Avinash Baji

Distal Femoral Fixation : A Biomechanical Comparison of Trigen Retrograde Intramedullary (I.M.) Nail, Dynamic Condylar Screw (DCS), and Locking Compression Plate (LCP) Condylar Plate

BACKGROUNDnThe purpose of this study was to establish if there are biomechanical differences between implants in stiffness of construct, microdisplacement, and fatigue failure in a supracondylar femoral fracture model.nnnMETHODSnA retrograde intramedullary (i.m.) nail, dynamic condylar screw (DCS), and locked condylar plate (LCP) were tested using 33-cm long synthetic femurs. A standardized supracondylar medial segmental defect was created in the distal femur bone models. A gap away from the distal joint axis and parallel to the knee axis was created for axial testing of the specimens (Arbeitsgemeinschaft fur Osteosynthesefragen [AO] type 33-A) and a T-fracture (33-C) was created for the fatigue testing of the specimens. Peak displacements were measured, and analysis was done to determine construct stiffness and gap micromotion in axial loading. Cyclic loading was performed for fatigue testing.nnnRESULTSnIt was observed that there were statistically significant differences in micromotion across the fracture gap and overall stiffness of various implant constructs. The stiffness of the i.m. nail, DCS, and LCP were 1,106, 750, and 625 N/mm, respectively. The average total micromotion across the fracture gap for the i.m. nail, DCS, and LCP were 1.96, 10.55, and 17.74 mm, respectively. In fatigue testing, the i.m. nail distal screws failed at 9,000 cycles, the DCS did not fail (80,000 cycles completed), and the LCP failed at 19,000 and 23,500 cycles.nnnCONCLUSIONSnWhen considering micromotion and construct stiffness, the i.m. nail had statistically significant higher stiffness and significantly lower micromotion across the fracture gap with axial compression. Hence, the i.m. nail tested had the greatest stability for type 33-A fractures. However, the nail demonstrated the least amount of resistance to fatigue failure with type 33-C fractures, whereas the DCS did not fail with testing in any pattern.

How Super Is Supercontraction? Persistent versus Cyclic Responses to Humidity in Spider Dragline Silk

SUMMARY Spider dragline silk has enormous potential for the development of biomimetic fibers that combine strength and elasticity in low density polymers. These applications necessitate understanding how silk reacts to different environmental conditions. For instance, spider dragline silk `supercontracts in high humidity. During supercontraction, unrestrained dragline silk contracts up to 50% of its original length and restrained fibers generate substantial stress. Here we characterize the response of dragline silk to changes in humidity before, during and after supercontraction. Our findings demonstrate that dragline silk exhibits two qualitatively different responses to humidity. First, silk undergoes a previously unknown cyclic relaxation–contraction response to wetting and drying. The direction and magnitude of this cyclic response is identical both before and after supercontraction. By contrast, supercontraction is a `permanent tensioning of restrained silk in response to high humidity. Here, water induces stress, rather than relaxation and the uptake of water molecules results in a permanent change in molecular composition of the silk, as demonstrated by thermogravimetric analysis (TGA). Even after drying, silk mass increased by∼ 1% after supercontraction. By contrast, the cyclic response to humidity involves a reversible uptake of water. Dried, post-supercontraction silk also differs mechanically from virgin silk. Post-supercontraction silk exhibits reduced stiffness and stress at yield, as well as changes in dynamic energy storage and dissipation. In addition to advancing understanding supercontraction, our findings open up new applications for synthetic silk analogs. For example, dragline silk emerges as a model for a biomimetic muscle, the contraction of which is precisely controlled by humidity alone.

Processing Methodologies for Polycaprolactone-Hydroxyapatite Composites: A Review

ABSTRACT Biodegradable implants have shown great promise for the repair of bone defects and have been commonly used as bone substitutes, which traditionally would be treated using metallic implants. The need for a second surgery exacerbated by the stress shielding effect caused by an implant has led researchers to consider more effective, synthetic biodegradable graft substitutes. The hierarchical structures commonly designed are inspired by nature in human bones, which consist of minerals such as hydroxyapatite, a form of calcium phosphate and protein fiber. The bone graft bio-substitutes should possess a combination of properties for the purpose of facilitating cell growth and adhesion, a high degree of porosity, which would facilitate the transfer of nutrients and excretion of the waste products, and the scaffold should have high tensile strength and high toughness in order to be consistent with human tissues. Blending of polycaprolactone and hydroxyapatite has demonstrated great potential as bone substitutes. It is essential to identify a standardized processing methodology for the composite, which would result in optimum mechanical property for the biocomposite. In this study, biocomposites made of polycaprolactone (PCL) and hydroxyapatite (HAP) are reviewed for their applications in bone tissue engineering. The processing methodologies are discussed for the purpose of obtaining the porosity and pore size required in an ideal tissue scaffold. The properties of the composite can be varied based on the change in pore size, porosity, and processing methodology. This paper reviews and evaluates the methods to produce the hydroxyapatite-polycaprolactone scaffolds.

Supercontraction Forces in Spider Dragline Silk Depend on Hydration Rate

Spider dragline silk is a model biological polymer for biomimetic research due to its many desirable and unusual properties. Supercontraction describes the dramatic shrinking of dragline silk fibers when wetted. In restrained silk fibers, supercontraction generates substantial stresses of 40-50 MPa above a critical humidity of approximately 70% relative humidity (RH). This stress may maintain tension in webs under the weight of rain or dew and could be used in industry for robotics, sensor technology, and other applications. Our own findings indicate that supercontraction can generate stress over a much broader range than previously reported, from 10 to 140 MPa. Here we show that this variation in supercontraction stress depends upon the rate at which the environment reaches the critical level of humidity causing supercontraction. Slow humidity increase, over several minutes, leads to relatively low supercontraction stress, while fast humidity increase, over a few seconds, typically results in higher supercontraction stress. Slowly supercontracted fibers take up less water and differ in thermostability from rapidly supercontracted fibers, as shown by thermogravimetric analysis. This suggests that spider silk achieves different molecular configurations depending upon the speed at which supercontraction occurs. Ultimately, rate-dependent supercontraction may provide a mechanism to tailor the properties of silk or biomimetic fibers for various applications.

Modelling of mechanical properties of electrospun nanofibre network

Electrospun nanofibres are widely investigated as extra-cellular matrix for tissue engineering and biomedical applications. Little is understood on the deformation mechanics of spun fibre mats. A model is developed to predict the deformation behaviour of randomly-oriented electrospun nanofibre network/mats with the fibre-fibre fusion and van der Waals interaction. The nanofibres in the mat are represented by chains of beads; the interactions between the beads are described by bonded (stretch, bending and torsion) and non-bonded (van der Waals) potentials. Stress-strain curves and dynamics fracture are predicted by this model. The results show that the fibre-fibre fusion has a significant effect on the tensile strength of the mats. Increasing the number of fusion points in the mat results in an increase in strength, but over-fusion may lead to lower fracture energy. The predicted stress-strain relationships are consistent with the experimental results.

Fracture Behavior of Hydroxyapatite-Filled Polycaprolactone

This paper reports some fracture characterization results of hydroxyapatite (HAP)-filled poly(e-caprolactone) (PCL). For the fracture toughness tests, the HAP concentration was steadily increased. The effect of HAP phase in PCL on the fracture and tearing toughnesses was investigated. The techniques of essential work of fracture (EWF) and tear strength were attempted. T-peel test was also used to evaluate the adhesive bond strength between HAP and PCL components using compression molded PCL-HAP-PCL laminates. Little is reported on the interfacial adhesion properties between bioactive components in scaffold development. The influence of PCL layer thickness (1.25, 2.5 and 3.5 mm) on adhesive strength between HAP and PCL was investigated. The adhesion between HAP and PCL components was found to be relatively strong; however, the thickness of PCL layers did not significantly influence the adhesive strength.Copyright

Techniques to Assess Adhesive Strength and Toughness of Hydroxyapatite-Filled Poly(ε-Caprolactone)

The fracture toughness of hydroxyapatite (HAP)-filled poly(ε-caprolactone) (PCL) composites was determined. Also, the adhesive strength between HAP and PCL was evaluated using T-peel tests on PCL-HAP-PCL sandwiches with various thicknesses of the PCL layer. The interfacial adhesion between HAP and PCL components was found to be relatively high, but it did not depend significantly on the PCL layer thickness. The effect of the amount of the HAP phase in PCL on the fracture toughness of the composite was investigated using double edge notched tension (DENT) specimens. The specific essential work of fracture (we) was found to decrease with increase in HAP concentration. This testing procedure showed promise in quantifying the tearing resistance and rising R-curve behavior common in natural materials and it can probably be used with other biomaterials that exhibit post-yield deformation. Introduction Biodegradable polymers reinforced with inorganic fillers such as HAP have been widely investigated for potential applications in tissue engineering [1,2]. Most studies on tissue scaffolds concentrate on processing techniques: the only mechanical properties measured are tensile strength and stiffness. Little is known about how to design tough and mechanically robust tissue scaffolds mimicking human bones and tissues. Studies to characterize fracture toughness of natural materials like cortical [3-5] bones were done in the past, but similar toughness values [3,4] were not attained from biomimetic equivalents [6]. Understanding the structures of natural materials and translating them into the design of scaffolds is important to optimize the properties of bone analogue materials. This study uses the concepts of essential work of fracture (EWF) to study the toughness of composites [7,8] and a T-peel test to determine the interfacial adhesion between HAP and PCL components [9-11]. The interfacial work of adhesive fracture between HAP and PCL has not been previously studied in detail. Such data can provide guidance for the design of scaffolds in future. Work of Fracture Measurements The EWF concept was first introduced by Cotterell and Reddell [7] to assess fracture toughness of metals, followed by ductile thermoplastics [8,11,12]. Gent [13] independently developed a similar technique to assess the tear strength of semi-crystalline polymers. Following the Mai-Cotterell scheme, the work expended in the fracture process zone (FPZ) work per unit of crack growth is a measure of the fracture toughness. It is defined as the essential work of fracture (we). Total fracture work includes (i) the essential work consumed in the FPZ, We, and (ii) non-essential work performed in the outer plastic zone (OPZ), Wp. For a given thickness, We is proportional to the ligament length (l=B-a), where B is the specimen width and a the crack size, Wp to the square of the ligament length (l 2 ). Hence, t l w lt w W p e f 2 β + = . (1) l w w lt W w p e f f β + = = . (2) where wf is the specific total fracture work (= Wf/lt), β the geometry-dependent plastic zone shape factor, wp the specific non-essential plastic work and t the thickness of the specimen. we and βwp can Key Engineering Materials Online: 2007-03-15 ISSN: 1662-9795, Vols. 334-335, pp 549-552 doi:10.4028/www.scientific.net/KEM.334-335.549