While it is known that musculotendon units adapt to their load environments, there is only a limited understanding of tendon adaptation in vivo. Here we develop a computational model of tendon remodeling based on the premise that mechanical damage and tenocyte-mediated tendon damage and repair processes modify the distribution of its collagen fiber lengths. We explain how these processes enable the tendon to geometrically adapt to its load conditions. Based on known biological processes, mechanical and strain-dependent proteolytic fiber damage are incorporated into our tendon model. Using a stochastic model of fiber repair, it is assumed that mechanically damaged fibers are repaired longer, whereas proteolytically damaged fibers are repaired shorter, relative to their pre-damage length. To study adaptation of tendon properties to applied load, our model musculotendon unit is a simplified three-component Hill-type model of the human Achilles-soleus unit. Our model results demonstrate that the geometric equilibrium state of the Achilles tendon can coincide with minimization of the total metabolic cost of muscle activation. The proposed tendon model independently predicts rates of collagen fiber turnover that are in general agreement with in vivo experimental measurements. While the computational model here only represents a first step in a new approach to understanding the complex process of tendon remodeling in vivo, given these findings, it appears likely that the proposed framework may itself provide a useful theoretical foundation for developing valuable qualitative and quantitative insights into tendon physiology and pathology.

This paper presents a framework for modelling biological tissues based on discrete particles. Cell components (e.g. cell membranes, cell cytoskeleton, cell nucleus) and extracellular matrix (e.g. collagen) are represented using collections of particles. Simple particle to particle interaction laws are used to simulate and control complex physical interaction types (e.g. cell-cell adhesion via cadherins, integrin basement membrane attachment, cytoskeletal mechanical properties). Particles may be given the capacity to change their properties and behaviours in response to changes in the cellular microenvironment (e.g., in response to cell-cell signalling or mechanical loadings). Each particle is in effect an 'agent', meaning that the agent can sense local environmental information and respond according to pre-determined or stochastic events. The behaviour of the proposed framework is exemplified through several biological problems of ongoing interest. These examples illustrate how the modelling framework allows enormous flexibility for representing the mechanical behaviour of different tissues, and we argue this is a more intuitive approach than perhaps offered by traditional continuum methods. Because of this flexibility, we believe the discrete modelling framework provides an avenue for biologists and bioengineers to explore the behaviour of tissue systems in a computational laboratory.

The dynamics of today's electronics industry introduces enormous pressure on chip designers to come up with chip designs in a very limited time. This is due partly to the short life cycle of application specific products in the marketplace. The availability of powerful graphics processors and microprocessors with processing powers comparable to minicomputers has introduced several stand alone workstations into the design arena. Designer productivity is improved to a great extent by the provision of computer-aided design tools that take a product from concept all the way through test at a single design station. This improvement in productivity is very much felt while designing less complex chips. However, corporations that turn out highly complex chips like 32-bit microprocessors and their peripheral chips still rely heavily on the processing powers of powerful mainframes or supercomputers. It is design cases such as these that will question the adequacy and practicability of introducing workstations into the culture of the compcenter-based design methodologies. To them, workstations represent yet another level of computing power which has to be integrated into the design environment.

The need for high performance sources for optical fiber telecommunications systems has resulted in the development of laser transmitters that incorporate high efficiency coupling optics, sophisticated control electronics, and high-speed modulation circuits. Present thinking on system development indicates the need for transmitters to have, in addition, much improved spectral characteristics. This paper reviews the present status of laser transmitters and describes likely future trends.