This paper introduces a concept that allows the creation of low-resistance composites using a network of compliant conductive aggregate units, connected through contact, embedded within the composite. Due to the straight-forward fabrication method of the aggregate, conductive composites can be created in nearly arbitrary shapes and sizes, with a lower bound near the length scale of the conductive cell used in the aggregate. The described instantiation involves aggregate cells that are approximately spherical copper coils-of-coils within a polymeric matrix, but the concept can be implemented with a wide range of conductor elements, cell geometries, and matrix materials due to its lack of reliance on specific material chemistries. The aggregate cell network provides a conductive pathway that can have orders of magnitude lower resistance than that of the matrix material - from 1012 ohm-cm (approx.) for pure silicone rubber to as low as 1 ohm-cm for the silicone/copper composite at room temperature for the presented example. After describing the basic concept and key factors involved in its success, three methods of implementing the aggregate into a matrix are then addressed – unjammed packing, jammed packing, and pre-stressed jammed packing – with an analysis of the tradeoffs between increased stiffness and improved resistivity.
The proposed research effort explores the development of active cells -simple contractile electromechanical units that can be used as the material basis for larger articulable structures. Each cell, which might be considered a "muscle unit", consists of a contractile Nitinol SMA core with conductive terminals. Large numbers of these cells might be combined and externally powered to change phase, contracting to either articulate with a large strain or increase the stiffness of the ensemble, depending on the cell design. Unlike traditional work in modular robotics, the approach presented here focuses on cells that have a simplistic design and function, are inexpensive to fabricate, and are eventually scalable to sub-millimeter sizes, working towards our vision of robot structures that can be custom-fabricated from large numbers of general cell units, similar to biological structures.
This paper introduces a technique of inducing bulk conductivity in a polymer. The technique uses coiled copper ‘cells’ embedded into a polymer during fabrication which can subsequently create highly redundant series-parallel networks. The preceding body of work aimed to improve the conductivity of non-conducting polymers by embedding particulates (of metal, carbon, etc.) into the polymer, or by altering the polymerization chemistry to incorporate conductive elements. The technique described here keeps the process independent of the specific polymer chosen by not relying on the polymerization chemistry to aid in the incorporation of the cells. The embedding drastically lowers the resistivity of the polymer, from 1012 Ω -cm (approx.) for pure silicone rubber to less than 50 Ω -cm for the composite at room temperature: a drop of 12 orders of magnitude. A secondary consideration of this paper is the mechanical stiffness changes brought about by the embedding of metal inside a flexible polymer. Although the connected network of copper cells allows the rubber to be highly conductive in bulk, the cells are themselves compliant and thus have minimal effect on the stiffness of the cured silicone rubber.
While the process of hand preshaping during grasping has been studied for over a decade, there is relatively little information regarding the organization of digit contact timing (DCT). This dearth of information may be due to the assumption that DCT while grasping exhibits few regularities or to the difficulty in obtaining information through traditional movement recording techniques. In this study, we employed a novel technique to determine the time of digit contacts with the target object at a high precision rate in normal healthy participants. Our results indicate that, under our task conditions, subjects tend to employ a radial to ulnar pattern of DCT which may be modulated by the shape of the target object. Moreover, a number of parameters, such as the total contact time, the frequency of first contacts by the thumb and index fingers and the number of simultaneous contacts, are affected by the relative complexity of the target object. Our data support the notion that a great deal of information about the object's physical features is obtained during the early moments of the grasp.
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