Co-reporter:P. G. Dixon;K. E. Semple;A. Kutnar
European Journal of Wood and Wood Products 2016 Volume 74( Issue 5) pp:633-642
Publication Date(Web):2016 September
DOI:10.1007/s00107-016-1047-9
The flexural properties in the longitudinal direction for natural and thermo-hydro-mechanically densified Moso bamboo (Phyllostachys pubescens Mazel) culm wall material are measured. The modulus of elasticity (MOE) and modulus of rupture (MOR) increase with densification, but at the same density, the natural material is stiffer and stronger than the densified material. This observation is primarily attributed to bamboo’s heterogeneous structure and the role of the parenchyma in densification. The MOE and MOR of both the natural and densified bamboo appear linearly related to density. Simple models are developed to predict the flexural properties of natural bamboo. The structure of the densified bamboo is modelled, assuming no densification of bamboo fibers, and the flexural properties of densified bamboo are then predicted using this structure and the same cell wall properties of that of the natural material modelling. The results are then compared with those for two analogous structural bamboo products: Moso bamboo glulam and scrimber.
Co-reporter:Marc Borrega;Patrik Ahvenainen;Ritva Serimaa;Lorna Gibson
Wood Science and Technology 2015 Volume 49( Issue 2) pp:403-420
Publication Date(Web):2015 March
DOI:10.1007/s00226-015-0700-5
Balsa, with its low density and relatively high mechanical properties, is frequently used as the core in structural sandwich panels, in applications ranging from wind turbine blades to racing yachts. Here, both the cellular and cell wall structure of balsa are described, to enable multi-scale modeling and an improved understanding of its mechanical properties. The cellular structure consists of fibers (66–76 %), rays (20–25 %) and vessels (3–9 %). The density of balsa ranges from roughly 60 to 380 kg/m3; the large density variation derives largely from the fibers, which decrease in edge length and increase in wall thickness as the density increases. The increase in cell wall thickness is predominantly due to a thicker secondary S2 layer. Cellulose microfibrils in the S2 layer are highly aligned with the fiber axis, with a mean microfibril angle (MFA) of about 1.4°. The cellulose crystallites are about 3 nm in width and 20–30 nm in length. The degree of cellulose crystallinity appears to be between 80 and 90 %, considerably higher than previously reported for other woods. The outstanding mechanical properties of balsa wood in relation to its weight are likely explained by the low MFA and the high cellulose crystallinity.
Co-reporter:P.G. Dixon, P. Ahvenainen, A.N. Aijazi, S.H. Chen, S. Lin, P.K. Augusciak, M. Borrega, K. Svedström, L.J. Gibson
Construction and Building Materials 2015 90() pp: 11-17
Publication Date(Web):15 August 2015
DOI:10.1016/j.conbuildmat.2015.04.042
•Longitudinal MOE and MOR of bamboo material from three species are investigated.•At a given density, the MOE of Guadua is higher than those of two other species.•All three species’ MOR can be described by a single linear density relationship.•Ultrastructural measurements suggest the solid cell wall of Guadua is stiffest.Bamboo is an underutilized resource widely available in countries with rapidly developing economies. Structural bamboo products, analogous to wood products, allow flexibility in the shape and dimensions of bamboo structural members. Here, the ultrastructure, microstructure, cell wall properties and flexural properties of three species of bamboo (Moso, Guadua and Tre Gai) are compared. At a given density, the axial modulus of elasticity of Guadua is higher than that of Moso or Tre Gai, which are similar; ultrastructural results suggest that Guadua has a higher solid cell wall stiffness. At a given density, their moduli of rupture are similar.
Co-reporter:Zubaidah Mohammed Ali and Lorna J. Gibson
Soft Matter 2013 vol. 9(Issue 5) pp:1580-1588
Publication Date(Web):07 Dec 2012
DOI:10.1039/C2SM27197D
Crystalline nanofibrillar cellulose has remarkable mechanical properties: a Young's modulus of about 130 GPa and a tensile strength in the range of 750–1000 MPa. Recently, there has been increasing interest in exploiting these exceptional properties in engineering composites and foams. Here, we compare measurements of the mechanical properties of nanofibrillar cellulose composites and foams with their potential properties based on models for composites and foams. We find that current NFC foams do not yet reach their potential and suggest modifications to their microstructure for improved mechanical performance.
Co-reporter:Biraja P. Kanungo, Lorna J. Gibson
Acta Biomaterialia 2010 Volume 6(Issue 2) pp:344-353
Publication Date(Web):February 2010
DOI:10.1016/j.actbio.2009.09.012
Abstract
Collagen–glycosaminoglycan scaffolds for the regeneration of skin have previously been fabricated by freeze-drying a slurry containing a co-precipitate of collagen and glycosaminoglycan. The mechanical properties of the scaffold are low (e.g. the dry compressive Young’s modulus is roughly 30 kPa and the dry compressive strength is roughly 5 kPa). There is interest in using these scaffolds for tendon and ligament regeneration where there is a need for improved mechanical properties. Previous attempts to increase the mechanical properties of the scaffold by increasing the solid volume fraction of the scaffolds were limited by the increasing viscosity of the slurry, making it more difficult to mix and giving inhomogeneous scaffolds. Our recent work on mineralized collagen–glycosaminoglycan scaffolds used a vacuum filtration technique to increase the volume fraction of solids in the slurry, thereby increasing the density and mechanical properties of the scaffolds. In this work, we used this technique to fabricate collagen–glycosaminoglycan scaffolds with dry densities between 0.0076 and 0.0311 g cm−3 and pore sizes between 250 and 350 μm, values appropriate for soft tissue growth. The compressive Young’s modulus and strength in the dry state increased from 32 to 127 kPa and from 5 to 19 kPa, respectively, with increasing density. The tensile Young’s modulus in the dry state increased from 295 to 3.1 MPa with increasing density. Finally, we showed that the attachment of cells onto the scaffold was directly proportional to the specific surface area of the scaffold, which defines the total internal surface area per volume of scaffold.
Co-reporter:Brendan A. Harley;Andrew K. Lynn;Zachary Wissner-Gross;William Bonfield;Ioannis V. Yannas
Journal of Biomedical Materials Research Part A 2010 Volume 92A( Issue 3) pp:1078-1093
Publication Date(Web):
DOI:10.1002/jbm.a.32387
Abstract
There is a need to improve current treatments for articular cartilage injuries. This article is the third in a series describing the design and development of an osteochondral scaffold based on collagen-glycosaminoglycan and calcium phosphate technologies for regenerative repair of articular cartilage defects. The previous articles in this series described methods for producing porous, three-dimensional mineralized collagen-GAG (CGCaP) scaffolds whose composition can be reproducibly varied to mimic the composition of subchondral bone, and pore microstructure and mineral phase can be modified. This article describes a method, “liquid-phase cosynthesis,” that enables the production of porous, layered scaffolds that mimic the composition and structure of articular cartilage on one side, subchondral bone on the other side, and the continuous, gradual or “soft” interface between these tissues: the tidemark of articular joints. This design enables the layered scaffolds to be inserted into the subchondral bone at an osteochondral defect site without the need for sutures, glue, or screws, with a highly interconnected porous network throughout the entire osteochondral defect. Moreover, the differential moduli of the osseous and cartilaginous compartments enable these layered scaffolds to exhibit compressive deformation behavior that mimics the behavior observed in natural articular joints. © 2009 Wiley Periodicals, Inc. J Biomed Mater Res, 2010
Co-reporter:Brendan A. Harley;Andrew K. Lynn;Zachary Wissner-Gross;William Bonfield;Ioannis V. Yannas
Journal of Biomedical Materials Research Part A 2010 Volume 92A( Issue 3) pp:1066-1077
Publication Date(Web):
DOI:10.1002/jbm.a.32361
Abstract
This paper is the second in a series of papers describing the design and development of an osteochondral scaffold using collagen–glycosaminoglycan and calcium phosphate technologies engineered for the regenerative repair of articular cartilage defects. The previous paper described a technology (concurrent mapping) for systematic variation and control of the chemical composition of triple coprecipitated collagen, glycosaminoglycan, and calcium phosphate (CGCaP) nanocomposites without using titrants. This paper describes (1) fabricating porous, three-dimensional scaffolds from the CGCaP suspensions, (2) characterizing the microstructure and mechanical properties of such scaffolds, and (3) modifying the calcium phosphate mineral phase. The methods build on the previously demonstrated ability to vary the composition of a CGCaP suspension (calcium phosphate mass fraction between 0 and 80 wt %) and enable the production of scaffolds whose pore architecture (mean pore size: 50–1000 μm), CaP phase chemistry (brushite, octacalcium phosphate, apatite) and crosslinking density (therefore mechanical properties and degradation rate) can be independently controlled. The scaffolds described in this paper combine the desirable biochemical properties and pore architecture of porous collagen–glycosaminoglycan scaffolds with the strength and direct bone-bonding properties of calcium phosphate biomaterials in a manner that can be tailored to meet the demands of a range of applications in orthopedics and regenerative medicine. © 2009 Wiley Periodicals, Inc. J Biomed Mater Res, 2010
Co-reporter:Andrew K. Lynn;Serena M. Best;Ruth E. Cameron;Brendan A. Harley;Ioannis V. Yannas;William Bonfield
Journal of Biomedical Materials Research Part A 2010 Volume 92A( Issue 3) pp:1057-1065
Publication Date(Web):
DOI:10.1002/jbm.a.32415
Abstract
This is the first in a series of articles that describe the design and development of a family of osteochondral scaffolds based on collagen-glycosaminoglycan (collagen-GAG) and calcium phosphate technologies, engineered for the regenerative repair of defects in articular cartilage. The osteochondral scaffolds consist of two layers: a mineralized type I collagen-GAG scaffold designed to regenerate the underlying subchondral bone and a nonmineralized type II collagen-GAG scaffold designed to regenerate cartilage. The subsequent articles in this series describe the fabrication and properties of a mineralized scaffold as well as a two-layer (one mineralized, the other not) osteochondral scaffold for regeneration of the underlying bone and cartilage, respectively. This article describes a technology through which the chemical composition—particularly the calcium phosphate mass fraction—of triple coprecipitated nanocomposites of collagen, glycosaminoglycan, and calcium phosphate can be accurately and reproducibly varied without the need for titrants or other additives. Here, we describe how the mineral:organic ratio can be altered over a range that includes that for articular cartilage (0 wt % mineral) and for bone (75 wt % mineral). This technology achieves the objective of mimicking the composition of two main tissue types found in articular joints, with particular emphasis on the osseous compartment of an osteochondral scaffold. Exclusion of titrants avoids the formation of potentially harmful contaminant phases during freeze-drying steps crucial for scaffold fabrication, ensuring that the potential for binding growth factors and drugs is maintained. © 2009 Wiley Periodicals, Inc. J Biomed Mater Res, 2010
Co-reporter:Karolina A. Corin, Lorna J. Gibson
Biomaterials 2010 Volume 31(Issue 18) pp:4835-4845
Publication Date(Web):June 2010
DOI:10.1016/j.biomaterials.2010.01.149
The contractile behavior of cells is relevant in understanding wound healing and scar formation. In tissue engineering, inhibition of the cell contractile response is critical for the regeneration of physiologically normal tissue rather than scar tissue. Previous studies have measured the contractile response of cells in a variety of conditions (e.g. on two-dimensional solid substrates, on free-floating tissue engineering scaffolds and on scaffolds under some constraint in a cell force monitor). Tissue engineering scaffolds behave mechanically like open-cell elastomeric foams: between strains of about 10 and 90%, cells progressively buckle struts in the scaffold. The contractile force required for an individual cell to buckle a strut within a scaffold has been estimated based on the strut dimensions (radius, r, and length, l) and the strut modulus, Es. Since the buckling force varies, according to Euler's law, with r4/l2, and the relative density of the scaffold varies as (r/l)2, the cell contractile force associated with strut buckling is expected to vary with the square of the pore size for scaffolds of constant relative density. As the cell density increases, the force per cell to achieve a given strain in the scaffold is expected to decrease. Here we model the contractile response of fibroblasts by analyzing the response of a single tetrakaidecahedron to forces applied to individual struts (simulating cell contractile forces) using finite element analysis. We model tetrakaidecahedra of different strut lengths, corresponding to different scaffold pore sizes, and of varying numbers of loaded struts, corresponding to varying cell densities. We compare our numerical model with the results of free-floating contraction experiments of normal human dermal fibroblasts (NHDF) in collagen-GAG scaffolds of varying pore size and with varying cell densities.
Co-reporter:Biraja P. Kanungo, Lorna J. Gibson
Acta Biomaterialia 2009 Volume 5(Issue 4) pp:1006-1018
Publication Date(Web):May 2009
DOI:10.1016/j.actbio.2008.11.029
Abstract
Mineralized collagen–glycosminoglycan scaffolds have previously been fabricated by freeze-drying a slurry containing a co-precipitate of calcium phosphate, collagen and glycosaminoglycan. The mechanical properties of the scaffold are low (e.g. the dry Young’s modulus for a 50 wt.% mineralized scaffold is roughly 780 kPa). Our previous attempt to increase the mechanical properties of the scaffold by increasing the mineralization (from 50 to 75 wt.%) was unsuccessful due to defects in the more mineralized scaffold. In this paper, we describe a new technique to improve the mechanical properties by increasing the relative density of the scaffolds. The volume fraction of solids in the slurry was increased by vacuum-filtration. The slurry was then freeze-dried in the conventional manner to produce scaffolds with relative densities between 0.045 and 0.187 and pore sizes of about 100–350 μm, values appropriate for bone growth. The uniaxial compressive stress–strain curves of the scaffolds indicated that the Young’s modulus in the dry state increased from 780 to 6500 kPa and that the crushing strength increased from 39 to 275 kPa with increasing relative density. In the hydrated state, the Young’s modulus increased from 6.44 to 34.8 kPa and the crushing strength increased from 0.55 to 2.12 kPa; the properties were further increased by cross-linking. The modulus and strength were well described by models for cellular solids.
Co-reporter:Biraja P. Kanungo, Emilio Silva, Krystyn Van Vliet, Lorna J. Gibson
Acta Biomaterialia 2008 Volume 4(Issue 3) pp:490-503
Publication Date(Web):May 2008
DOI:10.1016/j.actbio.2008.01.003
Abstract
Mineralized collagen–glycosaminoglycan scaffolds designed for bone regeneration have been synthesized via triple co-precipitation in the absence of a titrant phase. Here, we characterize the microstructural and mechanical properties of these newly developed scaffolds with 50 and 75 wt.% mineral content. The 50 wt.% scaffold had an equiaxed pore structure with isotropic mechanical properties and a Ca–P-rich mineral phase comprised of brushite; the 75 wt.% scaffold had a bilayer structure with a pore size varying in the through-thickness direction and a mineral phase comprised of 67% brushite and 33 wt.% monetite. The compressive stress–strain response of the scaffolds was characteristic of low-density open-cell foams with distinct linear elastic, collapse plateau and densification regimes. The elastic modulus and strength of individual struts within the scaffolds were measured using an atomic force microscopy cantilevered beam-bending technique and compared with the composite response under indentation and unconfined compression. Cellular solids models, using the measured strut properties, overestimated the overall mechanical properties for the scaffolds; the discrepancy arises from defects such as disconnected pore walls within the scaffold. As the scaffold stiffness and strength decreased with increasing overall mineral content and were less than that of natural, mineralized collagen scaffolds, these microstructural/mechanical relations will be used to further improve scaffold design for bone regeneration applications.
Co-reporter:Brendan A. Harley, Janet H. Leung, Emilio C.C.M. Silva, Lorna J. Gibson
Acta Biomaterialia 2007 Volume 3(Issue 4) pp:463-474
Publication Date(Web):July 2007
DOI:10.1016/j.actbio.2006.12.009
Abstract
Tissue engineering scaffolds are used extensively as three-dimensional analogs of the extracellular matrix (ECM). However, less attention has been paid to characterizing the scaffold microstructure and mechanical properties than to the processing and bioactivity of scaffolds. Collagen–glycosaminoglycan (CG) scaffolds have long been utilized as ECM analogs for the regeneration of skin and are currently being considered for the regeneration of nerve and conjunctiva. Recently a series of CG scaffolds with a uniform pore microstructure has been developed with a range of sizes of equiaxed pores. Experimental characterization and theoretical modeling techniques have previously been used to describe the pore microstructure, specific surface area, cell attachment and permeability of these variants. The results of tensile and compressive tests on these CG scaffolds and of bending tests on the individual struts that define the scaffold network are reported here. The CG scaffold variants exhibited stress–strain behavior characteristic of low-density, open-cell foams with distinct linear elastic, collapse plateau and densification regimes. Scaffolds with equiaxed pores were found to be mechanically isotropic. The independent effects of hydration level, pore size, crosslink density and relative density on the mechanical properties was determined. Independent control over scaffold stiffness and pore size was obtained. Good agreement was observed between experimental results of scaffold mechanical characterization and low-density, open-cell foam model predictions for uniform scaffolds. The characterized scaffold variants provide a standardized framework with defined extracellular environments (microstructure, mechanics) for in vitro studies of the mechanical interactions between cells and scaffolds as well as in vivo tissue engineering studies.
Co-reporter:Fergal J. O’Brien, Brendan A. Harley, Ioannis V. Yannas, Lorna Gibson
Biomaterials 2004 Volume 25(Issue 6) pp:1077-1086
Publication Date(Web):March 2004
DOI:10.1016/S0142-9612(03)00630-6
The cellular structure of collagen-glycosaminoglycan (CG) scaffolds used in tissue engineering must be designed to meet a number of constraints with respect to biocompatibility, degradability, pore size, pore structure, and specific surface area. The conventional freeze-drying process for fabricating CG scaffolds creates variable cooling rates throughout the scaffold during freezing, producing a heterogeneous matrix pore structure with a large variation in average pore diameter at different locations throughout the scaffold. In this study, the scaffold synthesis process was modified to produce more homogeneous freezing by controlling of the rate of freezing during fabrication and obtaining more uniform contact between the pan containing the CG suspension and the freezing shelf through the use of smaller, less warped pans. The modified fabrication technique has allowed production of CG scaffolds with a more homogeneous structure characterized by less variation in mean pore size throughout the scaffold (mean: 95.9 μm, CV: 0.128) compared to the original scaffold (mean: 132.4 μm, CV: 0.185). The pores produced using the new technique appear to be more equiaxed, compared with those in scaffolds produced using the original technique.
Co-reporter:J.-S Huang, L.J Gibson
Materials Science and Engineering: A 2003 Volume 339(1–2) pp:220-226
Publication Date(Web):2 January 2003
DOI:10.1016/S0921-5093(02)00152-1
The power-law creep of open-cell Voronoi foams is calculated using finite element analysis. The results are used to determine the geometrical constants in the power-law creep model for an open-cell foam with a random microstructure. In some foams, individual struts may be missing, either through fracture, as is the case in some metallic foams, or through resorption, as is the case in osteoporotic trabecular bone. Analysis of the effect of random removal of struts within the foam on the creep rate indicates that it can have a dramatic effect: removal of only a few percent of the struts can increase the creep rate by one to two orders of magnitude.
Co-reporter:W.S Sanders, L.J Gibson
Materials Science and Engineering: A 2003 Volume 352(1–2) pp:150-161
Publication Date(Web):15 July 2003
DOI:10.1016/S0921-5093(02)00890-0
Hollow-sphere foams provide an alternative microstructure for low-density metal structures with the potential for improved properties. In a recent companion paper, the mechanical properties of hollow-sphere foams with simple cubic packing (SC) were shown to be close to the theoretical values for closed-cell foams and well above the measured modulus and strength of metallic closed-cell foams. Here, we analyze the elastic moduli and initial yield strength of body-centered cubic and face-centered cubic (FCC) packings of hollow-sphere foams. In general, the FCC packing gives the highest values of moduli and strengths.
Co-reporter:W.S. Sanders, L.J. Gibson
Materials Science and Engineering: A 2003 Volume 347(1–2) pp:70-85
Publication Date(Web):25 April 2003
DOI:10.1016/S0921-5093(02)00583-X
Models for the mechanical properties of foams suggest that closed-cell foams should have significantly higher modulus and strength than open-cell foams, especially at low relative densities. In practice, as a result of defects in their microstructure, the measured properties of closed-cell metallic foams are similar to those of open-cell foams of the same relative density. Hollow sphere foams provide an alternative microstructure with the potential for improved properties for low-density metal structures. Here, we analyze the elastic moduli and initial yield strength of hollow sphere foams. The results indicate that their theoretical values of moduli and strength are intermediate to those for open- and closed-cell foams. With suitable manufacturing techniques, hollow sphere foams have the potential for improved mechanical properties compared with existing metallic foams.
Co-reporter:T.M. Freyman, I.V. Yannas, L.J. Gibson
Progress in Materials Science 2001 Volume 46(3–4) pp:273-282
Publication Date(Web):2001
DOI:10.1016/S0079-6425(00)00018-9
A major goal of tissue engineering is to synthesize or regenerate tissues and organs. Today, this is done by providing a synthetic porous scaffold, or matrix, which mimics the body's own extracellular matrix, onto which cells attach, multiply, migrate and function. Porous scaffolds are currently being developed for regeneration of skin, cartilage, bone, nerve and liver. The microstructures of many porous scaffolds ressemble that of an engineering foam. In this paper, we describe the microstructural requirements for porous scaffolds, review several processes for making them and show typical microstructures. Clinical studies have found that a collagen-based scaffold for skin regeneration reduces wound contraction during the healing process, reducing scar formation. The process of wound contraction is not well understood. Here, we describe the measurement of contraction of collagen-based scaffolds by fibroblasts in vitro using a cell force monitor.
Co-reporter:J.C Wallach, L.J Gibson
Scripta Materialia 2001 Volume 45(Issue 6) pp:639-644
Publication Date(Web):28 September 2001
DOI:10.1016/S1359-6462(01)01073-9
The effect of randomly removing members of a three-dimensional truss material on the Young's modulus and compressive strength has been calculated numerically. The results indicate that this structure is more tolerant to this type of defect than open-cell foams.
Co-reporter:E.W. Andrews, L.J. Gibson
Materials Science and Engineering: A 2001 Volume 303(1–2) pp:120-126
Publication Date(Web):15 May 2001
DOI:10.1016/S0921-5093(00)01854-2
The creep response of cellular solids is sensitive to the details of the microstructure. Here, finite element simulations were used to model the steady state, secondary creep rate of several two-dimensional cellular solids: a Voronoi honeycomb, representing a structure with a random variation in cell shape; a plane section of a micrograph of a commercially available closed-cell aluminum foam; and the same plane section of the foam, but with the curvature of the cell walls suppressed. The solid was assumed to follow power-law creep. Both periodic boundary conditions and boundary conditions corresponding to a finite size sample were analyzed. The results of the models are compared with the analytical results for a regular hexagonal honeycomb of the same relative density. The creep rates of all of the other structures are higher than that of the regular hexagonal honeycomb. The results indicate that the details of the microstructure can have a significant effect on the creep rate, and thus the lifetime, of the cellular solid. Cell wall curvature plays the most significant role, but the distribution of cell shape and size also influences the creep rate.
Co-reporter:Marc Borrega, Lorna J. Gibson
Mechanics of Materials (May 2015) Volume 84() pp:75-90
Publication Date(Web):1 May 2015
DOI:10.1016/j.mechmat.2015.01.014
•Mechanical properties of balsa are related to its cellular structure and modeled based on deformation and failure mechanisms.•In axial compression, bending, and torsion, the elastic modulus and strength increase linearly with density.•In radial compression, the modulus and strength increase nonlinearly with density, due to the combined contribution of fibers and rays.•The cell wall elastic modulus determined by nanoindentation is about half of that extrapolated from mechanical models.Balsa wood is one of the preferred core materials in structural sandwich panels, in applications ranging from wind turbine blades to boats and aircraft. Here, we investigate the mechanical behavior of balsa as a function of density, which varies from roughly 60 to 380 kg/m3. In axial compression, bending, and torsion, the elastic modulus and strength increase linearly with density while in radial compression, the modulus and strength vary nonlinearly. Models relating the mechanical properties to the cellular structure and to the density, based on deformation and failure mechanisms, are described. Finally, wood cell-wall properties are determined by extrapolating the mechanical data for balsa, and are compared with the reduced modulus and hardness of the cell wall measured by nanoindentation.
Co-reporter:Caitlin S. Sample, Alan K. Xu, Sharon M. Swartz, Lorna J. Gibson
Journal of Insect Physiology (March 2015) Volume 74() pp:10-15
Publication Date(Web):1 March 2015
DOI:10.1016/j.jinsphys.2015.01.013
•We did nanoindentation tests on the surface and interior layers of wing membranes.•The Young’s moduli and hardness varied across the surface and interior.•The Young’s moduli of the interior was higher than that of the dorsal surface.•The hardnesses of the interior and surface layers were comparable.Many insect wings change shape dynamically during the wingbeat cycle, and these deformations have the potential to confer energetic and aerodynamic benefits during flight. Due to the lack of musculature within the wing itself, the changing form of the wing is determined primarily by its passive response to inertial and aerodynamic forces. This response is in part controlled by the wing’s mechanical properties, which vary across the membrane to produce regions of differing stiffness. Previous studies of wing mechanical properties have largely focused on surface or bulk measurements, but this ignores the layered nature of the wing. In our work, we investigated the mechanical properties of the wings of the house cricket (Acheta domesticus) with the aim of determining differences between layers within the wing. Nanoindentation was performed on both the surface and the interior layers of cross-sectioned samples of the wing to measure the Young’s modulus and hardness of the outer- and innermost layers. The results demonstrate that the interior of the wing is stiffer than the surface, and both properties vary across the wing.Download full-size image
Co-reporter:Sardar Malek, Lorna Gibson
Mechanics of Materials (December 2015) Volume 91(Part 1) pp:226-240
Publication Date(Web):1 December 2015
DOI:10.1016/j.mechmat.2015.07.008
•Analyzed effect of node region on elastic properties, analytically and numerically.•Obtained more accurate predictions for elastic properties of hexagonal honeycombs.•Both analytical and numerical results describe experimental data well.•At high relative densities the effect of the nodes becomes significant.We investigate the elastic behavior of periodic hexagonal honeycombs over a wide range of relative densities and cell geometries, using both analytical and numerical approaches. Previous modeling approaches are reviewed and their limitations identified. More accurate estimates of all nine elastic constants are obtained by modifying the analysis of Gibson and Ashby (1997) to account for the nodes at the intersection of the vertical and inclined members. The effect of the nodes becomes significant at high relative densities. We then compare the new analytical equations with previous analytical models, with a numerical analysis based on a computational homogenization technique and with data for rubber honeycombs over a wide range of relative densities and cell geometries. The comparisons show that both the new analytical equations and numerical solutions give a remarkably good description of the data. The results provide new insights into understanding the mechanics of honeycombs and designing new cellular materials in the future.
Co-reporter:Brendan A.C. Harley, Hyung-Do Kim, Muhammad H. Zaman, Ioannis V. Yannas, Douglas A. Lauffenburger, Lorna J. Gibson
Biophysical Journal (15 October 2008) Volume 95(Issue 8) pp:
Publication Date(Web):15 October 2008
DOI:10.1529/biophysj.107.122598
Cell migration plays a critical role in a wide variety of physiological and pathological phenomena as well as in scaffold-based tissue engineering. Cell migration behavior is known to be governed by biochemical stimuli and cellular interactions. Biophysical processes associated with interactions between the cell and its surrounding extracellular matrix may also play a significant role in regulating migration. Although biophysical properties of two-dimensional substrates have been shown to significantly influence cell migration, elucidating factors governing migration in a three-dimensional environment is a relatively new avenue of research. Here, we investigate the effect of the three-dimensional microstructure, specifically the pore size and Young's modulus, of collagen-glycosaminoglycan scaffolds on the migratory behavior of individual mouse fibroblasts. We observe that the fibroblast migration, characterized by motile fraction as well as locomotion speed, decreases as scaffold pore size increases across a range from 90 to 150 μm. Directly testing the effects of varying strut Young's modulus on cell motility showed a biphasic relationship between cell speed and strut modulus and also indicated that mechanical factors were not responsible for the observed effect of scaffold pore size on cell motility. Instead, in-depth analysis of cell locomotion paths revealed that the distribution of junction points between scaffold struts strongly modulates motility. Strut junction interactions affect local directional persistence as well as cell speed at and away from the junctions, providing a new biophysical mechanism for the governance of cell motility by the extracellular microstructure.
Co-reporter:Brendan A. Harley, Toby M. Freyman, Matthew Q. Wong, Lorna J. Gibson
Biophysical Journal (15 October 2007) Volume 93(Issue 8) pp:
Publication Date(Web):15 October 2007
DOI:10.1529/biophysj.106.095471
Cell-mediated contraction plays a critical role in many physiological and pathological processes, notably organized contraction during wound healing. Implantation of an appropriately formulated (i.e., mean pore size, chemical composition, degradation rate) three-dimensional scaffold into an in vivo wound site effectively blocks the majority of organized wound contraction and results in induced regeneration rather than scar formation. Improved understanding of cell contraction within three-dimensional constructs therefore represents an important area of study in tissue engineering. Studies of cell contraction within three-dimensional constructs typically calculate an average contractile force from the gross deformation of a macroscopic substrate by a large cell population. In this study, cellular solids theory has been applied to conventional column buckling relationships to quantify the magnitude of individual cell contraction events within a three-dimensional, collagen-glycosaminoglycan scaffold. This new technique can be used for studying cell mechanics with a wide variety of porous scaffolds that resemble low-density, open-cell foams. It extends previous methods for analyzing cell buckling of two-dimensional substrates to three-dimensional constructs. From data available in the literature, the mean contractile force (Fc) generated by individual dermal fibroblasts within the collagen-glycosaminoglycan scaffold was calculated to range between 11 and 41 nN (Fc = 26 ± 13 nN, mean ± SD), with an upper bound of cell contractility estimated at 450 nN.
Co-reporter:Sardar Malek, Lorna J Gibson
International Journal of Solids and Structures (15 May 2017) Volumes 113–114() pp:118-131
Publication Date(Web):15 May 2017
DOI:10.1016/j.ijsolstr.2017.01.037
The hierarchical structure of the hardwood balsa (Ochroma pyramidale) is modelled sequentially at various length scales to describe the contribution of each to its elastic moduli. The model takes into account the main features of the microstructure at each length scale, including the orthotropic properties of cellulose microfibrils, the microfibrillar angle, the thickness of cell wall layers, as well as the geometry and arrangement of the cells: the rays, fibres and vessels. Similar to some multi-scale models proposed for softwoods, at each length scale, a representative unit cell of the material is identified and the effective stiffness matrix is determined using finite element analysis with appropriate periodic boundary conditions. A comprehensive search in the literature was conducted to obtain input values for the model. The results of the model, over a wide range of densities, gives a good description of experimentally measured values. The model highlights the significance of cellulose crystallinity, microfibrillar angle and the ray and fibre cell geometries (including the density) in determining the overall elastic constants of balsa. In the future, the model can be used as a tool to design lightweight cellular composites with optimized cell wall composition and cell geometry.
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