Hi Class! This week, we talked about crystal structures versus glassy structures. For crystals, we know there have to be nucleus forming followed by diffusion and subsequent growth to form very ordered atomic structures. And Prof. Gonzalez mentioned if we have a highly viscous liquid, complex crystal structure, and cooled rapidly, formation of glasses will be favorable. After class, I learned that normally crystalline metals can also form glassy structures if they are cooled down very rapidly. I am gonna introduce in this week’s blog this new kind of materials, metallic glasses.

In the past, small batches of amorphous metals with high surface area configurations (ribbons, wires, films, etc.) have been produced through the implementation of extremely rapid rates of cooling. This was initially termed “splat cooling” by doctoral student W. Klement at Caltech, who showed that cooling rates on the order of millions of degrees per second is sufficient to impede the formation of crystals, and the metallic atoms become “locked into” a glassy state. Amorphous metal wires have been produced by sputtering molten metal onto a spinning metal disk. More recently a number of alloys have been produced in layers with thickness exceeding 1 millimeter. These are known as bulk metallic glasses (BMG).  Figure 1 shows the TTT diagram of a metallic glass. Crystallization rate is determined by the competing effects of undercooling and reaction kinetics. Note supercooled liquid region exists between the melting and glass transition temperatures over time periods not exceeding that required for crystal nucleation.

Figure 1.Schematic time temperature transformation (TTT) diagram for metallic glasses.

Lacking the dislocations and grain boundaries inherent in crystalline materials, metallic glasses exhibit physical properties representative of a completely new paradigm in materials science. Without the premature deformation of slip, elastic strain may regularly approach 2% thereby facilitating strength and hardness values which are far beyond those of crystalline metals. (Figure 2) According to literatures, the Young’s Modulus of metallic glasses can exceed that of metals. Further to such desirable mechanical properties, metallic glasses exhibit a full range of toughness values, low mechanical damping, good corrosion resistance and high magnetic permeability coupled with low coercivity to give superior soft magnetic properties.

Figure 2. Schematic representation of room temperature yield (metals, composites and polymers) or flexural strength (ceramics) as a function of modulus

1. Klement, Jr., W.; Willens, R. H.; Duwez, Pol (1960). “Non-crystalline Structure in Solidified Gold-Silicon Alloys”. Nature. 187(4740): 869. 
2. Liebermann, H.; Graham, C. (1976). “Production of Amorphous Alloy Ribbons and Effects of Apparatus Parameters on Ribbon Dimensions”. IEEE Transactions on Magnetics. 12(6): 921. 
3. Burgess, Tim, and Michael Ferry. “Nanoindentation of metallic glasses.” Materials Today12.1-2 (2009): 24-32.

Hi Class! This week, we learned the interaction of nano-or micro-scale biomaterials with cells and the interaction of cells/tissues/organism with large scale biomaterials at both the cellular level and the tissue/host level. In this blog, I am going to talk about cell interactions with nano-patterned implants with a focus on implants made of metallic materials.

Metallic materials are often used for fabricating orthopedic and dental implants because their surfaces are biocompatible with tissues at the target area. Some commonly used surface modification methods for metals include grid-blasting, acid etching, chemical grafting, ionic implantation and calcium phosphate coatings. These strategies for modifying the nature of this interface usually involve changes to the surface at the nanometer level, thereby affecting protein adsorption, cell–substrate interactions, and tissue development.

Figure 1: Tissue – dental implant interactions at both gingival and bone sites.

For example, anodization of titanium implants can yield features of nanometer size dimensions . This is achieved by developing a titanium oxide layer using a platinum counter electrode in acidic solutions at a potential of 5-25 V. Through changing experimental conditions (i.e. potential, temperature, electrolyte), scientists have produced an oxide layer a few microns in thickness and composed of a regular array of nanometer sized pores with diameters in the 30-100 nm range, growing perpendicular to the titanium surface. The pore sizes are compatible with those of proteins such as fibronectin (FN) and vitronectin which play an important role in cell adhesion, as discussed in class.

In a nutshell, careful preparation of standardized nanostructured surfaces with repetitive topography may elicit protein adsorption, cell response and cell differentiation. Further research aiming at correlating cell behavior and tissue integration in the future will help us understand the role of surface nanostructures in these biological responses. It is possible to imagine controlling peri-implant tissue healing by changing the surface properties at the nanometer scale.

References:

[1] Lavenus, Sandrine, et al. “Cell interaction with nanopatterned surface of implants.” Nanomedicine 5.6 (2010): 937-947.

[2] Ratner, Buddy D., et al. Biomaterials science: an introduction to materials in medicine. Elsevier, 2004.