The mechanical and thermal properties of frog slow muscle fibres

1. A study has been made of the mechanical behaviour and the heat production of frog slow muscle fibres in iliofibularis nerve-muscle preparations at 20 degrees C.2. The slow fibre isometric tension and its rate of development increase with stimulation frequency, the increases beyond 30 Hz being relatively small. Relaxation rate also increases with stimulation frequency. The tension-length curve and maximum isometric tension (250 mN.mm(-2)) are similar to those of twitch fibres. The maximum shortening velocity is estimated to be 0.11 tonus bundle lengths per second.3. For contractions up to 60 sec at 30-50 Hz the slow fibre heat rate is steady at 6 mJ.g(-1).sec(-1). Slow fibres produce aerobic recovery heat with a time course similar to that of twitch fibres.4. The accuracy of the results is discussed, and a comparison is made with the properties of twitch fibres. It is concluded that the tension-producing reactions are thirty times slower in the slow fibres.     

Local effects of impaired mechanical properties of collagen on bone formation and resorption

Summary To study the relationship between the mechanical properties of collagen and the bone turnover, 2-week-old Balb/C mice were fed on a diet containing 0.25% Beta aminopropionitrile (B-APN), a potent inhibitor of collagen crosslink formation, for 3 weeks. Mandibular incisor socket was selected for the analysis of bone formation and resorption parameters. Plastic embedded sections stained with toludine blue and cut at 4 µm were used to analyze the average area of bone lamellae, bone-forming surface, and the number of osteoblasts/mm of forming surface. Similar sections were used to localize acid phosphatase on resorbing surfaces and within the osteoclasts, while bone alkaline phosphatase was determined by a colorimetric method. Morphometric analyses showed that the area of newly formed bone lamellae, total forming surface, number of osteoblasts and the Alk. Pase activity were significantly lower in B-APN-fed mice as compared to the controls. There was a concomitant smaller, but significant, reduction in total resorption surface, active resorption surface and the number of osteoclasts. These results suggest that the regulation of bone formation and resorption at this site, which is independent of systemic regulation, is influenced by the mechanical properties of the collagenous matrix, which in turn may have a significant effect on the existing pool of bone-forming cells, but may not influence the recruitment of new cells.     

Effects of manganese on the electrical and mechanical properties of frog skeletal muscle fibres

1. The effects of Mn on the electrical and mechanical properties of frog muscle fibres have been studied.2. In normal saline 10 or 20 mM-Mn hyperpolarized the fibres and had no effect on the membrane resistance. In isotonic K(2)SO(4) saline, Mn increased the membrane resistance indicating that this agent reduced the conductance to K.3. The action potential is prolonged by Mn while the overshoot amplitude is unaffected. The threshold of the action potential is shifted to more positive values of membrane potential.4. The isometric twitch is reduced by 45% in 10 mM-Mn; this effect is observed within 8 sec of the application.5. Mn (10 mM) reduced K contractures induced by 40 or 75 mM-K (constant [K].[Cl] product) and shifted to the right in a parallel manner the curve tension vs. log K concentration. The calculated mechanical threshold for K contractures was shifted from -48 to -33 mV.6. Caffeine contractures (3-4 mM) and supramaximal K contractures (190 mM-K) were unaffected by 10 mM indicating that contractile proteins and the ability of the sarcoplasmic reticulum to release Ca are not impaired.7. It is concluded that Mn is mainly affecting the excitation-contraction coupling by altering the mechanical threshold. Since Mn reduces the permeability to Ca in several excitable membranes, it is suggested that the mechanical threshold depends on the entry of Ca to the muscle.     

Effect of embedded optical fibres on the mechanical properties of cochlear electrode arrays

Incorporating optical fibres in cochlear electrode arrays has been proposed to provide sensors to help minimise insertion trauma and also for the delivery of light in optical nerve stimulation applications. However, embedding an optical fibre into an electrode array may change its stiffness properties, which can affect the level of trauma during insertion. This report uses measurements of buckling and deflection force to compare the stiffness properties of a range of cochlear electrode arrays (Nucleus straight array, rat array, cat array and guinea pig array) with custom arrays containing an embedded optical fibre. The cladding diameters of the optical fibres tested were 125 µm, 80 µm and 50 µm. The results show that the stiffness of the optical-fibre-embedded arrays is related to the diameter of the optical fibre. Comparison with wired arrays suggests optical fibres with a diameter of 50 µm could be embedded into an electrode array without significantly changing the stiffness properties of the array.     

The influence of micropore size on the mechanical properties of bulk hydroxyapatite and hydroxyapatite scaffolds

While recognized as a promising bone substitute material, hydroxyapatite (HA) has had limited use in clinical settings because of its inherent brittle behavior. It is well established that macropores (～100 μm) in a HA implant, or scaffold, are required for bone ingrowth, but recent research has shown that ingrowth is enhanced when scaffolds also contain microporosity. HA is sensitive to synthesis and processing parameters and therefore characterization for specific applications is necessary for transition to the clinic. To that end, the mechanical behavior of bulk microporous HA and HA scaffolds with multi-scale porosity (macropores between rods in the range of 250–350 μm and micropores within the rods with average size of either 5.96 μm or 16.2 μm) was investigated in order to determine how strength and reliability were affected by micropore size (5.96 μm versus 16.2 μm). For the bulk microporous HA, strength increased with decreasing micropore size in both bending (19 MPa to 22 MPa) and compression (71 MPa to 110 MPa). To determine strength reliability, the Weibull moduli for the bulk microporous HA were determined. The Weibull moduli for bending increased (became more reliable) with decreasing pore size (7 to 10) while the Weibull moduli for compression decreased (became less reliable) with decreasing pore size (9 to 6). Furthermore, the elastic properties of the bulk microporous HA (elastic modulus of 30 GPa) and the compressive strengths of the HA scaffolds with multi-scale porosity (8 MPa) did not vary with pore size. The mechanisms responsible for the trends observed were discussed.     

Biophysical and mechanical properties of red and white muscle fibres in fish

1. The electrical properties of red muscle fibres of the silver carp (Carassius auratus (Linné)) were investigated and compared with those of white fibres. The resting membrane potential of red muscle was -73·1 mV, and of white muscle -82·4 mV. The effective resistance of fibre, time constant and space constant were 1·43 MΩ, 26·6 msec and 2·1 mm for red muscle and 1·0 MΩ, 48·4 msec and 1·9 mm for white muscle.

2. The membrane capacitance in red muscle was 2·55 μF/cm² and in white muscle was 7·23 μF/cm², though the sarcoplasmic reticulum and the transverse tubular system were developed equally in both tissues.

3. No rectification could be observed in the current—voltage relation in both the muscles. However, in excess potassium solutions, both types of muscle showed anomalous rectification.

4. In red muscle it was rare to trigger a spike by nerve stimulation or by field stimulation of the tissue. However, in white muscle, there appeared graded responses and occasionally spikes were generated with overshoot potentials.

5. Excess potassium concentration produced contracture in the red muscle, but even forty times of normal potassium never produced contracture in the white muscle.

     

Formation of continuous collagen fibres: evaluation of biocompatibility and mechanical properties

Reconstituted collagen fibres have potential applications in repair of soft and hard tissues. Preliminary studies conducted in our laboratory suggest that discontinuous reconstituted type I fibres have strengths similar to those of fibres teased from tendons. The purpose of this paper is to report a method for continuous collagen fibre production and the properties of fibres produced. Ultimate tensile mechanical properties and biocompatibility of continuous type I collagen fibres were studied and compared with the properties of fibres produced manually (discontinuous fibres). In general, continuously made cyanamide cross-linked fibres show slightly inferior mechanical properties and faster biodegradation rates compared with manually made fibres because of minor differences in the fibreformation protocol introduced by design constraints. However, continuous and discontinuous fibres crosslinked with glutaraldehyde had comparable properties. These results demonstrate that production of 50 microns diameter continuous collagen fibre is possible.     

The influence of material and design features on the mechanical properties of transpedicular spinal fixation implants

This study describes the finding and performance of mechanical strength and corrosion testing procedures for comparative examination of multiple internal transpedicular spine fixators. Seven different implant models from five different manufacturers were compared regarding their bending strength and fatigue resistance. Because of the unacceptably high levels of time and material that they require, ISO and ASTM testing standards are not applicable to comparative testing. In addition, there is a lack of knowledge about clinically defined and proven strength-limit values. Therefore, actual standard testing procedures have been modified and extended to corrosion testing. Overall, the effort necessary to obtain reproducible comparative data has been reduced significantly. Although a reduced number of implants of each type were available for destructive testing, the results revealed fundamental differences in the tested implants between different materials and design features. During fatigue testing some of the implants showed poor corrosion properties. Because spinal fixation implants tend to be used as long-duration implants, corrosion testing as well as comparative strength testing with clinically successful implant models should be performed as preclinical evaluations.     

The influence of age on adaptive bone formation and bone resorption

Bone is a tissue with enormous adaptive capacity, balancing resorption and formation processes. It is known that mechanical loading shifts this balance towards an increased formation, leading to enhanced bone mass and mechanical performance. What is not known is how this adaptive response to mechanical loading changes with age. Using dynamic micro-tomography, we show that structural adaptive changes of trabecular bone within the tibia of living mice subjected to two weeks of in vivo cyclic loading are altered by aging. Comparisons of 10, 26 and 78 weeks old animals reveal that the adaptive capacity diminishes. Strikingly, adaptation was asymmetric in that loading increases formation more than it reduces resorption. This asymmetry further shifts the (re)modeling balance towards a net bone loss with age. Loading results in a major increase in the surface area of mineralizing bone. Interestingly, the resorption thickness is independent of loading in trabecular bone in all age groups. This data suggests that during youth, mechanical stimulation induces the recruitment of bone modeling cells whereas in old age, only bone forming cells are affected. These findings provide mechanistic insights into the processes that guide skeletal aging in mice as well as in other mammals.     

Adhesive and mechanical properties of hydrogels influence neurite extension

Photopolymerizable polyethylene glycol (PEG) hydrogels conjugated with bioactive ligands were examined for their use as scaffolds in peripheral nerve regeneration applications. The bioactivity and mechanical properties of PEG hydrogels can be tailored through the integration of bioactive factors (adhesion ligands, proteolytic sites, growth factors) and the alteration of PEG concentrations, respectively. For peripheral nerve regeneration, it will be important to determine the type and concentration of the bioactive molecules required to improve neurite extension. In this study, cell adhesion ligands (RGDS, IKVAV, and YIGSR) were covalently attached to PEG hydrogels. Both the type and concentration of cell adhesion ligand used affected neurite extension. Extension from PC12 cells was greater on hydrogels with RGDS incorporated than IKVAV, and the optimal concentration for each ligand was different. Cells adhered to but did not extend neurites on hydrogels with YIGSR. Cells did not adhere to hydrogels containing RGES. Furthermore, different combinations of these ligands affected neurite extension to different degrees. The mechanical properties of the hydrogels also significantly affected neurite extension. PC12 cells grown on more flexible hydrogels exhibited the greatest degree of neurite extension. PEG hydrogels have thus been developed with varying biochemical and mechanical properties that may enhance nerve regeneration.     

Separating the influence of the cortex and foam on the mechanical properties of porcupine quills

Lightweight thin cylinders filled with a foam have applications as collapsible energy absorbers for crashworthy and flotation applications. The local buckling compressive strength and Young’s modulus are dependent on material and geometrical properties. Porcupine quills have a thin cortex filled with closed-cell foam, and are entirely composed of α-keratin. The cortex carries the majority of the compressive load, but the foam is able to accommodate and release some of the deformation of the cortex during buckling. The presence of the foam increases the critical buckling strength, buckling strain and elastic strain energy absorption over that of the cortex. Good agreement is found between experimental results and modeled predictions. A strain distribution map of the foam close to the buckled cortex demonstrates that the deformation of the cells plays an important role in accommodating local buckling of the cortex. The robust connection between the foam and cortex results in superior crushing properties compared to synthetic sandwich structure where the foam normally separates from the shell. The foam/cortex construction of the quill can guide future biomimetic fabrications of light weight buckle-resistant columns.     

The influence of non-enzymatic glycosylation and formation of fluorescent reaction products on the mechanical properties of rat tail tendons

Mechanical stability was examined in rat tail tendons after in vitro incubation in glucose at pH 7.4 using buffer systems of either phosphate or tris(hydroxymethyl)aminomethan. In the phosphate buffer system glucose and fluorescent compounds were found to be attached to the collagen molecules and the maximum 'stress' of the tendons was increased. In the tris(hydroxymethyl)aminomethan buffer system glucose was attached to the collagen molecules, but only small amounts of fluorescent compounds were attached to the collagen molecules and no changes in mechanical parameters were recorded. Initial incubation of tendons in a high concentration of glucose followed by incubation in either phosphate or tris(hydroxymethyl)aminomethan buffer solutions resulted in equal attachment of glucose to the collagen, but only the collagen reincubated in phosphate buffer developed a relatively high amount of fluorescent compounds and an increase in maximum mechanical 'stress' compared to the collagen reincubated in tris(hydroxymethyl)aminomethan buffer. This shows that the non-enzymatic glycosylation per se does not necessarily influence the mechanical properties of collagen. Additional reactions like transformation of glucose into fluorescent compounds or browning reaction products by Maillard's reaction seem to be essential.     

Characterization of the mechanical properties of tough biopolymer fibres from the mussel byssus of Aulacomya ater

Byssus fibres are tough biopolymer fibres produced by mussels to attach themselves to rocks. In this communication, we present the mechanical properties of the byssus from the South American mussel Aulacomya ater which have not been previously reported in the literature. The mechanical properties of the whole threads were assessed by uniaxial tensile tests of dry and hydrated specimens. Elastoplastic and elastomeric stress-strain curves were found for byssal threads from A. ater in the dry and hydrated state, respectively. The results obtained from mechanical tests were modelled using linear, power-law-type and Mooney-Rivlin relationships. These methods for dealing with tensile measurements of mussel byssus have the potential to be used with other stretchy biomaterials.     

The influence of urgency on decision time

A fruitful quantitative approach to understanding how the brain makes decisions has been to look at the time needed to make a decision, and how it is affected by factors such as the supply of information, or an individual's expectations. This approach has led to a model of decision-making, consistent with recent neurophysiological data, that explains the observed variability of reaction times and correctly predicts the effects of altered expectations. Can it also predict what happens when the urgency of making the response changes? We asked subjects to make eye movements to low-visibility targets either as fast or as accurately as possible, and found that the model does indeed predict the timing of their responses: the degree of urgency seems to influence the criterion level at which a decision signal triggers a response.     

Modification with alkyl chains and the influence on thermal and mechanical properties of aromatic hyperbranched polyesters

All‐aromatic hyperbranched polyesters with hydroxy endgroups were functionalized with aliphatic n‐alkyl carboxylic acids. The length of the n‐alkyl chain as well as the degree of modification were varied and the resulting, partially amphiphilic polymers were characterized by differential scanning calorimetry (DSC). With both increasing degree of modification and increasing length of the alkyl chain the glass transition temperature decreases due to reduced intermolecular hydrogen bonding. When the alkyl chains start to crystallize T_g of the hyperbranched polymers increases again. The mechanical properties of the former brittle hyperbranched polyester were improved by modification with C12 chains and a stable free standing film was obtained by compression molding. The film was investigated by means of dynamic mechanic analysis (DMA) and microscopy, exhibiting a low temperature thermal transition and phase separation within the scale of a light microscope. Furthermore melt rheology measurements were performed on the starting polymer and on the C12 modified product. The complex viscosity is reduced strongly by the modification of the aromatic hyperbranched polyester.     

The response of laryngeal afferent fibres to mechanical and chemical stimuli

1. We have recorded afferent activity from ;single fibres' dissected from the superior laryngeal nerve of anaesthetized cats.2. Units which responded to gentle mechanical stimulation of the larynx epithelium were chosen for study.3. Receptors with myelinated fibres were grouped according to their spontaneous activity. Group 1 fibres had little or no spontaneous activity: group 2 fibres had constant and continuous spontaneous activity.4. Group 1 fibres had a wide range of adaptation rates. Their conduction velocities lay between 3.0 and 30 m/sec. The receptors were generally stimulated by ammonia and distilled water and often by CS riot control agent, 5 and 10% CO(2), 200 ppm SO(2), and cigarette smoke. Histamine, phenyl diguanide, graphite dust, 100 ppm SO(2) and saline drops did not generally excite the fibres.5. Group 2 fibres were slowly adapting. Their conduction velocities ranged between 8.0 and 26.5 m/sec. Ammonia usually, and distilled water sometimes, excited these fibres while 5 and 10% CO(2) mixtures inhibited them. A minority of group 2 fibres were pH sensitive, inhibited by acids and stimulated by alkaline buffers. Cigarette smoke had complex actions, either excitation, inhibition or, at different times, both. Histamine, P.d.g., CS, SO(2), saline drops and dust had no action on these fibres.6. Recordings were made from one unmyelinated fibre (conduction velocity 1.9 m/sec) which responded to stroking of the epithelium with a thread and to histamine, P.d.g. and ammonia vapour applied to the epithelium.7. We consider the site, method of excitation and reflex actions of the different receptors described.