Lubrication mechanisms of dispersed carbon microspheres in boundary through hydrodynamic lubrication regimes

HYPOTHESIS

Carbon microspheres have been shown to reduce friction and surface wear at relatively low speeds and high applied loads (i.e., within the boundary lubrication regime). We hypothesize that in dilute colloidal lubricating systems there is an interplay between the size of the carbon microspheres and the lubrication gap size, which determines the dominant lubricating mechanism of the system.

EXPERIMENTS

A 60 wt% aqueous glycerol solution was used as the base lubricant and compared to various carbon particle-based lubricant formulations ranging in particle concentrations from 0.05 to 0.30 vol%. The tribological properties of the various lubricant formulations were tested on a pin-on-disk tribometer. A simplified Stribeck plot was produced to understand the changing mechanism of lubrication over a wide range of conditions.

FINDINGS

The Stribeck curves show that the carbon microspheres assist lubrication by a rolling mechanism primarily in the boundary lubrication regime. A 0.20 vol% carbon-based lubricant formulation showed the best friction reduction compared to the base lubricant. Increasing speed increases the lubricating gap between the friction pair beyond the size of the particles, thereby nullifying the rolling mechanism of the particles. We introduce a modified specific film thickness parameter to determine the lubrication regime in a particle-lubricant system.

The load-bearing mechanism of plant wings: A multiscale structural and mechanical analysis of the T. tipu samara

Spinning winged fruits ("helicopter" samaras) generate significant lift forces at relatively low velocities, which enable the wind to disperse them across long distances. The biological material of the samara sustains the aerodynamic loadings and maintains the physical shape of the samara in the air via a yet unknown load-bearing mechanism. Here, positing that this mechanism fundamentally originates from the macro-to-microscale structural and mechanical characteristics of the samara, we use sub-micron computer tomography, electron microscopy, and multi-scale mechanical experiments to map the structural and mechanical characteristics of the tipu tree (Tipuana tipu) samara down to the micrometer length scale. Then, using theoretical models, we characterize the multiscale structural-mechanical principles of the samara and use these principles to disclose the underlying load-bearing mechanism. We found that the structural motifs of the tipu tree samara are closely analogous to various other types and forms of winged fruits, suggesting that this load-bearing mechanism is widespread in plant wings. The structural-mechanical principles governing the samara bear unconventional design concepts, which pave the way toward the development and engineering of small-scale wing elements for miniature aviation platforms with specialized mechanical capabilities. STATEMENT OF SIGNIFICANCE: The biomaterial of plant wings grants them mechanical resistance to flight forces during wind dispersal. "Helicopter seeds" demonstrate an intricate load-bearing mechanism that spans three structure-functional scales of their biomaterial. This mechanism appears widespread in plant wings and may promote novel micro-engineering design guidelines for futuristic flight materials and small-scale aviation platforms.

Mandibular Angle Fractures

Angle fractures are the most common among the mandibular fractures. History and physical examination are crucial in guiding time course and specifics of management. Computed tomography (CT) has become the gold standard for diagnosis of mandible fractures, offering advantages for both surgical planning and assessing dental involvement. Currently the use of a single monocortical plate with the Champy technique for osteosynthesis is used preferentially for noncomminuted fractures of the mandibular angle. Other load-sharing options for plating include strut plates, malleable plates, and geometric or 3D plates. Load-bearing options remain viable for comminuted fractures or other complex circumstances.

A pilot biomechanical study comparing a novel, intramedullary Nail/Plate construct to standard Dual-Plate fixation of intra-articular C2.3 distal humerus fractures

BACKGROUND

The gold-standard treatment for intra-articular distal humerus fractures (DHFs) is dual-plate/dual-column fixation, though optimal orientation is not yet established. With a superior method not yet identified, we propose a load-sharing construct, combining absolute stability (extramedullary plate fixation) for distal articular fragments and relative stability (load-sharing intramedullary nail) for the metaphyseal segment. The purpose of this pilot study was to evaluate the biomechanical performance of a novel implant compared to orthogonal dual-plating.

MATERIALS AND METHODS

Ten fresh-frozen matched-pairs of human cadaveric upper extremities with no prior elbow pathology/surgery were used. Pairs were randomized into two groups: Dual-Plate (medial and posterolateral) or novel Nail/Plate (cross-locked medial nail and posterolateral plate). AO/ASIF type 13-C2.3 multifragmentary fractures with simulated metaphyseal comminution. Biomechanical testing included stiffness (MPa) and load to failure (Newtons) in axial (100 cycles at 3 Hz at 20 N increments from 20 to 100 N) and coronal (varus/valgus; 4,000 cycles from 50N-100 N at 3 Hz) planes. Failed specimens were not analyzed and mechanisms were identified. For all failures, mechanisms were identified and reviewed by three consultant surgeons for revision vs. immobilization, to attempt to recreate a real-world scenario. All outcomes were compared between groups.

RESULTS

During stiffness testing, zero Nail/Plate specimens failed, but two (20%) Dual-Plate specimens failed (mechanisms: fracture diastasis; bone collapse and intussusception into osteotomy, yielding articular congruency loss). For remaining samples, Nail/Plate (n = 10) coronal (varus/valgus) stiffness was comparable to Dual-Plate (n = 8) constructs (41.5 vs. 39.0 MPa, p = 0.440). Remaining Dual-Plate constructs had greater axial overall stiffness than Nail/Plate (118.3 ± 48.3 vs. 95.6 ± 34.7 MPa, p = 0.020). Failure loads were comparable between Nail/Plate and Dual-Plate constructs (1,327.8 vs. 1,032.4 N, p = 0.170). Individual nail yield strength ranged from 1,101.1-1,124.4 N (n = 2). In review of all failures, the most common overall mechanism was fracture/osteotomy site posterolateral plate bending. Revision recommendation rate was comparable between constructs (Nail/Plate, 22.2% vs. Dual-Plate, 44.4%, p>0.05).

CONCLUSIONS

The novel Nail/Plate construct demonstrated non-inferior coronal (varus/valgus) stiffness, despite producing lower axial stiffness than orthogonal dual-plating, potentially due to the load-sharing cross-locked design. Considering comparable biomechanical performance, with no failures and comparable recommendations for revision, this novel construct warrants further evaluation as an alternative to the gold-standard, dual-plate fixation method for intra-articular distal humerus fractures.

LEVEL OF EVIDENCE

N/A.

Strontium ion reinforced bioceramic scaffold for load bearing bone regeneration

Bone defects in load bearing areas require bone reconstruction with strong biomaterial having mechanical characteristics like cortical bone. Bioceramics are biomaterials that support bone formation as well as provide adequate mechanical properties. A strontium substitution of the bioceramic is expected to further increase its bioactivity by enhancing osteogenesis and protect the bone from osteoclastic resorption. The study involves development, characterization and in vivo testing of a newly developed strontium substituted hydroxyapatite based bioceramic scaffold (SrHAB) with sufficient biomechanical properties. Optimal concentration of strontium ion required for enhanced osteogenic differentiation was identified by comparing three compositions of SrHAB scaffold; namely Sr10HAB, Sr30HAB and Sr50 HAB for their Alkaline phosphatase activity in vitro. The selected Sr10HAB scaffold demonstrated in vivo bone formation with osteogenic differentiation of stromal derived mesenchymal stem cells (MSC) from human and ovine sources in ectopic and ovine models. Thus, Sr10HAB scaffold has a potential for application in load bearing bone requirements of orthopaedics and dentistry.

Treatment of the surgical neck fracture of the humerus with a novel external fixator in the elderly with osteoporosis: biomechanical analysis

Background

No consensus exists on the treatment of proximal humerus fractures, especially in the elderly patients with osteoporosis. This study was aimed to evaluate the biomechanical characteristics of a novel external fixator in treating two-part surgical neck fracture of the proximal humerus in the elderly patients with osteoporosis.

Methods

Sixteen female elderly humeral shaft specimens with osteoporosis were randomized into 2 groups. Models with the surgical neck two-part fracture of the proximal humerus were built, in which a novel external fixator (test group) and a clover plate (control group) were applied separately. In the test group, the fracture was firstly fixed with intersection pinning using 3 Schanz pins (3.5 mm), followed by the novel external fixation frame. In the control group, a clover plate and 6 cortical bone screws were applied. Biomechanical testing of the specimens was performed to assess the resistance to load bearing and torsional stress. The parameters of the two groups were compared using independent t-test.

Results

Ultimate bearing capacity and load bearing at different parts with the humerus rotation were higher (P < 0.05) in the external fixator group (145.16 ± 17.42 N and 140 N respectively) than those in the plate group (120.21 ± 13.15 N and 69.63 ± 25.16–90.78 ± 17.18 N respectively). As for resistance to torsional stress, plate’s torque fluctuated between 1 Nm and 5 Nm, while the external fixator’s torque values were more evenly (P < 0.01) distributed with the fluctuation within 1 Nm.

Conclusions

In the fixation of two-part humeral fracture in elderly patients with osteoporosis, the new external fixator seemed to be superior to plate fixation in load bearing and resistance to torsional stress.

β-TCP-polylactide composite scaffolds with high strength and enhanced permeability prepared by a modified salt leaching method

A modified particulate leaching method for fabrication of strong calcium phosphate-polymer composite scaffolds with improved pore interconnectivity is reported. The scaffolds were produced by mixing precompacted composite granules (β-TCP with 40vol% PLA) of different size and density with salt particles followed by high pressure consolidation (at room temperature or 120°C) and porogen dissolution. The scaffolds' compressive strength and Darcy's permeability were found to be inversely related and to be strongly dependent on the processing parameters. The use of precompacted granules instead of the loose β-TCP-PLA powder allowed us to increase permeability by three orders of magnitude while maintaining load bearing characteristics. Scaffolds with 50% porosity prepared from large (300-420μm) composite granules of β-TCP-40vol% PLA and salt porogen particles of comparable size exhibited the best combination of compressive strength (4-6MPa) and permeability (1.3-1.6×10(-10)m(2)) falling within the range of trabecular bone.

The influence of load carrying on the energetics and kinematics of terrestrial locomotion in a diving bird

The application of artificial loads to mammals and birds has been used to provide insight into the mechanics and energetic cost of terrestrial locomotion. However, only two species of bird have previously been used in loading experiments, the cursorial guinea fowl (Numida meleagris) and the locomotor-generalist barnacle goose (Branta leucopsis). Here, using respirometry and treadmill locomotion, we investigate the energetic cost of carrying trunk loads in a diving bird, the tufted duck (Aythya fuligula). Attachment of back loads equivalent to 10% and 20% of body mass increased the metabolic rate during locomotion (7.94% and 15.92%, respectively) while sternal loads of 5% and 10% had a greater proportional effect than the back loads (metabolic rate increased by 7.19% and 13.99%, respectively). No effect on locomotor kinematics was detected during any load carrying experiments. These results concur with previous reports of load carrying economy in birds, in that there is a less than proportional relationship between increasing load and metabolic rate (found previously in guinea fowl), while application of sternal loads causes an approximate doubling of metabolic rate compared to back loads (reported in an earlier study of barnacle geese). The increase in cost when carrying sternal loads may result from having to move this extra mass dorso-ventrally during respiration. Disparity in load carrying economy between species may arise from anatomical and physiological adaptations to different forms of locomotion, such as the varying uncinate process morphology and hindlimb tendon development in goose, guinea fowl and duck.

Mesenchymal stem cell proliferation and differentiation on load-bearing trabecular Nitinol scaffolds

Bone tissue regeneration in load-bearing regions of the body requires high-strength porous scaffolds capable of supporting angiogenesis and osteogenesis. 70% porous Nitinol (NiTi) scaffolds with a regular 3-D architecture resembling trabecular bone were produced from Ni foams using an original reactive vapor infiltration technique. The "trabecular Nitinol" scaffolds possessed a high compressive strength of 79 MPa and high permeability of 6.9×10(-6) cm2. The scaffolds were further modified to produce a near Ni-free surface layer and evaluated in terms of Ni ion release and human mesenchymal stem cell (hMSC) proliferation (AlamarBlue), differentiation (alkaline phosphatase activity, ALP) and mineralization (Alizarin Red S staining). Scanning electron microscopy was employed to qualitatively corroborate the results. hMSCs were able to adhere and proliferate on both as-produced and surface-modified trabecular NiTi scaffolds, to acquire an osteoblastic phenotype and produce a mineralized extracellular matrix. Both ALP activity and mineralization were increased on porous scaffolds compared to control polystyrene plates. Experiments in a model coculture system of microvascular endothelial cells and hMSCs demonstrated the formation of prevascular structures in trabecular NiTi scaffolds. These data suggest that load-bearing trabecular Nitinol scaffolds could be effective in regenerating damaged or lost bone tissue.