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A Comparison of micro CT with other techniques used in the characterization of scaffolds
1. Review
A Comparison of micro CT with other techniques
used in the characterization of scaffolds
Saey Tuan Ho, Dietmar W. Hutmacher
Biomaterials 27 (2006) 1362 - 1376
2. Contents
Introduction
Architectural and structural parameters
Theoretical method and SEM analysis
Mercury porosimetry
Gas pycnometry
Gas adsorption
Flow porosimetry
Micro CT
A micro CT study
Conclusion
3. Introduction
Crucial factors in scaffold design:
Structure
Architecture
Scaffold porosity and pore size:
Large surface area favors cell attachment and growth
Large pore volume is to accommodate and deliver
sufficient number of cells
High porosity is for easy diffusion of nutrients,
transport and for vascularization
Evaluation methodology:
Fast
Accurate
Non-destructive
4. Creation & Design
Creation: Two methodology
Design and fabrication
Fabrication followed by design optimization
Design: Two broad categories
Precise geometrical layout
Honeycombed scaffolds, woven textile meshes
Deposition via non-precise ways
Foams, Nano-fiber meshes
6. Molecular Transport
Vasculature growth & diffusion:
Pore network optimization
Main mode of transport
Exchange of oxygen
Nutrient
Metabolic wastes
Molecular signaling
Key property: Porosity
Cell seeding efficiency
Diffusion
Mechanical strength
7. Architectural and Structural Parameters
Porosity
Pore size
Surface area to volume ratio
Interconnectivity of pores
Anisotropy
Strut thickness
Cross sectional area
Permeability
8. Definition
Pore size : Average diameter of pores
Strut/Wall thickness : Average
diameter/thickness of scaffold struts
Anisotropy : A measure of the non -uniformity
in the alignment of scaffold struts
Cross-section area : A measure of the area in a
specified sectional plane of the scaffold
Permeability : A measure of the ease with
which fluid passes through the scaffold pores
9. Theoretical Method
Most of the methods are capable of
estimating porosity
There are two main approaches
Unit cube analysis
Mass technique
Other approaches
Archimedes Method
Liquid displacement method
10. Unit Cube Analysis
Porosity = (1 – Vf / VA) x 100%
Vf is scaffold material volume
VA is apparent scaffold cube volume
Vf=ПLd2
n1n2
VA = Lwh
d = Strut diameter
L = Strut length
w = Strut width
h = Scaffold height
n1 = Number of struts per layer
n2 = Number of layers per scaffold
11. Merits & Demerits
Commonly adopted for honeycombed scaffolds
Calculation assume uniform struts and layers
Cannot apply to scaffolds fabricated using extrusion
techniques
Fused deposition modeling
3D printing
Stereolithography
12. Mass Technique
Porosity = (1 – Vg / VA) x 100%
Vgis scaffold material volume
VA is apparent scaffold cube volume
Vg=mass / density of scaffold material
VA = Lwh
L = Strut length
w = Strut width
13. Merits & Demerits
Commonly adopted for scaffolds with controlled &
un-controlled geometries
Dependent on accuracy of linear measurements (L, w
& h) of the cube
Rough edges and inaccurate linear dimensions would
be a concern
14. Archimedes Method
Porosity = (M wet – M dry) / (M wet – M submerged)
• M dry = Dry mass of scaffold
• M wet = Mass of prewet scaffold
• M submerged = Mass of scaffold soaked in water
Inappropriate for hydrophobic scaffolds
15. Liquid Displacement Method
Porosity = (V1 – V3) / (V2 – V3)
V1 = Initial known volume of scaffold
V2 = Volume sum of ethanol & submerged scaffold
V3 = Volume of ethanol in bath after scaffold
removal
16. Scanning Electron Microscopy
Complements theoretical calculation of
porosity
Allows direct measurement of pore size and
wall thickness
Qualitative - Provides visual estimation of
interconnectivity, cross-section area and
anisotropy
Restricted to surface analysis
Layer fusion & edge effects
19. Mercury Porosimetry - Principle
Washburn Equation : DP = - 4γ cos θ
D = Pore diameter
γ = Surface tension of mercury
P = Applied pressure
θ = Contact angle between pore wall and mercury
Provides bulk volume, total open pore volume and
porosity
Measurable pore size range: 0.0018 to 400 µm
Does not account closed pores
Excess pressure may compress the sample
Calculation assume cylindrical pores
Destructive analysis
20. Gas Pycnometry
Vx = (PE Vc + PEVr – PCVC - PrVr) / (PE- PC)
Vx is scaffold volume
Vc is chamber volume
Pc is initial chamber pressure
Vr is reference chamber volume
Pr is reference chamber pressure
PE is pressure at equilibrium
21. Merits & Demerits
Measures scaffold material volume
Accuracy depends on absence of moisture
and volatile substances
Sample pre-treatment in vacuum oven
Does not account closed pores
Porosity to be calculated using unit cube
approach
Error in linear measurement may concern
23. Gas Adsorption - Principle
• Based on adsorption of gas molecules due to Van der
Waals & electrical forces
• Surface area is calculated using BET theory
• Pore size is derived using BJH method
Measurable pore size range: 0.35 to 400 nm
Relevant to nano - featured & nano - modified
scaffolds
From isotherms and hysteresis loop several key
parameters are elucidated
Does not account closed pores and scaffold volume
Not suitable for scaffolds with low specific surface
area
24. Flow Porosimetry
Non - destructive method
Compression in pore size is measurable
Measurable pore size range: 0.013 to 500 µm
Should be coupled with other techniques to
determine porosity
Does not account closed pores
25. Micro CT
History & Developments:
Feldkamp et al pioneered the system in
early 70’s
Used extensively to study trabecular
architecture
Explored for assessment of scaffolds,
regenerated tissue and vasculature
networks
27. Micro CT Scanning
Specimen is divided into series of 2D slices
Emergent x-rays are captured by detector
2D pixel map is created
Attenuation coefficient correlated to material
density
3D modeling program visualize the object
Accuracy depends on software & hardware
31. Merits
Non - destructive method
No sample pre-preparation
Precise quantitative & qualitative
information on 3D morphology
Finite element modeling as an alternative
to mechanical testing
32. Beam hardening
Artifacts created by metals
Thresholding is not always accurate
More time consumption for higher
resolution
Storage and processing of large data sets
Demerits
36. Conclusion
Evaluation of scaffold architecture is necessary
Potential and concerns of technique is crucial
Micro CT is a rather new technique but possess it’s
own merits & demerits
New technique and future advancements are
anticipated to address the demerits