Bundles Of Fibrils Research Articles

Understanding the cutting mechanisms of composite structured soft tissues

Arthroscopic debridement with shaver is a standard procedure for the treatment of meniscus injury. The tissue damage level and surface quality depend significantly on the performance of the instrument and clinical operation. Therefore, it is critical to understand the cutting mechanism which causes collagen fibril fracture, in order to optimize the instrument designing and surgical processing. Nevertheless, there are only limited studies focusing on the cutting characteristics related to arthroscopic debridement. In this study, the effects of arthroscopic shaver design and the cutting parameters were investigated and considered in a model to characterize the differences in cutting force and surface quality. Furthermore, the fracture mechanics was studied based on the basic characteristics of meniscus. The results showed that the fibril structure contributed significantly to meniscus failure. Meniscus is a special composite, comprising the combination of soft tissue and fibre-reinforced composite. The geometry of the cutting teeth is a significant factor affecting the cutting force, mainly reflecting the area of the created surface and the fracture toughness in different direction. High surface quality could be achieved by optimizing the shaver combination and cutting parameter. The cutting experiment and compressive test on elemental cutting tool indicated that delamination is the main reason of meniscus failure. It concentrates on the interface between fibril bundles due to the shear and tensile strain generated by compression load. Moreover, crack bridging and uncracked ligament bridging are also common phenomena that occur during the process of meniscus fracture. Findings in this study may provide guidance to orthopedic surgeons for selecting appropriate arthroscopic shavers.

Rejuvenation of Aged Human Skin by Injection of Cross-linked Hyaluronic Acid.

Dermal injection of chemically cross-linked hyaluronic acid (CL-HA) is a common procedure to smooth wrinkles and add fullness to the face. Due to its physical properties, CL-HA both fills space and exerts mechanical forces within the dermis. Dermal fibroblasts produce the collagen-rich extracellular matrix (ECM), which comprises the bulk of skin. Attachment to the ECM allows fibroblasts to achieve a stretched, morphology, which confers a functional phenotype that maintains collagen production. In aged/photoaged skin, collagen fibril fragmentation impairs fibroblast attachment, resulting in a collapsed morphology and reduced collagen production. This article describes investigations of the impact of CL-HA injection on fibroblast morphology and function in the aged/photoaged human skin. Fifty-three subjects, age 70 years or older, received a single injection of saline (vehicle control) and CL-HA (0.5 ml each) in separate adjacent skin sites on photodamaged forearm or sun-protected buttock skin. Full-thickness punch biopsies were obtained from injected skin sites at various times and analyzed for molecular and cellular changes. Injected CL-HA forms discreet pockets that localize to areas of the dermis that contain fragmented, loosely organized collagen fibrils. These CL-HA pockets fill space and apply mechanical forces on adjacent ECM that induce stretching of fibroblasts. This stretching is associated with increased collagen gene expression and deposition of mature collagen fibril bundles, which resemble those observed in young skin. CL-HA injected into aged/photoaged human dermis acts by both filling space and inducing production of collagen by dermal fibroblasts. Deposition of mature collagen, which remains in the skin for decades, likely confers long-term benefits. Reduced collagen production in aged/photoaged skin is an adaptive response of fibroblasts to ECM fragmentation, rather than inherent cellular aging mechanisms.

Luffa fibers as promising reinforcement for polymer composites: Mechanical characterization of NaOH treated and untreated dumbbell test-pieces with Weibull statistics

Different materials require specific test-piece dimensions and designations for their mechanical characterization, present either in ASTM, ISO, or DIN norms. Natural polymers or bio-fibers are materials that have lignocellulosic fibrils that can be mechanically characterized as single fibers, which tend to be bundles of thinner hollow fibrils. In this work, a new approach has been applied to the unique natural fiber Luffa cylindrica, also called loofah or scourer, as loofah dumbbell test-pieces (LDTs) similar in length to the Type IV ASTM D-638 samples for unreinforced or reinforced plastics. 60 LDTs of 120 mm and 40 mm in length and width respectively were cut by hand, with and without alkali treatment (aqueous 2%NaOH), in transversal and longitudinal directions, to be characterized using testing speeds of 1 mm/min and 50 mm/min, SEM, XRD, statistics, and Weibull analysis. The resultant force vs. elongation/stress vs. strain graphs revealed a high variability of LDTs, and mercerization was found to increase their mechanical resistance. SEM images confirmed a microtubular cellular structure, XRD exposed an increment in crystallinity after alkali treatment, and Weibull indicated the probability of failure in 10 samples per configuration.

Lignin containing cellulose nanofibers (LCNFs): Lignin content-morphology-rheology relationships

This study aims to investigate the relationship between lignin content, morphology, and rheology of lignin containing cellulose nanofibers (LCNFs). The morphology and rheology of LCNFs were dominated by lignin content. Lignin content had two-sides on mechanical fibrillation. At high lignin content (23.79 %), reduced efficiency of defibrillation resulted in large LCNFs connecting with lignin patches. LCNF suspensions exhibited low viscosity, weak gel behavior due to infirm fibril network. Small yield stress of 1.14 Pa suggested that fibril network was easily disrupted. At residual lignin of 6.52 %, fibril bundles were sensitive to defibrillation, producing long and flexible LCNFs with high capacity of entanglement. The entangled fibril network had high viscosity and strong gel like behavior. Creep compliance of 0.09 Pa−1 and large yield stress of 4.25 Pa indicated excellent resistance to deformation. The desired rheology can be tailored by lignin content, providing practical guidance on novel rheology-dependent LCNF based materials.

Elastic modulus mapping for bovine cortical bone from submillimeter- to submicron-scales using PeakForce Tapping atomic force microscopy

Bone is a composite material consisting of organic and inorganic components that are organized into hierarchical structures to provide load-bearing functions. This paper presents the results of PeakForce Tapping atomic force microscopy (AFM) scans on cut and polished bovine cortical bone specimens that were submerged in water. The elastic modulus map and surface morphology were obtained for various bone hierarchical structures from submillimeter- to submicron-scales. The elastic modulus of osteons (20.51 ± 6.85 GPa) was slightly lower than the interstitial bone (21.87 ± 5.48 GPa); they were both much greater than that of the cement lines (7.49 ± 4.23 GPa). The elastic modulus in the lamella structures varied periodically from higher values in thick sub-lamellae (21.49 ± 6.58 GPa) to the lower values in thin sub-lamellae (9.67 ± 2.69 GPa). The results also show relatively softer mineralized collagen fibril bundle arrays (12.94 ± 2.71 GPa) embedded in harder matrix materials (28.39 ± 5.75 GPa). The variations in the elastic modulus suggest different degrees of mineralization or different fibril orientations. The histograms of elastic modulus indicate the dominating compositions or dominating fibril orientations.

Approaching Theoretical Haze of Highly Transparent All-Cellulose Composite Films.

A highly transparent cellulose film with a high built-in haze is emerging as a green photonic material for optoelectronics. Unfortunately, attaining its theoretical haze still remains a challenge. Here, we demonstrate an all-cellulose composite film with a 90.1% transmittance and a maximal transmission haze of 95.2% close to the theoretical limit (∼100%), in which the entangled network of softwood cellulose fibers works as strong light scattering sources and regenerated cellulose (RC) with undissolved fibril bundles functions as a matrix to simultaneously improve the optical transparency and transmission haze. The underlying mechanism for the ultrahigh haze is attributed to microsized irregularities in the refractive index, arising primarily from the crystalline structure of softwood fibers, undissolved nanofibril bundles in RC, and a small number of internal cavities. Moreover, the resulting composite film presents a folding resistance of over 3500 times and good water resistance, and its application in a perovskite solar cell as an advanced light management layer is demonstrated. This work sheds light on the design of a highly transparent cellulose film with a haze approaching the theoretical limit for optoelectronics and brings us a step further toward its industrial production.

Three-dimensional structural interrelations between cells, extracellular matrix, and mineral in normally mineralizing avian leg tendon

The spatial-temporal relationship between cells, extracellular matrices, and mineral deposits is fundamental for an improved understanding of mineralization mechanisms in vertebrate tissues. By utilizing focused ion beam-scanning electron microscopy with serial surface imaging, normally mineralizing avian tendons have been studied with nanometer resolution in three dimensions with volumes exceeding tens of micrometers in range. These parameters are necessary to yield sufficiently fine ultrastructural details while providing a comprehensive overview of the interrelationships between the tissue structural constituents. Investigation reveals a complex lacuno-canalicular network in highly mineralized tendon regions, where ∼100 nm diameter canaliculi emanating from cell (tenocyte) lacunae surround extracellular collagen fibril bundles. Canaliculi are linked to smaller channels of ∼40 nm diameter, occupying spaces between fibrils. Close to the tendon mineralization front, calcium-rich deposits appear between the fibrils and, with time, mineral propagates along and within them. These close associations between tenocytes, tenocyte lacunae, canaliculi, small channels, collagen, and mineral suggest a concept for the mineralization process, where ions and/or mineral precursors may be transported through spaces between fibrils before they crystallize along the surface of and within the fibrils.

Flexor tendon grafts for pulley reconstruction – Morphological aspects

BackgroundPulleys are crucial to convert flexor tendon excursion into angular motion at the metacarpophalangeal and interphalangeal joints. Loss of pulley function can lead to significant impairment of hand function and may require surgical reconstruction. This reconstruction can be achieved using different flexor tendons grafts, such as the intrasynovial flexor digitorum superficialis (FDS) or the extrasynovial palmaris longus (PL). However, there is limited knowledge on the micromorphology of human pulleys and the suitability of flexor tendon grafts for their reconstruction remains elusive. MethodsIn the present cadaver study A2 and A4 pulleys were compared with FDS and PL tendons by means of scanning electron microscopy (SEM), histology and immunohistochemistry. Surface morphology, core structure and vascularization of the specimens were analyzed. ResultsSEM imaging of the gliding surfaces revealed morphological differences between tendons and pulleys. Moreover, the core structure of FDS samples was characterized by bundles of individual collagen fibrils whereas PL tendons exhibited a less hierarchical microstructure. In contrast, pulleys consisted of lamellar sheets of densely packed collagen fibrils. Finally, immunohistochemical analyses revealed that the flexor tendons and pulleys contain similar numbers of CD31+ microvessels, indicating a comparable tissue vascularization. ConclusionThis study provides novel SEM and immunohistochemical insights into the micromorphology of human pulleys and flexor tendon grafts. Intrasynovial flexor tendons may be particularly suitable for pulley reconstruction and preserving the paratenon may be crucial for graft revascularization.

In vitro Neo-Genesis of Tendon/Ligament-Like Tissue by Combination of Mohawk and a Three-Dimensional Cyclic Mechanical Stretch Culture System.

Tendons and ligaments are pivotal connective tissues that tightly connect muscle and bone. In this study, we developed a novel approach to generate tendon/ligament-like tissues with a hierarchical structure, by introducing the tendon/ligament-specific transcription factor Mohawk (MKX) into the mesenchymal stem cell (MSC) line C3H10T1/2 cells, and by applying an improved three-dimensional (3D) cyclic mechanical stretch culture system. In our developed protocol, a combination of stable Mkx expression and cyclic mechanical stretch synergistically affects the structural tendon/ligament-like tissue generation and tendon related gene expression. In a histological analysis of these tendon/ligament-like tissues, an organized extracellular matrix (ECM), containing collagen type III and elastin, was observed. Moreover, we confirmed that Mkx expression and cyclic mechanical stretch, induced the alignment of structural collagen fibril bundles that were deposited in a fibripositor-like manner during the generation of our tendon/ligament-like tissues. Our findings provide new insights for the tendon/ligament biomaterial fields.

Modulation of structure and optical properties micro-fibrils of plants by means of electrical, deformation and optical treatments

Variations of structure and optical properties of micro-fibrils of plants induced by optical, electrical, deformation and water treatments were studied in situ by means of optical polarization microscopy. Bundles of dry fibrils pulled out from nettle, maple and spruce were used for the experiments. Strong enhancement of the optical anisotropy in all of the fibrils has been found just after their wetting with water. Appearance of this anisotropy is attributed to orientation ordering of cellulose molecules in the neighbor layers of the walls of the fibrils. This ordering is explained by penetration of water molecules into the interfaces between cellulose layers and amorphous lignin polymers bound with cellulose molecules in the dry state of the fibrils. Relatively week electrical fields applied to the fibrils removes this anisotropy by pushing out the molecules of water from these interfaces. Evaporation of water returns the fibrils to optically isotropic state as well. The changes of the anisotropy of the fibrils are followed with their deformations These deformations induce internal electrical fields. Hence the interactions of the fibrils with water, electrical and deformation fields result in self-consistent propagation of electromechanical waves along the plants vessels. These waves are capable to transport feeding nutrients. Irradiation of wood components immersed in water by ultraviolet light induces dissolving of lignin and produces pure cellulose fibers. This phenomenon provides the development of new ecologically safety technology of production of cellulose.

A Role for Monosaccharides in Nucleation Inhibition and Transport of Collagen.

Background: Collagenous tissues are composed of precisely oriented, tightly packed collagen fibril bundles to confer the maximal strength within the smallest volume. While this compact form benefits mobility, it consequentially restricts vascularity and cell density to a minimally viable level in some regions. These tissues reside in a homeostatic state with an unstable equilibrium, where perturbations to structure or molecular milieu cause descension into a long-term compromised state. Several studies have shown that glycosaminoglycans are key molecules required for healthy tissue maintenance. Our long-term goal is to determine if glycosaminoglycans serve a critical function of stabilizing soluble monomeric collagen in the interstitial fluid that bathes tissue for immediate availability in tissue development and repair in vivo. Materials and Methods: To test glycosaminoglycan and collagen interactions at the most fundamental level, we have explored the effect of the monosaccharides that populate the glycosaminoglycans of the extracellular matrix on collagen assembly kinetics, pre-established matrix stability, and collagen incorporation into a preassembled matrix. Results: Results showed that monosaccharides increased the threshold concentration required for spontaneous polymerization by at least three orders of magnitude. When the monosaccharides were introduced to a pre-existing collagen network, fibrillar dissociation was undetectable. Fluorescent-labeling studies illustrated that in the presence of the saccharide solution, soluble collagen maintains the functional capacity to integrate into a pre-existing network. Conclusion: This work demonstrates a feasible role for glycosaminoglycans in supporting tissue remodeling and highlights the potential importance of age-related deterioration of glycosaminoglycan biosynthesis in reference to the homeostasis of collagen-based tissues.

Researches of Morphological Structure, Element Composition And Natural Leather Adsorption By Exposure to Laser Radiation

Annotation. Scanning electron microscopy studied microphotographs of a longitudinal section of a bundle of collagen fibrils of the reticular layer of a leather tanning product of chromium tanning from cattle hides. The distribution of chrome tanning agent over the leather thickness in the initial sample was studied. The effect of laser exposure on the microstructure of the leather is investigated. It was found that laser irradiation (input energy 40 J, exposure time 40 sec) leads to a slight increase in pore size and loosening of its structure. It was established that in the process of laser radiation, collagen fiber bundles first swell with their own capillary moisture, then they are separated, i.e. they break up into individual microfibrils. An elemental analysis of the distribution of chromium over the leather cross section showed that the chromium compound penetrates the entire thickness of the leather, however, the chromium distribution varies significantly with thickness. The effect of laser radiation on the process of sorption-desorption of water by the leather is revealed. From the data obtained it was found that laser leather training changes its ability to adsorb water vapor and desorb moisture from the environment.

Hierarchical Mechanisms of Lateral Interactions in High-Performance Fibers.

The processing conditions used in the production of advanced polymer fibers facilitate the formation of an oriented fibrillar network that consists of structures spanning multiple length scales. The irregular nature of fiber tensile fracture surfaces suggests that their structural integrity is defined by the degree of lateral (interfacial) interactions that exist within the fiber microstructure. To date, experimental studies have quantified interfacial adhesion between nanoscale fibrils measuring 10-50 nm in width, and the global fracture energy through applying peel loads to fiber halves. However, a more in-depth evaluation of tensile fracture indicates that fiber failure typically occurs at an intermediate length scale, involving fibrillation along interfaces between fibril bundles of a few 100s of nanometers in width. Interaction mechanisms at this length scale have not yet been studied, due in part to a lack of established experimental techniques. Here, a new focused ion beam-based sample preparation protocol is combined with nanoindentation to probe interfaces at the intermediate length scale in two high-performance fibers, a rigid-rod poly(p-phenylene terephthalamide) and a flexible chain ultrahigh molecular weight polyethylene fiber. Higher interfacial separation energy recorded in the rigid-rod fiber correlated with less intensive fibrillation during failure and is discussed in the context of fiber chemistry and processing. Power law scaling of the total absorbed interfacial separation energy at three different scales in the polyethylene fiber is observed and analyzed, and distinct energy absorption mechanisms, featuring a degree of self-similarity, are identified. The contribution of these mechanisms to the overall integrity of the fiber is discussed, and the importance of the intermediate scale is elucidated. Results from this study provide new insights into the mechanical implications of hierarchical lateral interactions and will aid in the development of novel fibers with further improved mechanical performance.

Nanocellulose Film Properties Tunable by Controlling Degree of Fibrillation of TEMPO-Oxidized Cellulose.

A fibrous 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized wood cellulose/water slurry was disintegrated with a magnetic stirrer or high-pressure homogenizer under various conditions to prepare TEMPO-oxidized cellulose (TOC)/water dispersions with different degrees of fibrillation. The turbidity value of the as-prepared dispersion was used as a measure of the degree of nanofibrillation of the fibrous TOC slurry in water. The fibrillated TOC/water dispersions with low degrees of fibrillation had cellulose nanonetwork (CNNeW) structures consisting of TOC nanofibrils (TOCNs), unfibrillated TOC fibers, and fibril bundles. The original TOC/water slurry and partly fibrillated TOC/water dispersions with low degrees of fibrillation were converted to a sheet and films, respectively, in a short time by membrane filtration, and they had low bulk densities and high porosities. Membrane filtration of an almost completely nanofibrillated TOC/water or TOCN dispersion took a long time, but the as-prepared TOCN films had the highest light transparency, tensile strength, Young's modulus, and work of fracture. The oxygen permeabilities of the films at 23°C and 50% relative humidity were as low as 1–2 ml μm m−2 day−1 kPa−1 among the films prepared from the fibrillated TOC/water dispersions with a wide turbidity range of 0.01–0.45. Therefore, TEMPO-oxidized CNNeW films with the versatile optical, porous, and mechanical properties but similarly low oxygen permeabilities can be prepared by controlling the degree of fibrillation of the TOC/water slurry (Graphical ).

Bundling of Collagen Fibrils Using Sodium Sulfate for Biomimetic Cell Culturing.

Collagen is the most abundant extracellular matrix protein. The concentrations, structural arrangement, and directionality of collagen depend on the type of tissue. Thick fibril bundles of collagen are observed in most collagenous tissues, including connective tissues, bones, and tendons, indicating that they play a critical role in many cell functions. In this study, we developed a new method to regulate collagen bundling without altering the protein concentration, temperature, or pH by using sodium sulfate to replicate bundled collagen fibrils found in vivo. Microstructure analysis revealed that both the thickness of the fibril bundles and the pore size of the matrix increased with the amount of sodium sulfate. In contrast, there was no significant change in the bulk mechanical stiffness of the collagen matrix. The modified collagen bundle matrix was used to investigate the responses of human cervical cancer cells by mimicking the extracellular environments of a tumor. Compared to the normal collagen matrix, cells on the collagen bundle matrix exhibited significant changes in morphology, with a reduced cell perimeter and aspect ratio. The cell motility, which was analyzed in terms of the speed of migration and mean squared displacement, decreased for the collagen bundle matrix. Additionally, the critical time taken for the peak turning angle to converge to 90° decreased, indicating that the migration direction was regulated by geometric cues provided by collagen bundles rather than by the intrinsic cell persistence. The experimental results imply that collagen bundles play an important role in determining the magnitude and direction in cancer cell migration. The proposed method of extracellular matrix modification can be applied to investigate various cellular behaviors in both physiological and pathological environments.

Chiral Reaction Field with Thermally Invertible Helical Sense that Controls the Helicities of Conjugated Polymers.

A chiral reaction field with thermally invertible helical sense enables control of the helicity of the reaction product, which is a central challenge in asymmetric synthesis that has yet to be overcome. A novel chiral compound comprising two types of chiral moieties with opposite helicities and temperature dependences is synthesized; this compound is added as a chiral dopant to a mixture of nematic liquid crystals to prepare a chiral nematic liquid crystal (N*-LC). The N*-LC containing the chiral dopant exhibits thermally invertible helicity to yield left- and right-handed helical senses at low and high temperatures, respectively. Interfacial polymerization of acetylene is achieved in the N*-LC by modulating the temperature. Helical polyacetylenes (H-PAs) that are synthesized at low (-12 °C) and high (28 °C) temperature show right- and left-handedness, respectively, in terms of the fibrils, fibril bundles, and spiral morphology. In addition, the helical sense of H-PA is opposite that of the N*-LC because of the peculiar polymerization mechanism for acetylene in the N*-LC. The current N*-LC is the first chiral reaction field that has not only the thermally invertible helical sense but also the chemical functions and stability needed to serve as the medium for polymer reactions.

(−)-Epigallocatechin gallate protected molecular structure of collagen fibers in sea cucumber Apostichopus japonicus body wall during thermal treatment

During thermal tenderization of sea cucumber body wall, protein oxidation always happens which disrupts its structural integrity. In this study, we investigated the effects of (̶̶−)-epigallocatechin gallate (EGCG, at 0, 5, 50, 125, 250 μg/mL) to protect the structure of heated collagen fibers (CFs) in sea cucumber Apostichopus japonicus body wall. Many disintegrated collagen fibrils and fibril bundles were observed in samples with no EGCG by scanning electron microscope, while the integrity of CFs was protected in samples with 125 μg/mL EGCG. Electron paramagnetic resonance results indicated heat-induced •OH in heated CFs were neutralized by EGCG, up to 83% in samples with 250 μg/mL EGCG. Chemical analysis of the soluble fragments in heated CFs showed plenty of proteins and glycosaminoglycan were released in heated CFs, and the addition of EGCG into the samples reduced their dissolution. Analysis of differential scanning calorimetry and thermogravimetry indicated EGCG could enhanced the thermostability of CFs. Fourier transform infrared spectroscopy results showed EGCG at 125 μg/mL prevented the conversion of α-helixes to β-sheets, and affected N–H bending coupled with C–N stretching. In conclusion, EGCG could protect the structure of CFs in a dosage dependent way, and 125 μg/mL was the most effective.

Callose-synthesizing enzymes as membrane proteins of Betula protoplasts secrete bundles of β-1,3-glucan hollow fibrils under Ca2+-rich and acidic culture conditions

Abstract Previously, it was reported that plant protoplasts isolated from Betula platyphylla (white birch) callus secreted bundles of hollow callose fibrils in acidic culture medium containing a high concentration of calcium ions (Ca2+). Here, the callose synthase was characterized from in situ and in vitro perspectives. Localization of callose synthases at the secreting site of callose fiber was indicated from in situ immunostaining observation of protoplasts. For in vitro analyses, membrane proteins were extracted from membrane fraction of protoplasts with a 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate (CHAPS) treatment. The CHAPS extract aggregated in the presence of a high concentration of Ca2+, suggesting that Ca2+ may promote the arrangement of callose synthases in the plasma membrane. The callose synthase activity was dependent on pH and Ca2+, similar to the callose synthase of Arabidopsis thaliana. However, the synthesized fibril products were longer than those produced by callose synthases of herbaceous plants. This is the first insight into the specific properties of callose synthases of woody plants that secrete of callose hollow fibers.

Diverticula in Male Lycorea halia Butterflies (Lepidoptera: Nymphalidae: Danaini: Itunina)\u2014Support Organs for Everted Hairpencils with Unique Ultrastructure

The involvement of the diverticula, a synapomorphy for Itunina, in protrusion and expansion of hairpencils by male Lycorea halia (Hübner, 1816) is demonstrated for the first time. They facilitate maintaining the haemolymph pressure necessary to keep the hairpencils everted. The diverticula are curved hook-like lobes, open to the body cavity and densely filled with tracheae and threads made by units of two staggered cells surrounding a central extracellular fibril bundle. Such complex structures, apparently metabolically active, have not been reported for insects previously and might indicate additional functions, but their functional role(s) remains a puzzle. When a male emerges from pupa, the diverticula are not yet formed; this happens only during the first protrusion of the hairpencils.

Mechanical behavior of cellular networks of fiber bundles stabilized by adhesion

In this work, we study the mechanical behavior of non-crosslinked networks of fibers that interact adhesively. Adhesion drives fiber organization into bundles and a network of fiber bundles forms as a result of this process. Bundles split and re-connect forming specific triangular features at all bundle intersections, with role in network stabilization. The structure of such networks has been discussed in the literature, but their mechanics remains largely unexplored. We show here that such networks are exceptionally stable, and despite the absence of crosslinks between fibers behave, at relatively small strains, essentially similar to crosslinked networks, in which the role of crosslinks is played by the triangular structures at bundle intersections. We also provide new results regarding the effect of the network architecture on the type of strain stiffening observed in tension. The results apply to carbon nanotube structures, such as buckypaper, and various connective biological tissue in which collagen fibrils form bundles and the tissue is a network of collagen fibril bundles.