Journal of Biomimetics, Biomaterials and Tissue Engineering Vol. 4

Abstract: This study explores degree of vinyl conversion (DVC), polymerization shrinkage (PS) and shrinkage stress (PSS) of the experimental amorphous calcium phosphate (ACP) composites intended for use as an endodontic sealer. Light-cure (LC), chemical cure (CC) or dual-cure (DC; combined light and chemical cure) resins comprised urethane dimethacrylate (UDMA), 2-hydroxyethyl methacrylate (HEMA), methacryloyloxyethyl phthalate (MEP) and a high molecular mass oligomeric co-monomer, poly(ethyleneglycol)-extended UDMA (PEG-U) (designated UPHM resin). To fabricate composites, a mass fraction of 60 % UPHM resin was blended with a mass fraction of 40 % as-made (am-ACP) or ground ACP (g-ACP). DVC values of copolymer (unfilled UPHM resin) and composite specimens were determined by infrared spectroscopy. Glass-filled composites were used as controls. PS and PSS of composites were determined by dilatometry and tensometry, respectively. LC copolymers attained extraordinary high DVC values at 24 h post-cure (95.7 %), compared to CC (52 %) and DC (79.3 %) copolymer specimens. While the DVC values of LC and DC am-ACP composites were reduced between 5 and 10 %, DVC values of DC g-ACP composites increased almost 8 % compared to the corresponding copolymers. High DVC attained in LC composites was, expectedly, accompanied with high PS values (on average 7 vol%). However, PSS developed in LC and especially DC composites did not exceed PSS values seen in other UDMA-based composites. Based on this initial evaluation, it is concluded that, DC, g-ACP filled UPHM composite shows promise as an endodontic sealer. However, further physicochemical evaluations, including water sorption, mechanical stability and ion release as well as a leachability studies need to be performed before this experimental material is tested for cellular responses and, eventually recommended for clinical utility.

Abstract: A Scanning Electron Microscopic study on the fish scale of Cyprinus carpio communis (freshwater carp), depicts remarkable structural and compositional characteristics which may be used as inspiration for novel biomaterial design. The fracture surface structure and sintered fish scale reveals an osseous layer on its dorsal side. It has a compact heterogeneous crystalloid-like structure of assorted shapes and sizes. The ventral side is made of orthogonally arranged mineralized needle like crystalloids template embedded in a fibrillary plate. Frozen scales revealed that this dorsal layer may contain high atomic number elements as it appeared bright with back scattered electron signals. The ventral side consists of collagen plates and a matrix, which are arranged orthogonally in a double-twisted, plywood-like structure. There are alternate crystalloid and matrix which are arranged orthogonally and forming 15-17 layers in between these two sides. This provides useful information of scale composition. Design features from the structure of the fish scale may be useful in the development of functional biomaterials, in various different fields including nano-composites, biopolymers and natural source of hydroxyapatite, used in applied therapeutic, pharmaceutical industries and semiconductor technology. The tolerance of the fish scale to high temperature and very low temperature cooling revealed unique characteristics for biomaterial fabrication. The orthogonally arranged ventral side plates of collagen embedded in proteoglycans may also prove to be a good source of scaffold material for cell culturing for tissue engineering.

Abstract: A porous tissue scaffold depends on its ability to provide functional balance between mechanical strength, pore properties and interconnectivity of pores. High porosity levels, typically greater than 90% and pore sizes above 100µm are required for tissue growth and fixation. Alumina is a stable and very strong bioceramic which, when doped with calcium and phosphate ions, can potentially combine bioactivity with high porosity and high strength. Highly porous alumina foams were synthesized through heat induced chemical breakdown of precursor salt solutions. Pore sizes achieved for foamed alumina with moderate mole fractions are generally larger than 100µm. Foamed alumina with mole fractions on the extreme high and low ends shows lower average pore sizes. Compressive strength of synthesized foams falls in the range of 100kPa to 230kPa, significantly higher than porous biodegradable polymer tissue scaffolds. The significance of this work is that scaffolds can be produced with the unique combination of high porosity, high strength and biocompatibility.

Abstract: Mechanical properties of bioceramics are poor and need to be improved for biomedical applications. In order to do this, bioceramics may be strengthened by bioresorbable polymers. In this study, the mechanical properties of poly(ε-caprolactone), PCL, coated dense bioceramic pellets made of silica-contained calcium phosphates were studied and analyzed using a statistical experimental design in conjunction with Taguchi methods for optimization. The aim of this experimental work was to maximize the pellet flexural strength and minimize the amount of deposited PCL. The most important factors affecting the strengthening of the ceramic pellets were evaluated. Four independent processing variables (a removal technique of an excess polymer solution, concentration of PCL in the solution, a heat treatment temperature and the number of dipping) with three levels of variability were tested using an L9 (34) orthogonal array. A statistical experimental design using the analysis of means and orthogonal array was applied to optimize the responses of these variables. The optimal conditions for achieving the maximal flexural strength of the coated pellets at the minimal amount of the deposited PCL were determined. A high quality dense bioceramic pellets with ~ 10.5 MPa flexural strength and ~ 80 μm thickness (~ 21 mg weight) of the deposited PCL coating were manufactured as a result.

Abstract: The nucleus of a spinal disc is seamlessly connective and protectively supportive of the joint within which it is enveloped. A range of nucleus prosthesis configurations have been proposed and applied with some success. Those that have demonstrated clinical efficacy have approximated physiological form and function using established biomaterials while preserving key anatomical structures. The minimally invasive biostable, biomimetic Columna Disc Device (CDD) partial spinal disc replacement has been developed to clinical trial stage. It mimics the geometry and response of the nucleus that it replaces. While the implant configuration and materials have been set, the geometry and interfacial properties of this prosthesis may be modulated to account for versatility in surgical deployment, implant stiffness, and subsequent long-term tissue remodelling response. FEA models were developed to study effects of implant jacket geometry and surface properties on implant deployment and biomechanics. Studded and dimpled textures provide a method for increasing surface area to diffuse jacket-filler interfacial stress and similar for the implant-tissue junction. Surface texture design elements observed in nature can protect against delamination and interlayer slippage. This is the case with adherent outer layers of human skin. A textured implant design is also proposed to guard against third body wear by housing debris remote of wear sites and by reducing sliding. The periodically varying strain fields provided by the textured jacket may also help mitigate for tears by diverting and arresting micro-fissures. Increasing friction at the implant-tissue interface to the point of tissue-attachment was shown to increase the stiffness of the implant in axial-loading. In contrast, increasing bulk surface area is expected to contribute to a decrease in implant stiffness. This is, however, dependent on the intimacy and properties of interfacing tissues.

Abstract: The manufacturing process for in vitro tissue culture products and medical devices relies on a validated sterilization route for ensuring product sterility, safety and performance. Two key aspects that contribute toward final sterilization validation are (1) the reliable estimation of product bioburden and (2) the development of a proficient sterile packaging system. Bioabsorbable composite systems and architecture of tissue scaffolds can lead to numerous challenges for bioburden testing and packaging design. This study is concerned with the development of bioburden assessment methods and packaging systems for Variotis™; a soft tissue engineering scaffold. A bioburden test method relying on mechanical agitation was established. Bioburden assessment was achieved by recovering Geobacillus stearothermophilus spore inoculant for analysis. A packaging system was developed which provides adequate protection for Variotis™ scaffolds while meeting other user/sterilization requirements for research grade product. The guidelines and design approaches included in this study are generally applicable to other tissue engineering scaffold and medical devices.

Abstract: Review of current Anterior Cruciate Ligament (ACL) anchor technologies indicates that many devices facilitate osteointegration but not soft tissue in-growth. The design and preliminary testing of a novel biomimetic in-situ dilating bioabsorbable ACL anchor for simultaneous soft and hard tissue attachment is the subject of this study. The anchor method for this concept has been developed to mimic the mechanical-key configuration observed in a hair root. Reviewed anchor devices are typically interference screw-based. Screw anchors can lead to unnecessary ligament pre-stress, tearing during deployment and poor graft-bone contact. This work demonstrates a new fixation concept specifically developed for use with devices consisting of temperature-sensitive glass-reinforced-glass (GRG) soft tissue conductive biomaterial. Ligament anchorage is accomplished by dilation of the device into the base of a hair-root shaped osteotomy where a ligament with a collar and self tightening knot is inserted beforehand. This method facilitates full ligament-to-bone contact at the osteotomy zone where critical physiological ligament anchorage develops. Ligament pull-out loads equivalent to published results for conventional anchors were achieved using graft analogue. Testing with porcine ligaments resulted in a substantial reduction in ligament pull-out loads. Tibia bone sample constraints combined with the unraveling of the ligament knot were identified as primary factors for low pull-out loads for the porcine ligament tests. Subsequent design iterations will employ a reduction in prototype dimensions in addition to the use of a suture to lock the ligament knot. The hair-root shaped osteotomy and ligament anchor knot elements of this approach may be translated to other fixation systems and methods. By improving macro-mechanical-key interaction between the anchor, bone and ligament, further increase in pull-out forces may be achieved without unnecessary ligament pre-stress and tear damage caused by conventional interference screw threads.