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Engineering Mechanics Research Group at TU Vienna

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TU Vienna
Hierarchical biomaterials mechanics
Christian Hellmich

November 2015

Problems

Biomimetics deals with  the application  of nature‐made 
"design  solutions"  to  the  realm  of  engineering.  In  this 
context, mimicking biological materials with  fine‐tuned 
mechanical  properties  has  been  on  the  agenda  of 
engineering research and development  for many years. 
The  premise  of  biomimetics  is  that  it  is  possible  to 
reduce diversity and complexity  of  biological materials 
to  a  number  of  'universal'  functioning  principles.  This 
requires  foremost  a  deep  understanding  of  the 
hierarchical  structure  of  biological  materials.  It  now 
appears that multi‐scale mechanics may hold the key to 
such  an  understanding  of  "building  plans"  inherent  to 
entire classes of material. 

Approach

Based  on  various  physical‐chemical  and  mechanical 
experiments,  our  focus  is  the  development  of  multi‐
scale mechanical models. These models mathematically 
and computationally quantify how basic building blocks 
of  biological  materials  (such  as  hydroxyapatite 
minerals,  collagen,  and  water  in  all  bones  found 
throughout  the  vertebrate  kingdom)  govern  the 
materials'  mechanical  properties  at  different  length 
scales,  from  a  few  nanometers  to  macroscopic  scales.  
Thereby,  multi‐scale  homogenization  theory  allows  us, 
at  each  scale,  to  identify  material  representations  that 
are as  simple as  possible;  but as  complex as  necessary 
for  reliable  computational  predictions  of  key  material 
properties,  such as  poro‐elasticity,  creep, and  strength. 
This can be seen as "reverse" biomimetics engineering: 
(civil)  engineering  methods  are  used  to  understand 
biological systems. 

Findings

One  of  our  key  findings  is  that  bone's  mechanical  properties are  governed  by  porous  polycrystals which the minerals build up as structural complement to the collagen fibrils found in all connective tissues (also  in  tendon,  cartilage,  skin).  These  polycrystals  are  not  only  central  to  the  magnitude  of  elastic  anisotropy  of  bone  materials;  but  also  to  their  tensile-to-compressive  strength ratio that  results  from  universal  failure  characteristics  of  differently  oriented  submicron‐sized  mineral  platelets.  The  perspective  thus  offered  by  micro‐ mechanics has opened, for the first time, a theoretical  understanding of bone mechanics, which is consistent with all major experimental observations.   The  developed  tools  have  also  driven  forward  our  understanding  of  hierarchical  materials  with  deep  roots  in  Civil  Engineering:    wood  and  concrete.  Indeed, there are interesting similarities between the  failure  of  extrafibrillar  minerals  and  that  of  cement  hydration products. 

Impact

Experimentally  validated  multi‐scale  models  for  hierarchical materials emerge as central design tools  for  tailoring  material  composition  and  morphology  that fulfill functional requirements (e.g. minimization  of  failure  risk).  This  is  true  for  classical  civil  engineering  problems  (e.g.  shotcrete  tunneling);  but  even  more  so  for  the  rapidly  growing  field  of  regenerative  medicine,  where  biomimetic  tissue  engineering  scaffolds  are  implanted  for  tissue  regeneration.  At  the  same  time,  such  models  open  new avenues for the interpretation of state‐of‐the‐art  imaging  techniques  such  as  (Micro)  Computer  Tomography.

Selected Publications

1.   Fritsch A., Hellmich Ch., Dormieux L., and Sanahuja 
J., "Mechanical behavior of hydroxyapatite 
biomaterials: an experimentally validated 
micromechanical model for elasticity and strength", 
J. Biomed. Mater. Res. 88A, 149‐161, 2009. 
2.  Hellmich Ch., Kober C., Erdmann B., 
"Micromechanics‐based conversion of CT data into 
anisotropic elasticity tensors, applied to FE 
simulations of a human mandible", Ann. Biomed.
Eng. 36(1), 108‐122, 2008.
3. Scheiner  S.,  Hellmich  Ch.,  "Continuum 
microviscoelasticity  model  for  aging  basic 
creep  of  early‐age  concrete",  J. Eng. Mech.
135(4), 307‐323, 2009. 
4. Scheiner S., Sinibaldi, R., Pichler, B., Komlev 
V.,  Renghini,  C.,  Vitale‐Brovarone,  C., 
Rustichelli  F., Hellmich  C.,  "Micromechanics 
of  bone  tissue  engineering  scaffolds,  based 
on  resolution  error‐cleared  computer 
tomography",  Biomaterials  30,  2411‐2419, 
2009. 


Core Competencies

  • Multiscale continuum micromechanics: elasticity, strength, and creep of bone, wood, concrete, and bioreplacement materials.
  • Ultrasonic and poro-mechanical testing
  • Applications in image-based biomedical diagnosis, implant design, and in civil engineering structures (e.g. tunnels)

TU Vienna Christian Hellmich
Christian Hellmich

Current Research Team Members: 
• Dr. Bernhard Pichler (Ph.D. 2003)
• Dr. Andreas Fritsch (Ph.D. 2009) 
• Christoph Kohlhauser (Ph.D.  Candidate) 
• Shafi Ullah (Ph.D.  Candidate) 
• Fabien Perus (S.M. Candidate, on leave from 
Ecole Polytechnique) 

Recent Graduates: 
• Dr. Stefan Scheiner (Ph.D. 2009), leaving for 
University of Western Australia, Perth 

Current Research Collaborations: 
• Multiscale  strength  project:  Prof.  Luc 
Dormieux,  Paris  Tech  (Ecole  des  Ponts), 
France. 
• Micromechanics‐driven  CT  data 
exploitation: Prof. Cornelia Kober, Hamburg 
University  of  Applied  Sciences,  Germany ; 
Prof.  Franco  Rustichelli,  Universita 
Politecnica delle Marche, Ancona, Italy ; Dr. 
Vladimir  Komlev,  Russian  Academy  of 
Sciences, Moscow 

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