolution measurements free of noise, traction forces can be significantly underestimated, while traction peaks are typically overestimated by 50% or more, even in the noise free case. Compared to this errors, which are inherent to the nature of the mechanical problem and its discretization, the effect of ignoring the out-of-plane displacement component, the interpolation scheme used between the discrete measurement points and the disregard of the geometrical non-linearities when using a nearly linear substrate material are less consequential. Nevertheless, a nonlinear elastic substrate, with strain-stiffening response and some degree of compressibility, can substantially improve the robustness of the reconstruction of traction forces over a wide range of magnitudes. This poses the need for a correct mechanical representation of these non-linearities during the traction reconstruction and a correct mechanical characterization of the substrate itself, especially for the large strain shear domain which is shown to plays a major role in the deformations induced by cells.

Confocal reference free traction force microscopy.

The mechanical wiring between cells and their surroundings is fundamental to the regulation of complex biological processes during tissue development, repair or pathology. Traction force microscopy (TFM) enables determination of the actuating forces. Despite progress, important limitations with intrusion effects in low resolution 2D pillar-based methods or disruptive intermediate steps of cell removal and substrate relaxation in high-resolution continuum TFM methods need to be overcome. Here we introduce a novel method allowing a one-shot (live) acquisition of continuous in- and out-of-plane traction fields with high sensitivity. The method is based on electrohydrodynamic nanodrip-printing of quantum dots into confocal monocrystalline arrays, rendering individually identifiable point light sources on compliant substrates. We demonstrate the undisrupted reference-free acquisition and quantification of high-resolution continuous force fields, and the simultaneous capability of this method to correlatively overlap traction forces with spatial localization of proteins revealed using immunofluorescence methods.