Dr. Ergin Tönük Middle East Technical University Department of Mechanical Engineering

1. Searching for Collaboration Opportunities between Biomechanics Laboratory at METU Department of Mechanical Engineering and Ankara University Department of Computer Engineering Dr. Ergin Tönük Middle East Technical University Department of Mechanical Engineering

2. Outline • Mechanics and Biomechanics • Biomechanics Research at the Mechanical Engineering Department, METU • KISS Motion and Gait Analysis System • Soft Tissue Testing System

3. Mechanics (1/2) • It is the physical science that deals with the behavior of materials under the action of forces. • Materials may either move or deform (or in real life mostly do both) if subjected to forces.

4. Mechanics (2/2) • For rigid body motion, laws of dynamics are well established and there are techniques available for analyzing multi-body dynamics. • For deformation, ranging from strength of materials or elementary fluid mechanics to continuum mechanics and various advanced numerical solution techniques (like finite element analysis) are available.

5. Biomechanics • Application of principles of mechanics to biological systems in order to • Understand what is going on in detail • Predict what might happen under predefined conditions • Use computer models to perform tests which are hard do realize physically

6. Motion and Gait Analysis System

7. Biomechanics Research at Mechanical Engineering Department, METU KISS Motion and Gait Analysis System (1/7) • KISS (Kinematic Support System/Kas İskelet Sistemi) is the first gait analysis system in Turkey • It is the only system developed by local people and still is the only laboratory in Turkey that works on gait and motion analysis methods • Besides performing referred patient experiments we work on • developing new gait analysis protocols, • developing new mechanical models for gait and other motion, • analyze gait patterns of various pathologies with clinicians, • work on different joint models etc.

8. Biomechanics Research at Mechanical Engineering Department, METU KISS Motion and Gait Analysis System (2/7) • Motion capture (kinematic data collection) • PAL camera with infrared-pass filter mounted on the lens • Infrared light source around the lens

9. Biomechanics Research at Mechanical Engineering Department, METU KISS Motion and Gait Analysis System (3/7) • Motion capture (kinematic data collection) • Retro-reflective markers attached on anatomical locations of the subject

10. Biomechanics Research at Mechanical Engineering Department, METU KISS Motion and Gait Analysis System (4/7) • Motion capture (kinematic data collection) • Linearization of camera lens distortions

11. Biomechanics Research at Mechanical Engineering Department, METU KISS Motion and Gait Analysis System (5/7) • Motion capture (kinematic data collection) • Calibration of cameras

12. Biomechanics Research at Mechanical Engineering Department, METU KISS Motion and Gait Analysis System (6/7) • Motion capture (kinematic data collection) • Data collection

13. Biomechanics Research at Mechanical Engineering Department, METU KISS Motion and Gait Analysis System (7/7) • Motion capture (kinematic data collection) • 3-D reconstruction of marker locations

14. Biomechanics Research at Mechanical Engineering Department, METU KISS Motion and Gait Analysis System What can we do? (1/4) • Camera linearizaton and calibration are performed separately • Most new systems perform both tasks together by dynamic calibration • Marker matching among camera images and in time are done using heuristic methods • Intelligent methods may be developed to track markers

15. Biomechanics Research at Mechanical Engineering Department, METU KISS Motion and Gait Analysis System What can we do? (2/4) • Polynomial fitting with Butterworth filter is used on the trajectories of markers in x, y and z coordinates independently and successive time derivatives are taken to determine accelerations • Better methods can be developed to filter out noise (which is amplified by time derivatives) • Marker identification is done manually • A “skeleton based” marker identification and tracking scheme can be developed

16. Biomechanics Research at Mechanical Engineering Department, METU KISS Motion and Gait Analysis System What can we do? (3/4) • Subject data is stored by the name • A database may be developed to retieve data by other information (like age, gender, pathology, etc.) • In colloboration with orthopaedists and physical therapists automated diagnosis guide may be developed

17. Biomechanics Research at Mechanical Engineering Department, METU KISS Motion and Gait Analysis System What can we do? (4/4) • The kinematic hardware of the system may be renewed using the experience gained • ...

18. Soft Biological Tissue Testing SystemIndenter

19. Deformable Solid Mechanics • In mechanical engineering we have very powerful tools (like finite element or boundary element modeling techniques) that help engineers to predict the internal force intensities (i. e. stresses) and measures of deformations (i. e. strains).

20. Deformable Solid Biomechanics • This powerful tool of engineering is not that powerful in biomechanics because engineering materials are mostly linear elastic. • Further, engineering materials are mostly subjected to small strains which can be well approximated with infinitesimal strain theory.

21. Deformable Solid Biomechanics • For conventional engineering materials, to identify the material properties one may perform extensive material tests. • For many common engineering materials these mechanical properties are already tabulated.

22. Deformable Solid Biomechanics • For biological materials, performing material tests is more complicated due to: • Large physiological strains commonly encountered • Nonlinear and non-elastic material behavior • Maintaining physiological conditions and homeostasis during experiments

23. Deformable Solid Biomechanics • Result: • Improperly identified or over-simplified material models used in the powerful tool of engineering • Non-realistic and non-predictive computer models • Finite element or boundary element techniques found limited use in biomechanics.

24. Bottleneck: Material Identification

25. In vivo Indentation Tests • In vivo • Easy to perform • Non-invasive • Diverse • Cyclic loading-unloading at different rates • Relaxation (with different initial rate) • Creep (with different initial rate)

26. In vivo Indentation Tests • Experiment results need further processing

27. In-Vivo Soft Tissue Testing System • For accurate computer modeling of soft tissue mechanical behavior we need to perform “material testing” on living soft tissues. • We have developed a soft tissue indenter to perform tests on soft tissues in vivo.

28. In vivo Indentation Tests

29. Indenter Tip Step Motor Load Cell Indenter Test Unit

30. 220 V~ Switching Power Supply 15 V DC 12 V DC  Data Acquisition Card USB Force 0-5 V DC 0~5 V DC Portable Computer Enable& Direction V/F Converter 1~1000 Hz Step Motor Driver Card Control Box Centronix Connector Loadcell Step Motor Soft Tissue Interface Test Unit Non-Rotational Bearing Indenter Test System

31. 220 V~ Switching Power Supply 15 V DC 12 V DC  Data Acquisition Card USB Force 0-5 V DC 0~5 V DC Portable Computer Enable& Direction V/F Converter 1~1000 Hz Step Motor Driver Card Control Box Centronix Connector Loadcell Step Motor Soft Tissue Interface Test Unit Non-Rotational Bearing Indenter Test System

32. Preconditioning Indentation Test Results2 mm/s Cyclic LoadingRaw Data

33. F d Indentation Test Results2 mm/s Cyclic LoadingProcessed Data

34. Indentation Test ResultsMaterial Behavior ? Inverse Finite Element Method

35. Inverse Finite Element Method • Geometry is known • Boundary conditions are known • Material constants (and material constitutive law) are unknown • System response is known

36. Inverse Finite Element Method • Construct a finite element model • Apply appropriate boundary conditions • Select a material law (suitable for soft tissues) and make a guess about material coefficients • Obtain the response of ‘virtual’ soft tissue and compare it with the experimental one • Update the material coefficients

37. Inverse Finite Element Method

38. Inverse Finite Element Method

39. Inverse Finite Element Method

40. Inverse Finite Element Method

41. Inverse Finite Element Method

42. Elastic Material Model • James-Green-Simpson hyperelastic material model (modified for axisymmetric loading*): • W: Strain energy density per unit undeformed volume • I: Invariant of Green-Lagrange finite strain tensor *TÖNÜK, E., SILVER-THORN, M. B., “Nonlinear Elastic Material Property Estimation of Lower Extremity Residual Limb Tissues”. IEEE, Transactions on Rehabilitation Engineering Vol 11, No 1, pp. 43-53, March 2003

43. Inelastic Material Model • Viscoelastic extension of James-Green-Simpson material model*: • W0: Initial strain energy density per unit undeformed volume • t1 and t2 short and long term relaxation constants • d1 and d2 short and long term relaxation magnitudes * TÖNÜK, E., SILVER-THORN, M. B., Nonlinear Viscoelastic Material Property Estimation of Lower Extremity Residual Limb Tissues, ASME Journal of Biomechanical Engineering v. 126, pp. 289-300, April 2004.

44. Inverse Finite Element Method (Relaxation)

45. Inverse Finite Element Method (Creep)

46. Ongoing Research • Experimental Procedure • Verification of indenter test protocols • Effect of indenter tip geometry • Ways to obtain cleaner data • Material Model • Different strain energy functions • Different inelastic material models

47. Goal • Accurate finite element models of mechanical interaction of soft tissue with its environment

48. Possible Collaboration Areas • Experimental Procedure • Verification of indenter test protocols • Problems related with hardware (USB, A/D and D/A card, step motor driver card, step motor etc.) • Problems related with software (open loop control in cyclic and relaxation tests, closed loop control in creep tests, security and load limiting issues etc.) • Ways to obtain cleaner data • Intelligent data processing and filtering • Data storage in a database • ...

49. Photo: Ergin Tönük, Sabuncupınar, 18 November 2006 http://E40003.me.metu.edu.tr Thank You!