Nacre, the material of molusc shells is a wonderfully tough material. That is, it takes a good deal of energy to cause it to fracture. This is because nacre is composed of thin sheets of ceramic interleaved with even thinner sheets of organic ‘glue’ Though one sheet is easy to fracture the crack immediately stops when it hits the soft, gluey interface, and it takes another load of energy to start the crack in the next layer. Manufacturing nacre-like materials has been a goal for 25 years and has been acheived with very limited success. A new method may change that, if it can be scaled up. Antoni Tomsia of Berkeley Lab’s Materials Sciences Division and a team of ressearhers there have used the properties of freezing salt water to make a layered composit. When sea water freezes, particulate matter runs ahead of the freezing front with only a few particles being actually trapped in ice. These particles are trapped in between layers of freezing ice crystals and so the result is a layered composite of water ice and particulates. By carefully controlling the freezing rate of a clay/seawater mix the team was able to control the thickness of the layers of mineral . When the water is freezing slowly the layers of particulate are relatively thick, on the order of tens or hundreds of micros.  When the temperature is dropped, freezing rate increases and there are more layers formed per millimeter – down to a 1 micron thick layer. Once the water is sublimated (Freeze dried) away this leaves a nicely layered ceramic. The final step is to allow a thin gluey material to trickle inot the pores in the material.

Berkeley Press Release

Science Magazine Abstract

 I have highlighted several advances in the field of biomaterials that are aimed at replacing destroyed or degraded bone with synthetic materials.  An article in Nature Materials should make it clear that those methods which temporarily replace bone, providing a scaffold for later re-invasion by osteocytes are the best bet for biomimetics companies.  The material properties of bone, particularly its ability to withstand fracture, are complex and depend on the particular orientation of collagen in the mineralized matrix.

Does bone crack like a ceramic or like a polymer? The distinction isn’t clear-cut and various researchers have reported different toughening mechanisms depending on the experimental conditions. The main reason lies in the anisotropy of bone, and on the fact that the mechanism is dependent on the orientation of the crack propagation relative to the alignment of the collagen fibrils. Having performed controlled crack-propagation experiments, Herwig Peterlik and colleagues were able to reveal great differences in the values of crack energy for propagation in different directions. As the propagation of a crack changes from being aligned to being perpendicular to the collagen fibrils, the behavior changes from brittle to quasiductile.

Nature Materials Abstract

Here is an example of an exploitative phase  biomimetics project.  A porous tantalum material is used as a direct replacement for bone in knees, hips and patellae. The advantages seem at least two fold. The metal is 80% porous which leads to a compliant, energy dissipating structure that mimics trabecular bone well. Second, the porous nature means there are plenty of gaps for live tissue to grow into. The cells will never replace the tantalum, but the trabecular metal is biocompatible and promotes invasion by neighboring tissue.

Trabecular metal website

This research is aimed at putting a coating on the titanium or cobalt steel implants that are used to repair massive damage to bone.  By layering the surface with a bone-like ceramic it is hoped that adhesion and biocompatibility will be improved.

Article

Robert Hyers of UMASS Amherst has patented a method for making a highly porous hydroxyapatite construct.  Why is this a good thing you might ask.  Well, bone is a porous hydroxyapatite construct — currently when someone needs a bigger piece of bone than their body can manufacture to repair a gap a section of cadaveric material or non-biological titanium or stainless steel is inserted.  It would be far better for everyone if a correctly sized chunk of non-reactive, biocompatible, porous material could be stuck in place, later to be repaired and remodeled by osteocytes.

This fabrication method is certainly an advance. I can’t tell yet whether it will be useful in anything like the near term.

Hyer’s lab web page