The University of Sydney Slide 1

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1 The University of Sydney Slide 1

2 SIMULATION DRIVEN BIOMEDICAL DESIGN Lecture 4 Presented by Dr Paul Wong AMME4981/9981 Semester 1, 2016 The University of Sydney Slide 2

3 Simulation Types There is more to finite element analysis than static structural If the physics can be discretised, there can be a computational solution The University of Sydney Slide 3

4 Multiphysics Modelling and Types of Analysis Physics DOF(s) - Typical Analysis Type Structural Displacement Static, transient, modal Thermal Temperature Static, transient Incompressible fluid flow Velocity Steady-state, transient Acoustic Acoustic pressure Frequency-domain Electromagnetics Diffusion Electric potential, magnetic vector potential Concentration Static, frequencydomain, transient Steady-state, frequencydomain The University of Sydney Slide 4

5 MODAL ANALYSES For natural frequencies (resonance) The University of Sydney Slide 5

6 Modal Analyses Used to study vibrational excitation Function of geometry and material property only Determining natural frequencies allows for some interesting applications The University of Sydney Slide 6

7 Tacoma Narrows Bridge The University of Sydney Slide 7

8 Modal Analyses in Biomedical Engineering Modal analyses enable measurement of osseointegration Natural frequency analysis of osseointegration for transfemoral implants Shao et al., Annals of Biomedical Engineering 35: , 2007 Varying levels of osseointegration result in different interfacial conditions, resulting in changes to the natural frequency Experimentally determine the resonant characteristics of the implant Small mechanical excitation Measure vibration response FFT (spectral analysis) Determine modal frequency The University of Sydney Slide 8

9 Osseointegration: Experimental Setup Changing the curing time of the silicone to mimic different levels of osseointegration The University of Sydney Slide 9

10 Osseointegration: In Vitro and In Vivo The University of Sydney Slide 10

11 Osseointegration: Results and Conclusions In vitro: Longer curing times higher natural frequency In vivo: Longer healing times higher natural frequency Higher natural frequency because of the increase in stiffness/stability of the interface between bone and implant. The University of Sydney Slide 11

12 Osseointegration: In Silico Relationship between the stiffness (i.e. Young s Modulus) and natural frequency Use this relationship to determine osseointegration in models Implant Interfacial layer with Young s modulus between 0.1 and 0.7 MPa Host Bone The University of Sydney Slide 12

13 The Peri-Implant Region Peri-implant region (up to 1mm from screw thread) is defined in the model Some damage occurs during implant insertion, resulting in changes to mechanical properties Healing and bone remodelling in this area determine osseointegration Peri-Implant Layer E 1 = GPa E 2 =16GPa E 3 =0.6GPa The University of Sydney Slide 13

14 Bone Remodelling Loop for Dental Implants Lateral incisor 2 nd pre-molar dental crown 2 nd molar Canine 1 st molar 1 st pre-molar Dental implant Fully clamped Sectional plane Lingual side y z x Fully clamped Sectional plane Buccal side FE model Based on CT scans Initial: k=0 Dynamic FEA Resonance Frequency k = k + 1 Update modulus Cancellous Cortical E cancellous E cortical Static FEA Calculate density change ( k) Strain Energy ( k) ( k) (1 ) t Density (k) ( k) Update density ( k 1) ( k) ( k) The University of Sydney Slide 14

15 Osseointegration: In Silico Results One month loop intervals Interface stabilises after 4 months measured by increase in resonant frequency Validation with experimental results Resonance Natural frequency (Hz) (Hz) Natural frequency 1st natural frequency 2nd natural frequency 3rd natural frequency Number of month month 1 month 12 Non-invasive measurement tools Resonance frequency (Hz) This study Glauser et al. [40] De Smet et al s [54]* The University of Sydney Slide 15

16 COUPLED ANALYSES For interdependent physics The University of Sydney Slide 16

17 Coupled Analysis Solving for more than one set of DOFs Two methods Load Transfer Direct Coupling Most commonly: Thermal-structural Fluid-structural interactions (FSI) Thermal-electric Off-the-shelf solutions are commonplace In many cases, in-house code is used for highly specific applications The University of Sydney Slide 17

18 Transient Analyses Transient (time-dependent) solvers are required for many biomedical applications Pulsatile flow Slow processes like diffusion, deposition Body kinematics Transient solutions can be obtained by two methods Implicit: enables larger time-steps, but implicit expression needs iterations u t = lim u t + t, x t, p t u(t, x t, p t ) t 0 t Explicit: smaller time-steps to prevent drift, but explicit expression for solution is easier to solve du dt = lim u t + t, x t + t, p t + t u(t, x t, p t ) t 0 t The University of Sydney Slide 18

19 Load Transfer: FSI during Pulsatile Blood Flow Incompressible fluid flow model Transfer shear stress Structural model The University of Sydney Slide 19

20 FSI of Valve in Blood Vessels The University of Sydney Slide 20

21 FSI during Pulsatile Breathing The University of Sydney Slide 21

22 Inhalation and Deposition in the Lung The University of Sydney Slide 22

23 Inhalation and Deposition in the Lung The University of Sydney Slide 23

24 VERIFICATION AND SAFETY The University of Sydney Slide 24

25 Computational Modelling for Regulatory Submissions Any device that interacts with the body must be approved by regulatory authorities prior to sale Currently, most submissions include evidence on efficacy and safety from in vitro and in vivo studies The FDA recently released draft guidance on the reporting of in silico studies as a complementary source of evidence, with the goal of reducing dependency on existing (and costly) sources The University of Sydney Slide 25

26 Computational Modelling for Regulatory Submissions The University of Sydney Slide 26

27 Simulation-based Verification of Electromagnetic Devices FEA can be used to verify electromagnetic compatibility and safety of devices Communications devices emit radiation at radio frequencies (RF) Specific Absorption Rate SAR = න sample σ(r) E(r) 2 FE model SAR ρ(r) Temp. Change The University of Sydney Slide 27

28 Verification of Cochlear Implant Stimulation Electrode array inserted into cochlea Bypass hair cells by electrically stimulating cochlear nerve fibres directly External Microphone Signal Processing Electrode Array Limited understanding in vivo justifies the development of computational models The University of Sydney Slide 28

29 Current Flows from Cochlear Implants The University of Sydney Slide 29

30 Verification of Stimulation Limits for Neuroprostheses Modelling electric field effects on electrochemistry Safe stimulation avoids or limits irreversible reactions Frequency-dependent behaviour Time-dependent analyses Non-linear boundary conditions to model reaction kinetics The University of Sydney Slide 30

31 Orthopaedic Implant Failure Modes Mode I (Tension, opening) Mode II (In-Plane Shear, Sliding) Mode III (Out-Of-Plane Shear, Tearing) The University of Sydney Slide 31

32 Fracture Mechanics Theory Griffith s Crack Theory is based on strain energy release rate (G). Irwin s modification utilises stress intensity factor (K). Geometric correction factor (Y) depending on geometry. a WW The University of Sydney Slide 32

33 Fracture of Dental Bridges Crack insertion and propagation based on stress values FE Modelling of Bridge Fracture (Li et al., 2006) Cracking Simulation in Dental Bridges (Li et al., 2006) 2 unit cantilever bridge 4 unit fixed bridge Discontinuous shape functions (XFEM), or discrete element method enables modelling of cracks The University of Sydney Slide 33

34 Mode I Fracture in a Dental Bridge The University of Sydney Slide 34

35 Implant Failure by Fatigue Cyclic loading Gait cycle (knee and hip replacements). Bending, torsion and compression of spine (spinal fusion, spinal disc prostheses). S-N curve data is required Static and dynamic loading is tested The University of Sydney Slide 35

36 Fatigue Criteria Senalp et al, Materials and Design 28 (2007) Goodman Theory Soderberg Theory Gerber Theory S e is the endurance limit, S u is ultimate tensile strength, S y is yield strength Safety factors with respect to the endurance limit are determined for different designs The University of Sydney Slide 36

37 What Did We Learn? The use of computational methods in biomedical engineering is favourable in: Research Product development These methods add value because they provide answers that are not easily measured in vivo Importance of understanding underlying biological phenomena There is more to FEA than static structural The University of Sydney Slide 37

38 GROUP PROJECTS The University of Sydney Slide 38

39 Group Projects 3D Printing Induction scheduled for Tuesday, 5th April Each group can claim up to $150 for 3D printing Mechanical Testing Safety paperwork required prior to lab induction Need lab coat, safety shoes, and safety glasses (available) The University of Sydney Slide 39