Comparison Of Micro Ct
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Transcript of Comparison Of Micro Ct
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
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
Introduction
Crucial factors in scaffold design: StructureArchitecture
Scaffold porosity and pore size:Large surface area favors cell attachment and growthLarge pore volume is to accommodate and deliver sufficient number of cellsHigh porosity is for easy diffusion of nutrients, transport and for vascularization
Evaluation methodology:FastAccurateNon-destructive
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
Various Scaffold Designs
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
Architectural and Structural Parameters
� Porosity
� Pore size
� Surface area to volume ratio
� Interconnectivity of pores
� Anisotropy
� Strut thickness
� Cross sectional area
� Permeability
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
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
Unit Cube Analysis
Porosity = (1 – Vf / VA) x 100%
� Vf is scaffold material volume� VA is apparent scaffold cube volume
� Vf =ПLd2n1n2
� 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
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
Mass Technique
Porosity = (1 – Vg / VA) x 100%
� Vg is scaffold material volume
� VA is apparent scaffold cube volume
� Vg = mass / density of scaffold material
� VA = Lwh
� L = Strut length
� w = Strut width
� h = Scaffold height
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
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
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
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
Layer Fusion & Edge Effects in SEM
Mercury Porosimetry
Mercury Porosimetry - PrincipleWashburn 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
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
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
Gas Adsorption
Gas Adsorption Process
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
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
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
Micro CT - Principle
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
2D Slicing & Reconstruction
3D Visualization
Various Specimens
Merits
� Non - destructive method
� No sample pre-preparation
� Precise quantitative & qualitative information on 3D morphology
� Finite element modeling as an alternative to mechanical testing
� 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
Summary of Individual Techniques
S---+--Permeability
+----Q-Cross-section area
+----Q-Anisotropy
+----+-Wall thickness
+++-++-Pore size
+----Q-Interconnect-ivity
+---+--Surface area
+-+++Q+Porosity
Micro CT
Flow Porosimetry
Gas Adsoprtion
Gas Pycnometry
Mercury Porosimetry
SEMTheorectic-al Method
Parameters
Q = Qualitative S = Dependent on software
A Micro CT Study
00/600/1200
00/900/1800
Angle
1009.0775.4570.60 + 0.67
Copolymer of PEG, PCL & PLA
1008.6574.9972.05 + 0.41
Copolymer of PEG, PCL & PLA
Interconnectivity (%)
Surface area/volume –
Mimics (mm2/mm3)
Porosity –Mimics
(%)
Porosity –Pycnometer
(%)
Material
3D Analysis of Copolymers
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