Micro Materials

Biomedical
 

Optimising the mechanical properties of biomedical devices is a key step in determining their clinical performance. The mechanical integrity and wear resistance of a biomaterial are vital to its continuing success in vivo, particularly for long term implantable devices, such as total joint replacements, which need to function effectively over periods of 20 years or more.
 

With real world tests of the performance of innovations such as artificial joints taking several months to complete, it is not usually feasible to test each potential development, and as such laboratory tests are necessary to help optimise the biomaterial performance. This is where the NanoTest system has proved so effective in industry and academic research. The NanoTest flexibility enables a wide range of mechanical and wear tests to be carried out on actual medical devices, with their complex geometries. Typical tests include:-

 

·         Nanoindentation (for hardness and modulus)

·        Nano-scratch and nanowear testing (for abrasive/sliding wear resistance and adhesion strength of coatings)

·         Nano-impact testing (for fatigue wear and accelerated wear testing)

 

The above tests can be carried out in a dry or fluid environment thus replicating the in-vivo conditions. Areas in which the biomedical nanoindention is currently in use include;

 

·         Evaluation of mechanical properties of skin and tissue samples

·         Assessment of  in-vivo performance of implants and prosthetics

·         Measurement of the strain rate sensitivity of interconnect technology in pacemakers,

·         Design of porous scaffold stiffness measurements for closer materials matching,

·         Forensic archaeology.

 

Nanoindentation on tissues in a fluid environment

 

Scientists at MIT have used their NanoTest to produce a pathway to promechanical materials characterization, stating that the NanoTest liquid cell ‘provides robust measurements of hydrated specimens and could be exploited for chemomechanical or poromechanical studies.’ And that it gives the ‘ability to measure properties of materials in liquids with nanoscale accuracy in forces and displacements opens new avenues in the nanocharacterization of porous solids.’ They performed experiments on numerous synthetic and biological materials including, Polyacrylamide (PAAm)-based hydrogels aswell as porcine skin and liver tissue all in a fully submerged liquid environment. Their work includes experiments in air and water for inert materials that would not react nor absorb water, their results demonstrate that the introduction of liquids does not effect the precision or accuracy of the results. 'This approach can also be used to determine the Egel as a function of synthesis conditions, i.e., as a function of mol%-bisacrylamide crosslinker which is commonly employed to tune this hydrogel stiffness to approximate tissues such as fat, brain, and muscle.’ - See further reading 1 for the full paper.

 

Fig 1: Measurement of elastic moduli of the various specimens used in the study, all show good agreement with literature values.

 

Nanoindentation on teeth for use in biological archaeology studies

 

  

The Micro Materials NanoTest has recently been employed by F Riede and J M Wheeler of Aarhus University, Denmark, and Cambridge University respectively to investigate the Laacher Lake hypothesis that tephra from the fall out of the Laacher Lake eruption, ~ 13,000 years ago. One of the more innovated techniques used in this paper is the use of a particle of tephra from Bettenroder Berg IX as a scratch indenter. Leveraging the flexibility of the NanoTest the authors attached a small piece of tephra to the scratch probe to perform a constantly increasing load scratches of 1–100 mN on the occlusal tip of a molar of Alces alces. This allowed them to ‘demonstrated the ability of a tephra particle to abrade dental enamel. These results strongly support the notion that LST would indeed have led to a steep increase in tooth wear during mastication’ - See further reading 2 for the full paper.

 

Fig 1 – Scratch performed using tephra fragment (upper) and a 10um radius spherical diamond indenter (lower) on a on the occlusal tip of a molar of Alces alces

 

Further Reading

 

1.	Nanoindentaion in fluid: a pathway to nanoscale poromechanical materials characterization, G. Constantinides1,  I. Kalcioglu, J.F. Smith, K.J. Van Vliet
2.	Testing the ‘Laacher See hypothesis’: tephra as dental abrasive, Felix Riede, Jeffrey M. Wheeler
3.	Investigation of the nanomechanical and tribological properties of dental materials. M S Al-Haik, S Trinkle, Dgarciaand F Yang Int. J. Theor. App. Multiscale Mechanics, 1 (2009) 1
4.	Combinatorial Material Mechanics: High throughput polymer synthesis and nanomechanical screening CA Tweedie, DG Anderson, R Langer and KJ Van Vliet, Adv. Mater. 17 (2005) 2599.
5.	A combinatorial library of photocrosslinkable and degradable materials DG Anderson, CA Tweedie, N Hossain, SM Navarro, DM Brey, KJ Van Vliet, R Langer and JA Burdick, Adv. Mater. DOI 10.1002/adma.200600529.
6.	Physical properties of ultrafast deposited micro- and nanothickness amorphous hydrogenated carbon films for medical devices and protheses, T Zaharia, JL Sullivan et al, Proc. IMechE Vol. 221 Part H: J. Engineering in Medicine, 2007, 161-172
7.	Hemocompatibility of low-friction boron-carbon-nitrogen containing coatings, MF Maitz, R Gago et al, Journal of Biomedical Materials Research. Part B, Applied Biomaterials, 2006 (77) 179-187.
8.	Poster: Mechanical Testing and Characterisation of Micro-Mouldings P. S. Allan, G. Greenway, P. R. Hornsby. [PDF]