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196 Biomimetics: Biologically Inspired Technologies

Figure 6.14 Stages in shape deposition manufacture of a small robot at Stanford’s Rapid Prototyping and
Biomimetic Robotics Labs.

Stanford’s SDM Rapid Prototyping Lab was shown in Figure 6.2. This SDM robot is extremely
robust, capable of receiving quite a pounding (Bailey et al., 2000). This approach promises benefits
over traditional, rigid mechanical engineering — wherein systems fail from shock, friction among
parts, and fatigue — factors reduced or absent in such elastomeric devices.
In order to improve robotic facial materials, the author and White (Hanson and White, 2004)
developed a series of fabrication techniques akin to SDM, but utilizing a sacrificial matrix perfused
with elastomer, to create controlled cellular pores in the elastomer. The collection of techniques is
dubbed structured porosity elastomer manufacturing (SPEM). In preliminary experiments using
these materials in robots, the materials have shown dynamic aesthetics vastly more akin to those of
the human face, while requiring only 1/20th the force to actuate relative to solid elastomers.
Emulation of human skin in appearance and properties is one of the most challenging aspects of
creating lifelike robotic faces. HER’s patent-pending ‘‘structured porosity elastomer manufactur-
ing’’ (SPEM) process results in materials that very closely approximate the properties of human
skin, in a way amenable to mass production. SPEM produces a composite elastomer material with a
controlled 3D chamber geometry. The process can produce chambers that are analogous to those in
open-cell foam. Alternately, the pores need not be spheroids and can instead be rectangular, star-
shaped, or any topology that is useful to achieve novel material properties. By implementing
hierarchical chamber sizes, SPEM can relieve localized nodes of stress accumulation in a foamlike
material, thereby increasing the overall elastic strain of the material. With this technique, the author
increased strain in a silicone SPEM foam from 280 to greater than 800% — a value that is 85% of
the solid constituent elastomer, one that enables facial expressions (Hanson and White, 2004)
(Figure 6.15). Realistic facial expressions require up to 400% strain.
Because the enhanced compression of SPEM materials more closely matches that of facial soft
tissues than do solid elastomers (Hanson and White, 2004), SPEM-based faces wrinkle and bunch
more naturalistically, as can be seen at www.human-robot.org. SPEM techniques extend the power
of SDM (Amon et al., 1996; Bailey et al., 2000; Hanson and White, 2004) and rapid prototyping
(Figure 6.16).

Figure 6.15 A cross-section of silicone SPEM, with 0.5 and 3 mm pores. This sample also demonstrates the
composite possibilities of the material, as the material transitions into nonporous solid silicone toward the left.