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ABSTRACT: A transient multi-physics model of the mitral heart valve has been developed, which allows simultaneous calculation of fluid flow and structural deformation. A recently developed contact method has been applied to enable simulation of systole (the stage when blood pressure is elevated within the heart to pump blood to the body). The geometry was simplified to represent the mitral valve within the heart walls in two dimensions. Only the mitral valve undergoes deformation. A moving arbitrary Lagrange-Euler mesh is used to allow true fluid-structure interaction (FSI). The FSI model requires blood flow to induce valve closure by inducing strains in the region of 10-20%. Model predictions were found to be consistent with existing literature and will undergo further development.
Computer Methods in Biomechanics and Biomedical Engineering 05/2012; · 0.85 Impact Factor
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ABSTRACT: STUDY DESIGN.: Mechanical testing of total disc arthroplasty (TDA). OBJECTIVE.: To compare the friction between a polymer socket-on-metal ball and metal socket-on-polymer ball TDA. SUMMARY OF BACKGROUND DATA.: A degenerate intervertebral disc can be replaced by TDA. The most common designs have a ball and socket articulation; the contact between the surfaces leads to friction. Friction needs to be minimized to prevent loosening and wear. One of the common material combinations in disc arthroplasty devices is the articulation of a metal socket on polymer ball. However, the combination of a polymer socket on metal ball (which is used in hip arthroplasty) has not been investigated for TDA. METHODS.: TDA models with either a polymer socket/metal ball or a metal socket/polymer ball were manufactured with ball radii of 10 and 14 mm, each with a radial clearance of 0.35 mm. Samples were tested using a spine simulator with a lubricant of diluted newborn calf serum. Each sample was subjected to an axial load of 1200 N; motions of flexion-extension, lateral bending, and axial rotation were then applied at frequencies of 0.25 to 2 Hz. Frictional torque was measured to compare the performance of the TDAs. RESULTS.: The frictional torque was found to be significantly higher for a disc with a metal socket/polymer ball than for a disc with a polymer socket/metal ball for both 10 and 14 mm radii in axial rotation, lateral bend, and extension. The frictional torque in flexion (0°-6°) was not found to be significantly different between the 2 different material combinations. However, when the flexion motion was reduced to 0° to 2°, frictional torque in the metal socket/polymer ball was found to be significantly higher than the polymer socket/metal ball. CONCLUSION.: TDA with a combination of a polymer socket/metal ball has lower friction than the conventional TDA with metal socket/polymer ball. This conclusion has implications in the design of TDA.
Spine 04/2012; 37(21):1834-8. · 2.08 Impact Factor
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ABSTRACT: Repair of the mitral valve is defined (loosely) as a procedure that alters the valve structure, without replacement, enabling the natural valve itself to continue to perform under the physical conditions to which it is exposed. As the mitral valve is driven by flow and pressure, it should be feasible to analyse and assess its function, failure and repair as a mechanical system. This article reviews the current state of mechanical evaluation of surgical repairs of the failed mitral valve of the heart. This review describes the anatomy and physiology of the mitral valve, followed by the failure of the mitral valve from a mechanical point of view. The surgical methods used to repair failed valves are introduced, while the use of engineering analysis to aid understanding of mitral valve repair is also reviewed. Finally, a section on recommendations for development and future uses of engineering techniques to surgical repair are presented.
Proceedings of the Institution of Mechanical Engineers Part H Journal of Engineering in Medicine 04/2012; 226(4):275-87. · 1.21 Impact Factor
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ABSTRACT: Twelve Cadisc-L devices were subjected to flexion (0°-6°) and extension (0° to -3°) motions at compressive loads between 500 N and 2000 N at a flexural rate between 0.25°/s and 3.0°/s.
To quantify the change in flexural properties of the Cadisc-L (elastomeric device), when subjected to increasing magnitudes of axial load and at different flexural rates.
The design of motion preservation devices, used to replace degenerated intervertebral discs, is commonly based on a low-friction, ball-and-socket-articulating joint. Recently, elastomeric implants have been developed that attempt to provide mechanical and motion properties that resemble those of the natural disc more closely.
Twelve Cadisc-L devices (MC-10 mm-9° and MC-10 mm-12° size) were supplied by Ranier Technology Ltd (Cambridge, United Kingdom). The devices were hydrated and tested using a Bose spinal disc-testing machine (Bose Corporation, ElectroForce Systems Group, Eden Prairie, MN) in Ringer's solution at 37°C. A static load of 500 N was applied to a device and it was then subjected to motions of 0° to 6° to 0° (flexion) and 0° to -3° to 0° (extension) at a flexural rate of 0.25°/s, 0.5°/s, 1.0°/s, 1.5°/s, 2.0°/s, and 3.0°/s. Tests were repeated at 1000 N, 1500 N, and 2000 N.
Regression analyses showed a significant (R > 0.99, P < 0.05) linear increase in bending moment and flexural stiffness with flexion and extension angles (at 1000 N and higher loads)-a significant (R > 0.994, P < 0.05) linear decrease in flexural stiffness in flexion and extension as flexural rate increased.
The bending moment of the Cadisc-L increased linearly with flexion and extension angles at 1000 N and higher loads. Flexural stiffness increased with compressive load but decreased with flexural rate.
Spine 02/2012; 37(15):E908-12. · 2.08 Impact Factor
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ABSTRACT: The mineral content of cancellous bone from sheep and pig vertebral bodies was determined by ashing at 800 degrees C. The results were compared with published results for human vertebral cancellous bone. The results for sheep (0.37 +/- 0.06 gcm(-3), mean +/- standard deviation) were not significantly different (p = 0.127) to those from pigs (0.33 +/- 0.03 gcm(-3)). The results from both species were significantly higher (p < 0.001) than those from human bones (0.15 +/- 0.02 gcm(-3)). This means that cancellous bone from sheep and pig vertebral bodies is not a good model for corresponding human bone. However, sheep and pig bone are useful, for example, for providing stringent tests of cutting instruments to be used in human spinal surgery.
Proceedings of the Institution of Mechanical Engineers Part H Journal of Engineering in Medicine 01/2012; 226(1):76-8. · 1.21 Impact Factor
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ABSTRACT: Total disc arthroplasty (TDA) can be used to replace a degenerated intervertebral disc in the spine. There are different designs of prosthetic discs, but one of the most common is a ball-and-socket combination. Contact between the bearing surfaces can result in high frictional torque, which can then result in wear and implant loosening. This study was designed to determine the effects of ball radius on friction. Generic models of metal-on-metal TDA were manufactured with ball radii of 10, 12, 14 and 16 mm, with a radial clearance of 0.015 mm. A simulator was used to test each sample in flexion-extension, lateral bending and axial rotation at frequencies of 0.25, 0.5, 0.75, 1, 1.25, 1.5, 1.75 and 2 Hz under loads of 50, 600, 1200 and 2000 N, in new born calf serum. Frictional torque was measured and Stribeck curves were plotted to illustrate the lubrication regime in each case. It was observed that implants with a smaller ball radius showed lower friction and showed boundary and mixed lubrication regimes, whereas implants with larger ball radius showed boundary lubrication only. This study suggests designing metal-on-metal TDAs with ball radius of 10 or 12 mm, in order to reduce wear and implant loosening.
Journal of biomechanics 12/2011; 45(3):504-9. · 2.66 Impact Factor
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ABSTRACT: Alginate hydrogels have been used widely in tissue engineering for cell encapsulation for several reasons: low toxicity, the ability to gel under gentle condition and compatibility with cells. In this study, we determined the effect of different concentrations of alginate on encapsulation of 3T3 fibroblast cells at two different cell seeding densities. Live/dead staining and MTT assay were performed at regular intervals up to 4 weeks. A Hoechst 33258 assay was done to validate the MTT results. There were more dead cells on day 1 for the higher concentrations of alginate while at, the lower concentration of alginate, cell proliferation and spheroid formation occurred more quickly. Furthermore, at low cell seeding density, cell proliferation was prolonged compared to the intermediate seeding density. In conclusion, by altering both alginate concentration and cell seeding density, proliferation and spheroid formation can be controlled.
Bio-medical materials and engineering 01/2011; 21(3):159-70. · 1.23 Impact Factor
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ABSTRACT: Screw fixation can be extremely difficult to achieve in osteoporotic (OP) bone because of its low strength. This study determined how pullout strength is affected by placing different bone screws at varying angles in normal and OP bone models. Pullout tests of screws placed axially, and at angles to the pullout axis (ranging from 10° to 40°), were performed in 0.09 g cm(-3), 0.16 g cm(-3) and 0.32 g cm(-3) polyurethane (PU) foam. Two different titanium alloy bone screws were used to test for any effect of thread type (i.e. cancellous or cortical) on the screw pullout strength. The cancellous screw required a significantly higher pullout force than the cortical screw (p<0.05). For both screws, pullout strength significantly increased with increasing PU foam density (p<0.05). For screws placed axially, and sometimes at 10°, the observed mechanism of failure was stripping of the internal screw threads generated within the PU foam by screw insertion. For screws inserted at 10°, 20°, 30° and 40°, the resistance to pullout force was observed to be by compression of the PU foam material above the angled screw; clinically, this suggests that compressed OP bone is stronger than unloaded OP bone.
Medical Engineering & Physics 10/2010; 32(8):822-8. · 1.62 Impact Factor
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ABSTRACT: Four medical grade silicones were swollen, until they reached equilibrium (i.e. constant mass) in eight liquids at 25 degrees C. The greatest swelling was obtained with n-heptane but the volume fraction, varphi, of the silicones in their swollen state was not significantly different (p<0.05) in this liquid than in cyclohexane. For each grade of silicone, varphi was plotted against delta(l), the liquid solubility parameter, for each liquid in which it was swollen. A second-order polynomial was plotted through the results; the minimum in this polynomial provided a value for the polymer solubility parameter, delta(p). The Flory polymer-liquid interaction parameter, chi, was calculated for the four best liquids, using Hildebrand's solubility parameter theory. An alternative method for calculating chi, directly from swelling measurements, was shown to produce physically unreasonable results. The cross-link density, upsilon, was calculated, from varphi and chi, for each grade of silicone, using the Flory-Rehner equation. Since the values of two parameters involved in Hildebrand's theory cannot be determined reliably and because the Flory-Rehner equation is an approximation, absolute values of upsilon cannot be obtained. However, the relative values of upsilon obtained were higher for the harder grades then for the softer grades and similarly, the grades with the higher Young's modulus had higher upsilon values.
Medical Engineering & Physics 05/2010; 32(4):298-303. · 1.62 Impact Factor
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Medical Engineering & Physics 03/2010; · 1.62 Impact Factor
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ABSTRACT: Articular cartilage is a viscoelastic material, but its exact behaviour under the full range of physiological loading frequencies is unknown. The objective of this study was to measure the viscoelastic properties of bovine articular cartilage at loading frequencies of up to 92 Hz.
Intact tibial plateau cartilage, attached to subchondral bone, was investigated by dynamic mechanical analysis (DMA). A sinusoidally varying compressive force of between 16 N and 36 N, at frequencies from 1 Hz to 92 Hz, was applied to the cartilage surface by a flat indenter. The storage modulus, loss modulus and phase angle (between the applied force and the deformation induced) were determined.
The storage modulus, E', increased with increasing frequency, but at higher frequencies it tended towards a constant value. Its dependence on frequency, f, could be represented by, E' = Alog(e) (f) + B where A = 2.5 +/- 0.6 MPa and B = 50.1 +/- 12.5 MPa (mean +/- standard error). The values of the loss modulus (4.8 +/- 1.0 MPa mean +/- standard deviation) were much less than the values of storage modulus and showed no dependence on frequency. The phase angle was found to be non-zero for all frequencies tested (4.9 +/- 0.6 degrees ).
Articular cartilage is viscoelastic throughout the full range of frequencies investigated. The behaviour has implications for mechanical damage to articular cartilage and the onset of osteoarthritis. Storage modulus increases with frequency, until the plateau region is reached, and has a higher value than loss modulus. Furthermore, loss modulus does not increase with loading frequency. This means that more energy is stored by the tissue than is dissipated and that this effect is greater at higher frequencies. The main mechanism for this excess energy to be dissipated is by the formation of cracks.
BMC Musculoskeletal Disorders 02/2009; 10:61. · 1.58 Impact Factor
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ABSTRACT: This paper investigates the effect of sterilisation by gamma irradiation (dose 2.5Mrad) on the following properties of polycaprolactone (PCL): (1) degradation rate (catalysed by lipase), (2) mechanical properties, (3) the ability of cells to attach and subsequently grow on its surface. Gel permeation chromatography (GPC) was used to determine the effects of gamma irradiation of weight average (M(w)) and number average (M(n)) molecular weights. Gamma irradiation significantly decreased the rate of degradation, although the rates depended on the initial mass of polymer; it also affected the appearance of the degraded specimens when they were examined by scanning electron microscopy. Irradiation also significantly increased the mechanical yield stress but not the failure stress of PCL. It caused a significant increase in M(w) and decrease in M(n) that could be attributed to chain scission and cross-linking. Chondrocyte attachment and growth on PCL was not significantly affected by gamma irradiation.
Medical Engineering & Physics 09/2008; 31(2):221-6. · 1.62 Impact Factor
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ABSTRACT: Unlike engineering fibre composite materials which comprise of fibres that are uniform cylindrical in shape, collagen fibrils reinforcing the proteoglycan-rich (PG) gel in the extra-cellular matrices (ECMs) of connective tissues are taper-ended (paraboloidal in shape). In an earlier paper we have discussed how taper of a fibril leads to an axial stress up-take which differs from that of a uniform cylindrical fibre and implications for fibril fracture. The present paper focuses on the influence of fibre aspect ratio, q (slenderness), and Young's modulus (stiffness), relative to that of the gel phase, E(R), on the magnitude of the axial tensile stresses generated within a fibril and wider implications on failure at tissue level. Fibre composite models were evaluated using finite element (FE) and mathematical analyses. When the applied force is low, there is elastic stress transfer between the PG gel and a fibril. FE modelling shows that the stress in a fibril increases with E(R) and q. At higher applied forces, there is plastic stress transfer. Mathematical modelling predicts that the stress in a fibril increases linearly with q. For small q values, fibrils may be regarded as fillers with little ability to provide tensile reinforcement. Large q values lead to high stress in a fibril. Such high stresses are beneficial provided they do not exceed the fracture stress of collagen. Modulus difference regulates the strain energy release density, u, for interfacial rupture; large E(R) not only leads to high stress in a fibril but also insures against interfacial rupture by raising the value of u.
Journal of Theoretical Biology 04/2007; 245(2):305-11. · 2.21 Impact Factor
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ABSTRACT: Statistical analysis of clinical radiologic data.
To develop an objective method for finding the number of sizes for a lumbar disc replacement.
Cluster analysis is a well-established technique for sorting observations into clusters so that the "similarity level" is maximal if they belong to the same cluster and minimal otherwise.
Magnetic resonance scans from 69 patients, with no abnormal discs, yielded 206 sagittal and transverse images of 206 discs (levels L3-L4-L5-S1). Anteroposterior and lateral dimensions were measured from vertebral margins on transverse images; disc heights were measured from sagittal images. Hierarchical cluster analysis was performed to determine the number of clusters followed by nonhierarchical (K-means) cluster analysis. Discriminant analysis was used to determine how well the clusters could be used to classify an observation.
The most successful method of clustering the data involved the following parameters: anteroposterior dimension; lateral dimension (both were the mean of results from the superior and inferior margins of a vertebral body, measured on transverse images); and maximum disc height (from a midsagittal image). These were grouped into 7 clusters so that a discriminant analysis was capable of correctly classifying 97.1% of the observations. The mean and standard deviations for the parameter values in each cluster were determined.
Cluster analysis has been successfully used to find the dimensions of the minimum number of prosthesis sizes required to replace L3-L4 to L5-S1 discs; the range of sizes would enable them to be used at higher lumbar levels in some patients.
Spine 01/2007; 31(25):2979-83; discussion 2984. · 2.08 Impact Factor
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ABSTRACT: The study aim was to compare mitral valve repair techniques in vitro. Rupture or elongation of the mitral valve chordae tendineae is a known cause of mitral regurgitation, and can be corrected by edge-to-edge repair, chordal replacement, or chordal transposition.
A test apparatus was used to apply pressure to porcine mitral valves. Mitral valve specimens were tested intact (n = 50), after they had been experimentally damaged, and after repair. Each test was repeated ten times. Experimental damage consisted of severing either the anterior leaflet strut, and attached marginal chordae (n = 30) or posterior leaflet chordae (n = 20). Valves with damaged anterior leaflets were repaired by either: (i) edge-to-edge repair; (ii) chordal replacement; or (iii) chordal transposition. Valves with damaged posterior leaflets were repaired by the first two techniques. Each repair method was repeated on ten specimens.
Mitral valves repaired using the edge-to-edge repair (p = 0.002) and chordal replacement (p = 0.038), after rupture to anterior leaflet chordae, recovered significantly better than specimens repaired by chordal transposition. There was no statistical difference in recovery between edge-to-edge repair and chordal replacement (p > 0.05). There was no statistical difference (p > 0.05) in the recovery of the pressure withstood by valves repaired by edge-to-edge repair and chordal replacement, after rupture of posterior leaflet chordae.
These results showed that edge-to-edge repair and chordal replacement are well suited for the repair of both the anterior and posterior leaflets.
The Journal of heart valve disease 05/2006; 15(3):375-81. · 0.81 Impact Factor
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ABSTRACT: The study aim was to understand the role of different mitral valve chordae tendineae, and how damage to them affects valve competence.
A test apparatus was used to apply pressure to porcine mitral heart valves that were intact and have had selected chords severed. Anterior leaflet strut and marginal chords were selectively severed, as were posterior leaflet basal and marginal chords. Commissural chords were also severed.
Severing anterior leaflet marginal chords (p = 0.018) and commissural chords (p = 0.018) significantly reduced mitral valve competence. Severing posterior leaflet marginal and basal chords, and anterior leaflet strut chords, had no significant effect in reducing the pressures that the valves could withstand. Severing a mixture of posterior leaflet basal and marginal chords significantly reduced the pressure withstood by the valves (p = 0.004).
The study results confirmed that anterior leaflet marginal chords, but not strut chords, are vital for valve competence. Commissural chords were also shown to be vital for mitral valve competence. Several posterior leaflet chords had to be severed to affect mitral valve competence.
The Journal of heart valve disease 10/2005; 14(5):603-9. · 0.81 Impact Factor
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ABSTRACT: Statistical methods allow the effects of uncertainty to be incorporated into finite element models. This has potential benefits for the analysis of biological systems where natural variability can give rise to substantial uncertainty in both material and geometrical properties. In this study, a simple model of the intervertebral disc under compression was created and analysed as both a deterministic and a stochastic system. Factorial analysis was used to determine the important parameters to be included in the stochastic analysis. The predictions from the model were compared to experimental results from 21 sheep discs. The size and shape of the distribution of the axial deformations predicted by the model was consistent with the experimental results given that the number of model solutions far exceeded the number of experimental results. Stochastic models could be valuable in determining the range and most likely value of stress in a tissue or implant.
Computer Methods in Biomechanics and Biomedical Engineering 09/2003; 6(4):243-8. · 0.85 Impact Factor
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ABSTRACT: Sternal dehiscence is a relatively rare but serious complication of sternal closure with an unacceptable mortality and morbidity rate. The 6 mm cannulated Sternum Screws are a novel approach that aims to prevent dehiscence. The screws are placed on either side of the sternotomy and wire threaded through the cannula and closed customarily. The Sternum Screws make the bone non-penetrable at the same time retaining the familiarity of conventional stainless steel wire. This novel technique was tested in sheep sterna to compare its efficacy to conventional wire closure. Using tensile testing in a testing machine, randomised controlled closures of the Sternum Screws and No. 5 stainless steel wires were evaluated until system failure. Seventeen matched pairs were tested. The Sternum Screw closure was on average 36% stronger, 284+/-43 N (mean+/-SD) compared to conventional wire closure alone, 215+/-38 N (mean+/-SD) [p<0.0001 by t-test]. System failure in 82% of Sternum Screw closures, however, was due to wire breakage or untwisting rather than the screw itself. In all these cases the screw remained intact in bone. 71% of conventional wire closures failed by dehiscing through the sterna. The mean forces required for wire dehiscence and wire failure in the Sternum Screws may be achieved in vivo during large coughs. The study shows there is merit in further evaluating the approach as a method of preventing dehiscence. It also highlights the use of alternative wiring techniques increase the tensile strength of the closure yet retain the familiarity and versatility of conventional wire.
Cardiovascular Surgery 02/2003; 11(1):85-9.
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ABSTRACT: To show the benefits of using a novel approach to closure of the median sternotomy through a mechanical model and mechanical testing. Simple cannulated screws are placed on either side of the sternotomy. Conventional stainless steel wire is passed through the cannula of each screw and the sternotomy is closed in the usual manner.
Hertzian contact analysis was used to estimate the stress between the wire and the sternum. Mechanical testing was used to compare using wire on its own with a sternum screw plus wire. Ten samples of balsa wood (sternum substitute) had wire placed through a hole in them, while a further ten samples were fitted with a cannulated screw and had wire passed through the screw cannula. The wire was connected to a materials testing machine, which applied tension to the wire until the wire or screw cut through the wood.
The analysis showed that the mean stress between the wire and the sternum decreases with increasing wire diameter. At low diameters of wire the stress in the sternum can be comparable to the failure stress of bone. Using a cannulated screw reduces the stresses in the sternum. The mechanical testing showed that the wire cut through the wood at a mean load of 104 N, whereas the sternum screw cut through the wood at a mean load of 209 N (p = 0.007, Mann-Whitney Test).
Closing a median sternotomy with cannulated screws plus wire should reduce the occurrence of sternal dehiscence.
The Heart Surgery Forum 02/2002; 5(1):69-74. · 0.42 Impact Factor
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ABSTRACT: The viscoelastic properties of cylinders (diameter 5 mm, height 2.2 ± 0.2 mm) of Elast-Eon™ 3, (a polyurethane with poly(dimethylsiloxane) and poly(hexamethylamine oxide) segments) were investigated before and after the specimens had undergone accelerated aging in saline solution at 70 °C for 38, 76 and 114 days (to simulate aging at 37 °C, for 1, 2 and 3 years, respectively). All sets of specimens were immersed in physiological saline solution at 37 °C during testing and the properties were measured using dynamic mechanical analysis (DMA). A sinusoidal cyclic compression of 40 N ± 5 N was applied over a frequency range, f, of 0.02–100 Hz. Values of the storage, E′, and loss, E″, moduli were found to depend on f; the dependence of E′ or E″ on the logarithm (base 10) of f was represented by a second-order polynomial. After accelerated aging, the E′ and E″ increased significantly (p < 0.05) and the specimens became darker and more opaque. SEM images showed that accelerated aging affected the surface morphology but ATR-FTIR spectra did not show any appreciable change in surface chemistry. DSC thermograms showed some slight changes in thermal properties following accelerated aging.
Materials Science and Engineering: C.
