1. Material Characterization Techniques/Methods
By:
Dr. Murtaza Najabat Ali (CEng MIMechE P.E.)

2. Surface Characterization Techniques
Introduction
Some of the main techniques to conduct surface characterization of biomaterials, are
Contact Angle Analysis
Light Microscopy
Electron Spectroscopy for Chemical Analysis (ESCA) or X-ray Photoelectron Spectroscopy (XPS)
Infrared Spectroscopy (IR)
Electron Microscopy SEM
Atomic Force Microscopy (AFM)

3. Surface Characterization Techniques
In order to understand how Degradative, Surface and Bulk properties affect the biological response to a biomaterial, it is important first to measure them
Quantitative procedures produce a numerical maeasure of a property (in absolute or relative units), whereas
Qualitative experiments give a general overview of the property of interest, without supplying a numerical value
For example, the act of observing a material under microscope is qualitative

4. Surface Characterization Techniques
Example 3.1
For each of the following methods for the determination of the degradation profile of a polymer, indicate whether it is quantitative or qualitative:
Polymer mass measurement at each time point
Visual Inspection at each time point

5. Surface Characterization Techniques
Introduction
Surface characterization of biomaterials is important, both
to determine the quality of surface OR surface treatment, and
to also provide information about the extent of protein adsorption to a biomaterial
There are surface characterization methods that record data from various depth

6. Contact Angle Analysis
Surface Characterization Techniques
Contact Angle Analysis
The contact angle is the angle at which a liquid/vapor interface meets a solid surface.
Contact angle analysis is used to provide overall information about the hydrophobicity of a surface.
Quick Revision of Basic concepts
If the liquid is very strongly attracted to the solid surface (for example water on a strongly hydrophilic solid) the droplet will completely spread out on the solid surface and the contact angle will be close to 0°.
Less strongly hydrophilic solids will have a contact angle up to 90°. On many highly hydrophilic surfaces, water droplets will exhibit contact angles of 0° to 30°.
If the solid surface is hydrophobic, the contact angle will be larger than 90°. On highly hydrophobic surfaces the surfaces have water contact angles as high as 150° or even nearly 180°

7. Contact Angle Analysis
Surface Characterization Techniques
Contact Angle Analysis
Young's equation can be used to determine the contact angles and is based on the surface energies of the three interfaces. At equilibrium three interfacial tensions corresponding to solid/vapor γSV , solid/liquid γSL , and liquid/vapor γLV interfaces are counterbalanced. Hence the contact angle (θ) can be evaluated from Young's equation which is given as
The above equation contains two unknowns i.e. γSV and γSL. γSV is often approximated using the critical surface tension γC. γC is determined by testing the material using various liquids with a range of γLV values. A plot of contact angle Vs. γLV is generated and extrapolated to 𝞱 = 0 (i.e. at complete spreading on the surface). The value of γLV at this point is called γC

8. Contact Angle Analysis
Surface Characterization Techniques
Contact Angle Analysis
Contact angle is one of the most sensitive and inexpensive surface analysis techniques and is capable of measuring ~3-20 Å deep
There are five common techniques that can be employed to measure the contact angle. The technique chosen depends principally on the geometry and location of the surface or coating to be studied. In all the methods, the contact angle (θ) is the angle of the liquid at the interface relative to the plane of the surface.  The output of contact angle analysis can only be a single number (i.e. either θ or γC)
Five ways that the contact angle (𝞱) can be measured. (A) Sessile or Static drop. (B) Wilhelmy plate method. (C) Captive air bubble method. (D) Capillary rise method. (E) Tilting substrate method

9. Contact Angle Analysis
Surface Characterization Techniques
Contact Angle Analysis
Direct inspection is the easiest goniometer technique to perform. A digital image of a drop profile is taken and a drawing software is used to draw the surface and tangent lines and measure the contact angle

10. Contact Angle Analysis
Surface Characterization Techniques
Contact Angle Analysis
Example 3.2:
Guided by the following image, derive Young’s equation for surface tension ?

11. Surface Characterization Techniques
Light Microscopy
Light microscopy is a relatively simple technique that is used as a first approach to gain primarily qualitative information about surface topography. The resolution of such instruments in most cases is around 0.2𝞵m
(a) Gross image of a compound microscope. On a compound microscope the light source is placed below the specimen and the specimen is viewed from above, (b) Light path of a compound microscope. The objective forms a magnified image of the object that is larger than the original object. This image is magnified many times by the eyepiece to form the large (inverted) virtual image

12. Surface Characterization Techniques
Light Microscopy
There are four basic components to a light microscope:
Source ---- produces white light
Lenses ---- glass lenses focus light beam and/or magnify image of sample
Sample stage holds sample securely
Detector ---- (camera or human eye) views and captures resulting image
Sample of a light microscopy image, in this case, a fibroblast attached to a substrate
Light microscopy is used exclusively for imaging, and provides in itself only qualitative assessment. In addition to imaging surface topography, this instrument is extremely important in histological analysis for biocompatibility assessment. Main advantage to light microscopy is that vacuum is not required to view samples, but it remain difficult to see thick or hydrated samples