Michael M. Porter

Structural Design Elements in Biological Materials: Application to Bioinspiration

Eight structural elements in biological materials are identified as the most common amongst a variety of animal taxa. These are proposed as a new paradigm in the field of biological materials science as they can serve as a toolbox for rationalizing the complex mechanical behavior of structural biological materials and for systematizing the development of bioinspired designs for structural applications. They are employed to improve the mechanical properties, namely strength, wear resistance, stiffness, flexibility, fracture toughness, and energy absorption of different biological materials for a variety of functions (e.g., body support, joint movement, impact protection, weight reduction). The structural elements identified are: fibrous, helical, gradient, layered, tubular, cellular, suture, and overlapping. For each of the structural design elements, critical design parameters are presented along with constitutive equations with a focus on mechanical properties. Additionally, example organisms from varying biological classes are presented for each case to display the wide variety of environments where each of these elements is present. Examples of current bioinspired materials are also introduced for each element.

Why the seahorse tail is square

The curious tale of the square tail Appendages in animals are typically round, but the seahorse tail has a square cross section. Porter et al. hypothesize that this shape provides better functionality and strength than a round cross section (see the Perspective by Ashley-Ross). Three-dimensional printed models show that square cross section shapes behave more advantageously when subjected to compressive forces. By allowing greater deformation without damage and accommodating twisting deformations, square appendages passively return to their original configurations. The added flexibility of the square cross section enhances the tails ability to grasp objects. Science, this issue 10.1126/science.aaa6683; see also p. 30 3D-printed models show that square profile seahorse tails have better crush resistance and grasping ability than do circular ones. [Also see Perspective by Ashley-Ross] INTRODUCTION Although the predominant shapes of most animal tails are cylindrical, seahorse tails are square prisms. The skeleton of their tails consists of a bony armor arranged into several ringlike segments composed of four L-shaped plates that surround a central vertebra. These plates articulate with specialized joints that facilitate bending and twisting, as well as resist vertebral fracture from crushing. Muscles attached to the vertebral column transmit forces to the bony plates to provide motion for grasping and holding on to objects such as sea grasses, mangrove roots, and coral reefs, which allows them to hide and rely on camouflage when evading predators and capturing prey. RATIONALE We hypothesize that the square cross-sectional architecture of a seahorse tail improves mechanical performance in prehension (grasping ability) and armored functions (crushing resistance), relative to a cylindrical one. To test this hypothesis, we evaluated the mechanics of two three-dimensional (3D)–printed prototypes composed of articulating plates and vertebrae that mimic the natural (square prism) and a hypothetical (cylindrical) tail structure. We compared the bending, twisting, and compressive behavior of the biomimetic prototypes to show that the square profile is better than the circular one for two integrated functions: grasping ability and crushing resistance. RESULTS Seahorse tails (and the prototypes) have three primary joints that enable motion: ball-and-socket, peg-and-socket, and gliding. The ball-and-socket joints connect adjacent vertebrae and constrain bending in both the square and cylindrical prototypes to the same degree, exhibiting a behavior similar to that of a natural seahorse tail. The peg-and-socket joints connect the plates of adjacent segments and substantially restrict twisting in the prototype with a square profile, as compared with the circular one. The square geometry limits excessive torsion and preserves articulatory organization, which could provide seahorses a natural safety factor against torsion-induced damage and assist in tail relaxation. Further, the square architecture is flat (increasing surface contact) and undergoes an exterior shape change when twisted, which could allow seahorses to grasp objects with more control. Gliding joints are present at the plate overlaps along all four sides of both prototypes. Under transverse compression and impact (with a rubber mallet), the plates of the square prototype slide past one another with one degree of translation freedom (analogous to the crushing behavior of a natural seahorse tail), exhibiting a response that is stiffer, stronger, and more resilient than its cylindrical counterpart, whose plates translate and rotate on impact. CONCLUSION Exploration of these biologically inspired designs provides insight into the mechanical benefits for seahorses to have evolved prehensile tails composed of armored plates organized into square prisms. Beyond their intended practical applications, engineering designs are convenient means to answer elusive biological questions when live animal data are unavailable (for example, seahorses do not have cylindrical tails). Understanding the role of mechanics in these prototypes may help engineers to develop future seahorse-inspired technologies that mimic the prehensile and armored functions of the natural appendage for a variety of applications in robotics, defense systems, or biomedicine. Engineering designs answer biological questions. 3D-printed models that mimic a seahorse tail were designed not only for potential engineering applications but also to answer the biological question, why might tails organized into square prisms be better than cylinders? A mechanical comparison of the prototypes shows that articulated square prisms perform better than do cylinders for grasping and resistance to crushing. Whereas the predominant shapes of most animal tails are cylindrical, seahorse tails are square prisms. Seahorses use their tails as flexible grasping appendages, in spite of a rigid bony armor that fully encases their bodies. We explore the mechanics of two three-dimensional–printed models that mimic either the natural (square prism) or hypothetical (cylindrical) architecture of a seahorse tail to uncover whether or not the square geometry provides any functional advantages. Our results show that the square prism is more resilient when crushed and provides a mechanism for preserving articulatory organization upon extensive bending and twisting, as compared with its cylindrical counterpart. Thus, the square architecture is better than the circular one in the context of two integrated functions: grasping ability and crushing resistance.

Highly deformable bones: unusual deformation mechanisms of seahorse armor.

Multifunctional materials and devices found in nature serve as inspiration for advanced synthetic materials, structures and robotics. Here, we elucidate the architecture and unusual deformation mechanisms of seahorse tails that provide prehension as well as protection against predators. The seahorse tail is composed of subdermal bony plates arranged in articulating ring-like segments that overlap for controlled ventral bending and twisting. The bony plates are highly deformable materials designed to slide past one another and buckle when compressed. This complex plate and segment motion, along with the unique hardness distribution and structural hierarchy of each plate, provide seahorses with joint flexibility while shielding them against impact and crushing. Mimicking seahorse armor may lead to novel bio-inspired technologies, such as flexible armor, fracture-resistant structures or prehensile robotics.

Structure and mechanical properties of selected protective systems in marine organisms

Marine organisms have developed a wide variety of protective strategies to thrive in their native environments. These biological materials, although formed from simple biopolymer and biomineral constituents, take on many intricate and effective designs. The specific environmental conditions that shape all marine organisms have helped modify these materials into their current forms: complete hydration, and variation in hydrostatic pressure, temperature, salinity, as well as motion from currents and swells. These conditions vary throughout the ocean, being more consistent in the pelagic and deep benthic zones while experiencing more variability in the nearshore and shallows (e.g. intertidal zones, shallow bays and lagoons, salt marshes and mangrove forests). Of note, many marine organisms are capable of migrating between these zones. In this review, the basic building blocks of these structural biological materials and a variety of protective strategies in marine organisms are discussed with a focus on their structure and mechanical properties. Finally, the bioinspired potential of these biological materials is discussed.

The armored carapace of the boxfish.

The boxfish (Lactoria cornuta) has a carapace consisting of dermal scutes with a highly mineralized surface plate and a compliant collagen base. This carapace must provide effective protection against predators as it comes at the high cost of reduced mobility and speed. The mineralized hydroxyapatite plates, predominantly hexagonal in shape, are reinforced with raised struts that extend from the center toward the edges of each scute. Below the mineralized plates are non-mineralized collagen fibers arranged in through-the-thickness layers of ladder-like formations. At the interfaces between scutes, the mineralized plates form suture-like teeth structures below which the collagen fibers bridge the gap between neighboring scutes. These sutures are unlike most others as they have no bridging Sharpeys fibers and appear to add little mechanical strength to the overall carapace. It is proposed that the sutured interface either allows for accommodation of the changing pressures of the boxfishs ocean habitat or growth, which occurs without molting or shedding. In both tension and punch testing the mineralized sutures remain relatively intact while most failures occur within the collagen fibers, allowing for the individual scutes to maintain their integrity. This complex structure allows for elevated strength of the carapace through an increase in the stressed area when attacked by predators in both penetrating and crushing modes.

Stiff, porous scaffolds from magnetized alumina particles aligned by magnetic freeze casting

Bone consists of a hard mineral phase and a compliant biopolymer phase resulting in a composite material that is both lightweight and strong. Osteoporosis that degrades spongy bone preferentially over time leads to bone brittleness in the elderly. A porous ceramic material that can mimic spongy bone for a one-time implant provides a potential solution for the future needs of an aging population. Scaffolds made by magnetic freeze casting resemble the aligned porosity of spongy bone. A magnetic field applied throughout freezing induces particle chaining and alignment of lamellae structures between growing ice crystals. After freeze drying to extract the ice and sintering to strengthen the scaffold, cubes from the scaffold center are mechanically compressed along longitudinal (z-axis, ice growth direction) and transverse (y-axis, magnetic field direction) axes. The best alignment of lamellar walls in the scaffold center occurs when applying magnetic freeze casting with the largest particles (350nm) at an intermediate magnetic field strength (75mT), which also agrees with stiffness enhancement results in both z and y-axes. Magnetic moments of different sized magnetized alumina particles help determine the ideal magnetic field strength needed to induce alignment in the scaffold center rather than just at the poles.

3D-printing and mechanics of bio-inspired articulated and multi-material structures

3D-printing technologies allow researchers to build simplified physical models of complex biological systems to more easily investigate their mechanics. In recent years, a number of 3D-printed structures inspired by the dermal armors of various fishes have been developed to study their multiple mechanical functionalities, including flexible protection, improved hydrodynamics, body support, or tail prehensility. Natural fish armors are generally classified according to their shape, material and structural properties as elasmoid scales, ganoid scales, placoid scales, carapace scutes, or bony plates. Each type of dermal armor forms distinct articulation patterns that facilitate different functional advantages. In this paper, we highlight recent studies that developed 3D-printed structures not only to inform the design and application of some articulated and multi-material structures, but also to explain the mechanics of the natural biological systems they mimic.

Generation and Utilization of Microbial Biomass Hydrolysates in Recovery and Production of Poly(3-hydroxybutyrate)

© 2013 Yu et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Generation and Utilization of Microbial Biomass Hydrolysates in Recovery and Production of Poly(3-hydroxybutyrate)

Synergistic structures from magnetic freeze casting with surface magnetized alumina particles and platelets

Magnetic freeze casting utilizes the freezing of water, a low magnetic field and surface magnetized materials to make multi-axis strengthened porous scaffolds. A much greater magnetic moment was measured for larger magnetized alumina platelets compared with smaller particles, which indicated that more platelet aggregation occurred within slurries. This led to more lamellar wall alignment along the magnetic field direction during magnetic freeze casting at 75 mT. Slurries with varying ratios of magnetized particles to platelets (0:1, 1:3, 1:1, 3:1, 7:1, 1:0) produced porous scaffolds with different structural features and degrees of lamellar wall alignment. The greatest mechanical enhancement in the magnetic field direction was identified in the synergistic condition with the highest particle to platelet ratio (7:1). Magnetic freeze casting with varying ratios of magnetized anisotropic and isotropic alumina provided insights about how heterogeneous morphologies aggregate within lamellar walls that impact mechanical properties. Fabrication of strengthened scaffolds with multi-axis aligned porosity was achieved without introducing different solid materials, freezing agents or additives. Resemblance of 7:1 particle to platelet scaffold microstructure to wood light-frame house construction is framed in the context of assembly inspiration being derived from both natural and synthetic sources.

Precision control of tensile properties in fabric for computational fabrication

This paper presents an exploration of computer-controlled embroidery design and stitching with the goal of making purposeful, precision changes to material properties of a base elastic textile. Two techniques are proposed for adding stitches through designed microstructural cells and stitch-level planning. For the latter, a novel path planning algorithm is proposed to serialize stitches that is similar to a greedy solution for the travelling salesman problem with a set of domain-specific constraints that dictate edge cost. We show the efficacy of the concept through a set of simple design examples that undergo mechanical load testing and discuss the value of the technique in future applications using computational design.