Properties Of Individual Fibers Research Articles

Chemo-mechanical ageing of paper: effect of acidity, moisture and micro-structural features

A multi-scale modeling framework is proposed for the prediction of the chemo-mechanical degradation of paper, with the particular aim of uncovering the key factors affecting the degradation process. Paper is represented as a two-dimensional, periodic repetition of a fibrous network unit cell, where the fibers are characterized by a moisture-dependent chemo-hygro-mechanical constitutive behavior. The degradation of paper occurs primarily as a result of the hydrolysis of cellulose, which causes a reduction of the degree of polymerization and a consequent decrease of the effective mechanical properties, ultimately leading to fiber embrittlement and a loss of material integrity. The interplay between the acidity of the paper, the ambient environmental conditions, and its chemo-mechanical degradation behaviour is a complex process. In the model, these interactions are accounted for by determining the coupled temporal evolution of the degree of polymerization, the acidity of the paper, and the moisture content, from which the time-dependent tensile strength of the paper is calculated. The internal stresses developing in the fibrous network under a change in moisture content lead to brittle fiber fracture once they reach the fiber tensile strength. The successive breakage of individual fibers results in damage development in the fibrous network, altering its effective constitutive properties. The temporal evolution of the effective hygro-mechanical properties of the fibrous network is calculated by employing asymptotic homogenization. For obtaining accurate model input, the strength and stiffness properties of individual fibers and the degree of polymerization of paper samples are measured at different ageing times by carrying out dedicated experiments. Subsequently, a series of numerical simulations is performed to analyze the chemo-mechanical degradation process of paper, highlighting the influence of the time-evolving acidity and moisture content. The numerical study further considers the effects of micro-structural features (i.e., the anisotropy of the fibrous network orientation and the fiber longitudinal elastic modulus) on the macroscopic degradation response of paper. The results of this work may help conservators of cultural heritage institutions determining optimal environmental conditions to limit or delay the time-dependent degradation of valuable historical paper artefacts.

Optimization of Polycaprolactone and Type I Collagen Scaffold for Tendon Tissue Regeneration.

Introduction Collagen synthesis is vital for restoring musculoskeletal tissues, particularly in tendon and ligamentous structures. Tissue engineering utilizes scaffolds for cell adhesion and differentiation. Although synthetic scaffolds offer initial strength, their long-term stability is surpassed by biological scaffolds. Combining polycaprolactone (PCL) toughness with collagen in scaffold design, this study refines fabrication via electrospinning, aiming to deliver enduring biomimetic matrices for widespread applications in musculoskeletal repair. Methods Electrospinning employed four solutions with varied collagen and PCL concentrations, dissolved in chloroform, methanol, and hexafluoro-2-propanol. Solutions were combined to yield 60 mg/mL concentrations with different collagen/PCL ratios. Electrospinning at 12-14kV voltage produced scaffolds, followed by vacuum-drying. Collagen coating was applied to PCL and 15% collagen/PCL scaffolds using a 0.1% collagen solution. SEM characterized fiber morphology, tensile testing was conducted to determine the mechanical properties of the scaffold, and Fourier-transform infrared (FTIR) spectroscopy analyzed scaffold composition. Atomic force microscopy (AFM) analyzed the stiffness properties of individual fibers, and a finite element model was developed to predict the mechanical properties. Cell culture involved seeding human bone marrow mesenchymal stem cells onto scaffolds, which were assessed through Alamar Blue assay and confocal imaging. Results Various scaffolds (100% PCL, PCL-15% collagen, PCL-25% collagen, PCL-35% collagen) were fabricated to emulate the extracellular matrix, revealing collagen's impact on fiber diameter reduction with increasing concentration. Tensile testing highlighted collagen's initial enhancement of mechanical strength, followed by a decline beyond PCL-15% collagen. FTIR spectroscopy detected potential hydrogen bonding between collagen and PCL. A finite element model predicted scaffold response to external forces which was validated by the tensile test data. Cell viability and proliferation assays demonstrated successful plating on all scaffolds, with optimal proliferation observed in PCL-25% collagen. Confocal imaging confirmed stem cell integration into the three-dimensional material. Collagen coating preserved nanofiber morphology, with no significant changes in diameter. Coating of collagen significantly altered the tensile strength of the scaffolds at the macro scale. AFM highlighted stiffness differences between PCL and collagen-coated PCL mats at the single fiber scale. The coating process did not significantly enhance initial cell attachment but promoted increased proliferation on collagen-coated PCL scaffolds. Conclusion The study reveals collagen-induced mechanical and morphological alterations, influencing fiber alignment, diameter, and chemical composition while emphasizing scaffolds' vital role in providing a controlled niche for stem cell proliferation and differentiation. The optimization of each of these scaffold characteristics and subsequent finite element modeling can lead to highly repeatable and ideal scaffold properties for stem cell integration and proliferation.

Vol4 num3-12 Surface modification of carbon fibres for its use as supercapacitors in electric vehicles

Carbon fibre reinforced composite materials show potential as energy storage components, due to the high electric conductivity of the carbon fibre. Supercapacitors store electrostatic energy over the electrode surface. Increasing the surface area of electrodes based on carbon fibre could lead to an increase in storage capacitance. Different methods to increase surface area of carbon fibre are studied with the aim of increasing energy storage capacitance of the fibres working as electrodes for structural electrochemical capacitors. Surface modification was carried out by the deposition of carbon nanoparticles (GNP) and porous carbon (CAG). Effectiveness of the different methods was analysed by measurements of BET surface area and microscopy characterisation. The effect on mechanical properties of individual fibres was analysed and storage capacitance was measured by electrochemical characterisation.

Modern concrete pipes: a review of reinforcement and new technologies

Abstract The paper discusses existing reinforcement, future reinforcement and new technologies for concrete pipes used in the sewage systems. Concrete pipes currently in use and under investigation are reviewed. Structural fibres, as the main reinforcement of concrete pipes, are known as an attractive alternative to the traditional steel bars. Steel, synthetic and basalt fibres have been considered. The latest research and mechanical properties of individual fibres are presented. Advances in fibre-reinforced concrete provide a new basis for the design of more efficient concrete pipes, especially those resistant to biological corrosion and with a longer service life. In the article, future non-corrosive reinforcement due to the reduction of steel reinforcement and corrosion protection linings has been proposed.

The transverse and longitudinal elastic constants of pulp fibers in paper sheets

Cellulose fibers are a major industrial input, but due to their irregular shape and anisotropic material response, accurate material characterization is difficult. Single fiber tensile testing is the most popular way to estimate the material properties of individual fibers. However, such tests can only be performed along the axis of the fiber and are associated with problems of enforcing restraints. Alternative indirect approaches, such as micro-mechanical modeling, can help but yield results that are not fully decoupled from the model assumptions. Here, we compare these methods with nanoindentation as a method to extract elastic material constants of the individual fibers. We show that both the longitudinal and the transverse elastic modulus can be determined, additionally enabling the measurement of fiber properties in-situ inside a sheet of paper such that the entire industrial process history is captured. The obtained longitudinal modulus is comparable to traditional methods for larger indents but with a strongly increased scatter as the size of the indentation is decreased further.

Effects of Fiber Density and Strain Rate on the Mechanical Properties of Electrospun Polycaprolactone Nanofiber Mats.

This study examines the effects of electrospun polycaprolactone (PCL) fiber density and strain rate on nanofiber mat mechanical properties. An automated track collection system was employed to control fiber number per mat and promote uniform individual fiber properties regardless of the duration of collection. Fiber density is correlated to the mechanical properties of the nanofiber mats. Young's modulus was reduced as fiber density increased, from 14,901 MPa for samples electrospun for 30 s (717 fibers +/– 345) to 3,615 MPa for samples electrospun for 40 min (8,310 fibers +/– 1,904). Ultimate tensile strength (UTS) increased with increasing fiber density, where samples electrospun for 30 s resulted in a UTS of 594 MPa while samples electrospun for 40 min demonstrated a UTS of 1,250 MPa. An average toughness of 0.239 GJ/m3 was seen in the 30 s group, whereas a toughness of 0.515 GJ/m3 was observed at 40 min. The ultimate tensile strain for samples electrospun for 30 s was observed to be 0.39 and 0.48 for samples electrospun for 40 min. The relationships between UTS, Young's modulus, toughness, and ultimate tensile strain with increasing fiber density are the result of fiber-fiber interactions which leads to network mesh interactions.

Effect of the Interfiber Bonding on the Mechanical Behavior of Electrospun Fibrous Mats

Electrospun fibrous mats, characterized by their large surface-to-volume ratios, have unique and beneficial properties for various applications. The micro or nanoscale architectures of these structures significantly affects the mechanical properties of the material. The lack of knowledge for predicting the mechanical behavior of electrospun fibrous mats may prevent applications utilizing the se mats from reaching their full potential. In this paper, we propose a new computational model to predict the mechanical behavior of an electrospun fibrous mat by considering its microstructure and the percentage of the cross-points that are bonded. The model of the electrospun mat with randomly distributed fibers is considered in uniaxial and biaxial tension. Three cases are studied: (1) no interaction in cross-points of intersecting fibers, (2) half of the cross-points are bonded, (3) all of the cross-points are bonded. The results show that along with the mechanical properties of individual fibers, the fusion bonding of fibers is a critical parameter for tuning the mechanical properties of the bulk material. In a predefined porosity, the interfiber fusion enhanced the stiffness of the mat by 60%, which is independent of the loading mode and the mechanical property of individual fibers. For all ranges of porosities, bonding increases the stiffness of the mat; however, the bonding is more effective at stiffening when the porosity of the mat is low.

Compression failure in dense non-woven fiber networks

Investigating the compression properties of randomly ordered fiber networks experimentally is difficult which has resulted in ongoing disputes as to the mechanisms controlling the compression strength in such materials. In this work, we investigated compression properties of randomly oriented fiber networks with a special emphasis on cellulose products such as paperboard. We numerically reconstructed the conditions of the short span compression test widely used to quantify the compression strength of paperboard. We found that the phenomenological failure mode of such networks is elasto-plastic buckling. The x-shaped failure mode observed in physical experiments appears when test specimen restraints are included in the model. The most significant improvements to sheet strength can be obtained by improving the elastic properties while the strain to failure is increased most by an improvement of the plastic yield and hardening properties of individual fibers. Bond breaks were confirmed to have a smaller influence on the overall response. Fiber level microscopic buckling was investigated in depth, providing quantitative estimates of the fraction of mass likely to buckle at the microscopic level. The analysis indicated that only a low to moderate number of load carrying fibers can be expected to buckle. The inherent strength reserve in non-ordered fiber networks was investigated by introducing hinge mechanisms throughout the network, and the effect was shown to be small for a small to moderate number of hinges.

Chemical Structure and Mechanical Properties of Wood Cell Walls Treated with Acid and Alkali Solution

The mechanical properties of individual fibers are related to the production and performance of papers and fiberboards. This paper examines the behavior of the microstructure constituents of wood subjected to acid and alkali treatments. The chemical structure and mechanical properties of wood cell walls with different acid or alkali treatments was analyzed. The results show that, compared with acid treatment, the crystal size and crystallinity index of cellulose increased after alkali treatment, resulting in an increase in the cell wall elastic modulus. The mechanical properties of the wood cell wall S2 region were higher than those of the compound middle lamella (CML) region. There was a topochemical effect between the CML and the S2 region in acid and alkali-treated samples, which provided a major threshold that facilitates the production/separation of wood fibers for better strength fiber properties.

Mechanical properties of subisostatic random networks composed of nonlinear fibers.

Fibrous protein networks provide structural integrity to different biological materials such as soft tissues. These networks display an unusual exponential strain-stiffening behavior when subjected to mechanical loads. This nonlinear strain-stiffening behavior has so far been explained in terms of the network microstructure and the flexibility of constituting fibers. Here, we conduct a comprehensive computational study to characterize the importance of material properties of individual fibers in the overall nonlinear mechanical response of random fiber networks. To this end, we consider three nonlinear material models, ranging from an almost linear form to a highly nonlinear one, for the fibers of subisostatic disordered networks. We characterize the amount of strain-stiffening as a function of bending rigidity of the fibers, the amount of nonlinearity of the fibers, and the connectivity of random networks. We find that networks composed of highly nonlinear fibers exhibit much more strain-stiffening than networks made up of linear fibers. Furthermore, the local strain distribution becomes more homogenous as the amount of nonlinearity in the material models increases. Increasing the network connectivity signifies the importance of the nonlinear material response of individual fibers in the overall mechanical behavior of networks. The constitutive behavior of fibers plays an important role in defining the failure response of networks particularly in the damage initiation and evolution. These important findings for how the mechanical response of individual fibers affects the overall mechanical properties of random networks could find applications in designing new biomimetic materials and could help scientists better understand the mechanical properties of biological materials.

Mechanical and electromechanical properties of piezoelectric ceramic fibers drawn by the alginate gelation method

AbstractPiezoelectric ceramic fibers are widely used in piezocomposite devices. The various methods that are used to draw ceramic fibers differ in the way the fiber form is obtained, which in return closely affects the density, uniformity and the properties of the fibers that are obtained at the end. In this study, the processing‐property relationship in the piezoceramic fibers drawn using the alginate gelation method is investigated, with a focus on the mechanical and electrical properties of individual fibers. Fibers with a Weibull strength of 65.3 MPa, remanent polarization of 22 μC/cm2 and a total bipolar field induced strain of 0.25% under an electric field of 2.5 kV/mm, piezoelectric coefficient of 300 pm/V and dielectric constant of 3323 were produced. 1‐3 piezocomposite devices prepared from these fibers had a 6 dB higher free‐field voltage sensitivity and a 50% wider bandwidth compared to a solid disk transducer of the same dimensions.

Mechanisms of Plastic Deformation in Collagen Networks Induced by Cellular Forces

Contractile cells can reorganize fibrous extracellular matrices and form dense tracts of fibers between neighboring cells. These tracts guide the development of tubular tissue structures and provide paths for the invasion of cancer cells. Here, we studied the mechanisms of the mechanical plasticity of collagen tracts formed by contractile premalignant acinar cells and fibroblasts. Using fluorescence microscopy and second harmonic generation, we quantified the collagen densification, fiber alignment, and strains that remain within the tracts after cellular forces are abolished. We explained these observations using a theoretical fiber network model that accounts for the stretch-dependent formation of weak cross-links between nearby fibers. We tested the predictions of our model using shear rheology experiments. Both our model and rheological experiments demonstrated that increasing collagen concentration leads to substantial increases in plasticity. We also considered the effect of permanent elongation of fibers on network plasticity and derived a phase diagram that classifies the dominant mechanisms of plasticity based on the rate and magnitude of deformation and the mechanical properties of individual fibers. Plasticity is caused by the formation of new cross-links if moderate strains are applied at small rates or due to permanent fiber elongation if large strains are applied over short periods. Finally, we developed a coarse-grained model for plastic deformation of collagen networks that can be employed to simulate multicellular interactions in processes such as morphogenesis, cancer invasion, and fibrosis.

Radiative characterization of random fibrous media with long cylindrical fibers: Comparison of single- and multi-RTE approaches

Radiative heat transfer is analyzed in participating media consisting of long cylindrical fibers with a diameter in the limit of geometrical optics. The absorption and scattering coefficients and the scattering phase function of the medium are determined based on the discrete-level medium geometry and optical properties of individual fibers. The fibers are assumed to be randomly oriented and positioned inside the medium. Two approaches are employed: a volume-averaged two-intensity approach referred to as multi-RTE approach and a homogenized single-intensity approach referred to as the single-RTE approach. Both approaches require effective properties, determined using direct Monte Carlo ray tracing techniques. The macroscopic radiative transfer equations (for single intensity or two volume-averaged intensities) with the corresponding effective properties are solved using Monte Carlo techniques and allow for the determination of the radiative flux distribution as well as overall transmittance and reflectance of the medium. The results are compared against predictions by the direct Monte Carlo simulation on the exact morphology. The effects of fiber volume fraction and optical properties on the effective radiative properties and the overall slab radiative characteristics are investigated. The single-RTE approach gives accurate predictions for high porosity fibrous media (porosity about 95%). The multi-RTE approach is recommended for isotropic fibrous media with porosity in the range of 79−95%.

Study of the tensile properties of individual multicellular fibres generated by Bacillus subtilis

Multicellular fibres formed by Bacillus subtilis (B. subtilis) are attracting interest because of their potential application as degradable biomaterials. However, mechanical properties of individual fibres remain unknown because of their small dimensions. Herein, a new approach is developed to investigate the tensile properties of individual fibres with an average diameter of 0.7 μm and a length range of 25.7–254.3 μm. Variations in the tensile strengths of fibres are found to be the result of variable interactions among pairs of microbial cells known as septa. Using Weibull weakest-link model to study this mechanical variability, we predict the length effect of the sample. Moreover, the mechanical properties of fibres are found to depend highly on relative humidity (RH), with a brittle–ductile transition occurring around RH = 45%. The elastic modulus is 5.8 GPa in the brittle state, while decreases to 62.2 MPa in the ductile state. The properties of fibres are investigated by using a spring model (RH < 45%) for its elastic behaviour, and the Kelvin–Voigt model (RH > 45%) for the time-dependent response. Loading-unloading experiments and numerical calculations demonstrate that necking instability comes from structural changes (septa) and viscoelasticity dominates the deformation of fibres at high RH.

Physicochemical properties of polycaprolactone/collagen/elastin nanofibers fabricated by electrospinning

Collagen and elastin are the two most abundant proteins in the human body, and as biomaterials offer fascinating properties to composite materials. More detailed investigations including these biomaterials within reinforced composites are still needed. This report describes physicochemical properties of fibers composed of collagen type I, collagen III, elastin and polycaprolactone (PCL). Prior to the electrospinning process, PCL was functionalized through covalent attachment of –NH2 groups by aminolysis reaction with hexamentilendiamine. The fibers were fabricated by electrospinning technique set up with a non-conventional collector. A morphological comparative study was developed at different rations of collagen type I, observing in some cases two populations of fibers. The diameters and morphology were analyzed by SEM, observing a wide array of nanostructures with diameters of ~310 to 693nm. Chemical characterization was assessed by FT-IR spectroscopy and the functionalized PCL was characterized through ninhydrin assay resulting in 0.36mM NH2/mg fiber. Swelling tests were performed for 24h, obtaining 320% for the majority of the fibers indicating morphological stability and good water uptake. In addition, contact angle analysis demonstrated adequate permeability and differences for each system depending mainly upon the type of biopolymer incorporated and the functionalization of PCL, ranging the values from 108° to 17°. Moreover, differential scanning calorimetry results showed a melting temperature (Tm) of ~60°C. The onset degradation temperatures (Td,onset) ranged between 115 and 148°C, and were obtained by thermogravimetric analysis. The local mechanical properties of individual fibers were quantified by atomic force acoustic microscopy. These results propose that the physicochemical and mechanical properties of these scaffolds offer the possibility for enhanced biological activity Thus, they have a great potential as candidate scaffolds in tissue engineering applications.

Brittle matrix composites with heterogeneous reinforcement: Multi-scale model of a crack bridge with rigid matrix

Abstract A mechanical multi-scale model describing the relationship between the crack-opening and composite bridging stress in brittle matrix composites with heterogeneous reinforcement is introduced. Unlike currently utilized models, it is able to reflect the heterogeneity of fibrous reinforcement. Mechanical, geometrical and bond properties of individual fibers (e.g. fiber surface roughness, radius, strength) are defined as random variables. The functional dependency between these random variables and the fiber stress within the composite cross-section is introduced using local equilibrium equations. The response of the composite to an applied uniaxial tensile load is evaluated by averaging the fiber stress contributions in a crack bridge. In particular, the model describes the behavior of a single crack bridge in a composite assuming the matrix to be rigid. The fiber bridging action is represented using the shear-lag model. Upon fiber rupture the global load sharing for stress redistribution is considered. With these assumptions the rules for asymptotic Daniels’ fiber-bundle models can be applied for the evaluation of macroscopic crack bridge behavior. We use the model to illuminate the effect of selected sources of heterogeneity (fiber breaking strain, fiber radius and bond strength) on the crack bridge response and to approximately predict the ultimate state of a multiply cracked composite. The model constitutes a basis for the multi-scale simulation of the strain-hardening response of brittle-matrix composites.

Impacts of Gin and Mill Cleaning on Medium-Long Staple Stripper-Harvested Cotton

The quality of upland cotton grown in the Texas Plains, which in good seasons constitutes nearly 40% of all U.S. cotton, has changed radically over the past decade. However, production practices, particularly pertaining to harvesting and ginning, remain largely unchanged, with a predominately stripper-harvested crop. Hence, the necessity to subject the lint to vigorous cleaning at the gin could adversely impact the use value of the cotton in high-value added applications more typically served by picker-harvested cotton. This research was conducted to provide a systemic analysis, combining perspectives from both the gin and the spinning mill, of the impacts of mechanical cleaning on the emerging stripper-harvested cotton with improved fiber properties that appear suitable for ring-spinning. Five cotton varieties grown over three seasons were processed through various sequences of gin and spinning mill cleaning processes. Analysis of the properties of individual fibers across the gin and spinning mill processing chain revealed significant interactions impacting fiber entanglements (neps), mean length, and short fiber content. Both neps and length distribution parameters appeared significantly impacted by fiber damage in both the ginning and spinning processes. However, the damage inflicted through both ginning and spinning may be a function of only fiber maturity. Mature cottons have lower fiber damage potential and appear to be less affected by the processes of lint cleaning and spinning preparation than immature cottons. The latter have higher fiber damage potential, which is reached even with minimal cleaning at the gin because if those immature fibers are preserved at the gin, they will ultimately break in spinning preparation.

Effects of high- and low-velocity resistance training on the contractile properties of skeletal muscle fibers from young and older humans

A two-arm, prospective, randomized, controlled trial study was conducted to investigate the effects of movement velocity during progressive resistance training (PRT) on the size and contractile properties of individual fibers from human vastus lateralis muscles. The effects of age and sex were examined by a design that included 63 subjects organized into four groups: young (20-30 yr) men and women, and older (65-80 yr) men and women. In each group, one-half of the subjects underwent a traditional PRT protocol that involved shortening contractions at low velocities against high loads, while the other half performed a modified PRT protocol that involved contractions at 3.5 times higher velocity against reduced loads. Muscles were sampled by needle biopsy before and after the 14-wk PRT program, and functional tests were performed on permeabilized individual fiber segments isolated from the biopsies. We tested the hypothesis that, compared with low-velocity PRT, high-velocity PRT results in a greater increase in the cross-sectional area, force, and power of type 2 fibers. Both types of PRT increased the cross-sectional area, force, and power of type 2 fibers by 8-12%, independent of the sex or age of the subject. Contrary to our hypothesis, the velocity at which the PRT was performed did not affect the fiber-level outcomes substantially. We conclude that, compared with low-velocity PRT, resistance training performed at velocities up to 3.5 times higher against reduced loads is equally effective for eliciting an adaptive response in type 2 fibers from human skeletal muscle.

The Mechanical Properties of Electrospun Collagen/PCL Single Fibers and Fibrous Scaffolds

Collagen/Poly-ε-caprolactone (PCL) blended fibers were electrospun from a solution of 2.5% wt./vol. bovine collagen, 2.5% wt./vol. PCL and 95% 1,1,1,3,3,3-hexaflouro-2-propanol (HFP). A combined atomic force microscopic (AFM)/optical microscopic technique was used to study the properties of individual fibers in buffer. Mechanical testing of fibers was done using the AFM to laterally stretch individual fibers suspended over 13.5μm wide groves in a transparent substrate. The optical microscope, located below the sample, was used to monitor the stretching process. Fibrous scaffolds of these blends were also electrospun to test the bulk mechanical properties. A total of 5ml of 5% wt./vol. solution of blended collagen/PCL was used to electrospin scaffolds with a 4.75mm inner diameter to be used to for blood vessel tissue replacements. Dots are applied to the surface of the scaffold which is then subjected to cyclic loading under wet conditions while video is being recorded of the test. When a replicable elastic mechanical behavior is achieved, the strain found in the loading part of the cycle is analyzed using non-contact video strain measurements. By testing the single fiber mechanical properties of these blends as well as the bulk mechanical properties of a fibrous scaffold we are able to determine the mechanical differences of the fibers on both a micro and macro scaffold. These results can be used to better develop tissue engineered scaffolds for blood vessel replacements.

Size Dictates Mechanical Properties for Protein Fibers Self-Assembled by the Drosophila Hox Transcription Factor Ultrabithorax

The development of protein-based materials with diverse mechanical properties will facilitate the realization of a broad range of potential applications. The recombinant Drosophila melanogaster transcription factor Ultrabithorax self-assembles under mild conditions in aqueous buffers into extremely extensible materials. By controlling fiber diameter, both the mechanism of extension and the magnitude of the mechanical properties can be varied. Narrow Ultrabithorax fibers (diameter <10 μm) extend elastically, whereas the predominantly plastic deformation of wide fibers (diameter >15 μm) reflects the increase in breaking strain with increasing diameter, apparently due to a change in structure. The breaking stress/strain of the widest fibers resembles that of natural elastin. Intermediate fibers display mixed properties. Fiber bundles retain the mechanical properties of individual fibers but can withstand much larger forces. Controlling fiber size and generating fiber superstructures is a facile way to manipulate the mechanical characteristics of protein fibers and rationally engineer macroscale protein-based materials with desirable properties.