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Glass sponges hold internal secrets to structural strength

Scientists often look to biology for inspiration and innovation, emulating the way that nature builds to advance human engineering. Creatures of the ocean’s depths are some of the most mysterious and fascinating subjects for study, due to the challenges of collecting them from very deep waters and for their unique adaptations for colonizing the sea floor. One such group is the hexactinellids, a collection of predominantly deep-sea sponges that produce elaborate skeletal systems of glass. Known as glass sponges, over the years their skeletal systems and their constituent elements (called spicules) have served as useful model systems for the design and fabrication of robust and damage tolerant structures.

Many hexactinellid sponges have evolved elaborate networks of hair-like skeletal elements (indicated by arrows) that form the basis of an anchoring system that secures the sponges in soft sediments of the sea floor. Figure adapted from Weaver, et al, 2010, Journal of Adhesion.

These skeletal systems can be remarkably complex, and one particular glass sponge, Venus’ Flower Basket (Euplectella apergillum), synthesizes a diagonally reinforced cage-like skeletal tube that forms a delicate latticework consisting of periodic open and closed spaces. Each of the major load-bearing spicules in this species exhibit a distinctive layered architecture, consisting of concentric cylinders of silica separated by thin organic inter-layers that radiate out from an inner solid core of silica. The concentric cylinders of silica decrease in thickness towards the outer periphery of the spicules.

Venus’ Flower Basket (Euplectella aspergillum) is a deep-sea sediment dwelling sponge with a grid-like skeletal architecture resembling a checkerboard pattern of open and closed latticework reinforced by diagonal bracings. Figure adapted from Weaver, et al, 2007, Journal of Structural Biology.

Much of our understanding of the architecture of Venus’ Flower Baskets has been due to the research of Wyss Institute Core Faculty member Joanna Aizenberg, Ph.D., who is also the Amy Smith Berylson Professor of Materials Science at SEAS and Professor of Chemistry and Chemical Biology in the Department of Chemistry and Chemical Biology, and Wyss Senior Research Scientist James Weaver, Ph.D. Working in collaboration with Daniel Morse from the University of California Santa Barbara and Peter Fratzl from the Max Planck Institute of Colloids and Interfaces, Aizenberg and Weaver have investigated this sponge for more than a decade, discovering architectural and optical properties that can be leveraged to improve construction, design, and fiber optic technology.

The main load bearing spicules from many hexactinellid sponges exhibit a distinctive laminated architecture (left) with a gradual reduction in silica layer thickness from the spicule core to its periphery (center), a design strategy which is highly effective in slowing crack propagation through these materials (right). Figure adapted from Weaver, et al, 2007, JSB and Weaver, et al, 2010, Journal of Adhesion.

These spicules are further surrounded by a layered silica cement that rigidifies the entire composite. Previously, the group demonstrated that this layered design strategy significantly increases the toughness of each spicule, and as cracks travel though this composite, they do so in a stair step-like fashion, with the thinnest outer layers significantly limiting the depth to which a newly formed crack can travel.

Now, a new paper co-authored by Aizenberg and Weaver in the Proceedings of the National Academy of Sciences reveals more secrets of the glass sponge’s internal structural properties: not only does the design pattern of decreasing concentric silica layers increase fracture resistance, but it also increases the ultimate load-bearing strength of each spicule.

The entire skeletal system of Euplectella aspergillum is surrounded by continuous layered silica cement, shown in both external (left and middle) and cross-sectional (right) views. The color-coded illustration of the sectioned skeleton (right) illustrates the continuity of the individual silica layers that binds the skeletal lattice together. Figure adapted from Weaver, et al, 2007, Journal of Structural Biology

Working with collaborators at Brown University, including the new paper’s corresponding author Haneesh Kesari, Ph.D, Assistant Professor of Bioengineering, and its first author, graduate student Michael Monn, the team was able to model the internal architecture of the layered spicules and compare it to layer measurements from more than one hundred sponge spicules to predict their carrying capacity for bearing different loads. The group’s results suggest that the layered spicule structure of decreasing concentric thickness is multifunctional and designed to prevent damage accumulation both before and after any initiation of cracking.

Using the long hair-like anchor spicules from Euplectella aspergillum as a model system (left), it was demonstrated that the gradual progression in reduced silica layer thickness from the spicule core to its outer surface matches extremely well with a layer pattern specifically optimized for maximum strength (right). Figure adapted from Monn, et al, 2015, Proceedings of the National Academy of Sciences.

The structural principles obtained from this study could be used for the design and fabrication of high-strength beams in load-bearing applications through the modification of their internal architecture, rather than their external geometry, opening up the possibility for new architectural design strategies of materials that are safer and more structurally sound.

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