A new method for extracting quantitative data from amplitude modulation dynamic force-distance measurements is developed. The method is based on the harmonic oscillator model of vibrating atomic force microscope cantilevers, and is capable of extracting both the conservative and dissipative parts of the tip-sample interaction from a measurement of oscillation amplitude and phase as a function of distance. Numerical simulations are used to demonstrate the validity of the method. Further proof of the accuracy of this method is provided by a measurement of electrostatic forces between an AFM tip and a graphite sample.

The UCD community has made this article openly available. Please share how this access benefits you. Your story matters! (@ucd_oa) Some rights reserved. For more information, please see the item record link above. TitleNano-textured self-assembled aligned collagen hydrogels promote directional neurite guidance and overcome inhibition by myelin associated glycoprotein The development of nerve guidance conduits is constantly evolving as the need arises for therapies for spinal cord injury. In addition to providing a path for regrowing axons to reconnect with their appropriate targets, the structural and biochemical cues provided by these conduits should be permissive for directional neurite outgrowth and be protective against inhibition in the vicinity of the injury site. Here, we adapted the use of iso-electric focusing to drive the alignment of supramolecular fibrils into self-assembled collagen hydrogels ($300 mm diameter), and tested those hydrogels for the ability to direct and enhance the migration of neurites. Structural characterization revealed anisotropic alignment of nanofibrillar aggregates ($20 nm diameter), arranged in micron-scale bundles ($1 to 2 mm diameter) similar to the hierarchical size scales observed in native tissues. Neurite outgrowth extended bidirectionally along the axes of aligned hydrogels. Furthermore, it was shown that, as opposed to poly-D-lysine, neurite outgrowth on aligned hydrogels is not inhibited in the presence of myelin-associated glycoprotein (p > 0.05). These results highlight for the first time a structural and biochemical role for iso-electrically aligned collagen hydrogels in controlling neuronal growth, and indicate that the short-term signaling associated with these hydrogels can be used in adjunct therapy following injury to the spinal cord.

a b s t r a c tMuscle contractions begin in early embryonic life, generating forces that regulate the correct formation of the skeleton. In this paper we test the hypothesis that the biophysical stimulation generated by muscle forces may be a causative factor for the changes in shape of the knee joint as it grows. We do this by predicting the spatial and temporal patterns of biophysical stimuli, where cell proliferation and rudiment shape changes occur within the emerging tissues of the joint over time. We used optical projection tomography (OPT) to create anatomically accurate finite element models of the embryonic knee at three time points (stages) of development. OPT was also used to locate muscle attachment sites and AFM was used to determine material properties. An association was found between the emergence of joint shape, cell proliferation and the pattern of biophysical stimuli generated by embryonic muscle contractions. Elevated rates of growth and cell proliferation in the medial condyle were found to co-localise with elevated patterns of biophysical stimuli including maximum principal stresses and fluid flow, throughout the time period studied, indicating that cartilage growth and chondrocyte proliferation in the epiphysis is potentially related to local patterns of biophysical stimuli. The development of the patella and articular cartilages, which is known to be affected by in ovo immobilisation, could be contributed to by specific patterns of fluid flow, pore pressure and stress in the joint interzone. This suggests that both cartilage growth and tissue differentiation in the embryonic joint is regulated by specific patterns of biophysical stimuli and that these stimuli are needed for the correct development of the joint.

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