Feature Article

Getting It Right on the Surface

Published: September 14, 2010
By: John Newman
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When it comes to characterizing orthopaedic materials, choosing the appropriate surface analysis technique is everything.

The orthopaedics industry has made great strides over the last few decades in the quality, functionality, and performance of orthopaedic devices. The same can be said for the analytical tools that are available to characterize orthopaedic devices and materials. Advances in instrumentation allow designers to observe smaller features, achieve better detection limits, and probe thinner layers. This article provides an overview of some analytical techniques that are important for research and development, production support, and failure analysis of orthopaedic materials.
 
Choosing the Right Technique
For cost-efficient problem solving and characterization of orthopaedic materials, the correct analytical tool must be used—i.e., one that has the correct depth of analysis, detection range, and analytical spot size, and is also applicable to the type of sample being studied.
 
Figure 1. The analytical resolution versus the detection limit. Click figure for larger image. Click here for a glossary of terms. 
Figure 1 lists detection range and analytical spot size for a variety of techniques. Techniques that have blue bubbles provide elemental information while those in red can also provide chemical or molecular information. Techniques above the chart are imaging-only (no compositional information) that can provide spatial resolution down to the angstrom scale. Techniques to the right of the chart are bulk-only techniques (i.e., no spatial resolution) some of which provide detection limits in the parts-per-billion to parts-per-trillion range.
 
Figure 2 shows the typical depths of analysis for different techniques. The depths of analysis range from only the top few atomic layers to many microns in depth. Matching the problem or characterization need with a technique that has an appropriate depth of analysis is critical to providing meaningful results. If a relatively deep analysis tool is used for characterizing a very thin contaminant, the chances are slim that the contaminant will actually be identified or even observed. Likewise, when characterizing the bulk of a material, results from a surface sensitive technique may be misleading because surfaces are often substantially different from the bulk.
 
There are various types of analyses that are important to the orthopaedics industry. Some of the most common include the following: 
 
  • Microscopy. 
  • Surface-sensitive analyses (depths of analysis of 10 nm or less).
  • Thin-film analyses (depths of analysis 10 nm–10 µm).
  • Bulk elemental measurements.
  • Crystallographic measurements.
Figure 2. Typical depths of analysis for different surface-analysis techniques. Click figure for larger image.
Multiple techniques are available within most of these classes, each of which have their own strengths and limitations. Except for the bulk elemental techniques, most of these analyses are relatively nondestructive. However, the sample preparation procedures used in cross-sectioning, the application of conductive coatings, and the cutting of samples to meet sample-handling requirements for particular instruments can all be destructive. Compositional depth profiling with surface-sensitive techniques is also destructive. 
 
Microscopy. Microscopy techniques provide high-magnification images of the sample surface to observe topography and features of interest. Microscopy can be performed on cross-sectioned samples to image buried features, interfaces, and crystalline structures. Techniques included in this class include scanning electron microscopy (SEM) and transmission electron spectroscopy (TEM). These techniques are essential for applications when resolutions greater than those provided by optical microscopy are required.
 
Figure 3. SEM Image and EDS spectrum from a contaminated region on a TiAlV orthopaedic implant. Click figure for larger image. 
SEM is one of the most popular analytical tools because of its ability to provide high-resolution images with an excellent depth of field. In SEM, a highly focused electron beam is scanned over the sample surface and secondary or backscattered electrons are collected. Secondary electrons provide primarily topographic information, whereas backscattered electrons provide information on atomic number variation. A common attachment on SEM instruments is an energy dispersive x-ray spectrometer (EDS), which can provide elemental information from the top micron(s) with detection limits in the low percent range. 
 
TEM and scanning TEM (STEM) are electron beam techniques that provide even higher spatial resolution than SEM. Besides the imaging mode of STEM, various other modes are available to provide information on average atomic number and differences in crystalline phase and orientation. Sample prep is complex for STEM, because the sample must be made thin enough that electrons can pass through it (<150 nm) while keeping sample damage to a minimum. Analytical techniques such as EDS and electron energy loss spectroscopy (EELS) can also be added to STEM systems to provide elemental information. 
 
Surface-Sensitive Techniques. For many orthopaedic materials issues, the region of interest is only the top few atomic layers of the sample. These applications include:
 
  • Surface contamination (stains, hazes, particles, etc.).
  • Cleaning efficacy.
  • Surface modification and passivation.
  • Antibacterial surface distribution.
  • Packaging adhesion failures.
  • Surface segregation of polymer additives.
  • Coating characterization.
For these types of applications, very specialized instruments are needed, including Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF–SIMS). AES and XPS analyze the top ~100 Å or less of a solid material while TOF–SIMS is even more surface sensitive and analyzes the top ~15 Å.
 
 
Figure 4. An XPS survey spectrum and Ti high-resolution scan from a contaminated region on TiAlV orthopaedic material. Click figure for larger image.
AES is an electron-beam technique used to determine the elemental composition of the top 50–100 Å and of features as small as ~25 nm. Combined with ion beam etching, AES provides compositional depth profile information that is useful for studying thin film and specific feature thicknesses. AES is primarily used on conducting and semiconducting materials. Surface metal oxide layer thickness measurements and precipitate or contamination analyses in metals are two common applications for AES.
XPS, also known as electron spectroscopy for chemical analysis (ESCA), provides both elemental and chemical composition of the outer 50–100 Å, with a minimum analytical spot size of approximately 10 µm. By bombarding the sample with x-rays and monitoring shifts in the binding energy of the emitted photoelectrons, XPS can provide chemical bonding information for both organic and inorganic materials. XPS is capable of analyzing insulating materials as well as conductors and is therefore often used to analyze polymers and other organic materials. Similar to AES, XPS can also be combined with ion beam etching to provide compositional depth profiles. When using newly developed buckyball C60 ion beams for depth profiling, subsurface layers of organic materials can be exposed without destroying their chemical structure.
 
TOF–SIMS focuses a submicron-pulsed beam of high-energy primary ions (i.e., Ga, Au, Bi, or C60) onto a sample surface, producing secondary ions in a sputtering process. Analyzing the mass-to-charge ratio of the secondary ions provides information about the elemental and molecular species present on the surface. With submicron spatial resolution, TOF–SIMS also provides 2-D molecular images of species of interest. This technique is especially useful for detecting thin layers of lubricants, surfactants, polymer additives, and other organic contaminants.
 
Thin-Film Analysis. Two of the most common techniques for chemical analysis of organic materials are Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. Both of these techniques involve the excitation and identification of vibrational bonds between atoms, thereby providing a spectral fingerprint that can be used to identify functional groups and thus compounds. 
 
FTIR provides information about chemical bonding and molecular structure by bombarding a sample with infrared light and monitoring the absorption or transmission of radiation at specific frequencies. Measuring the absorption or transmission spectrum produces a spectral fingerprint characteristic of the chemical structure of the molecule. The depth of analysis of FTIR is typically several microns; however, special attachments can reduce the depth to about 0.1 µm. Infrared spectra can be obtained from features as small as about 10 µm in diameter using an FTIR microscope.
 
Figure 5. TOF-SIMS spectrum from a contaminated region on TiAlV orthopaedic material. Click figure for larger image.
Similar to FTIR, Raman spectroscopy can determine the chemical structure of a sample and identify the compounds present by measuring molecular vibrations. However, Raman uses a laser as the excitation source and relies on the inelastic scattering of light to produce a spectrum. By focusing the laser below the surface of a sample, Raman spectra can be collected from different depths within a sample. This confocal mode of operation produces nondestructive depth profiles with roughly 1-µm depth resolution. Raman has better spatial resolution (1–2 µm) than FTIR and therefore enables the analysis of smaller features. Raman has the added advantage of being able to positively identify different forms of elemental carbon (diamond, graphite, hydrocarbons, etc.)
 
Bulk Elemental Measurements. At times, accurate quantification of the elements present in the bulk of a sample is needed, possibly to identify an alloy. Techniques available for bulk elemental analysis include inductively coupled plasma optical emission spectroscopy (ICP–OES), inductively coupled plasma mass spectrometry (ICP–MS), and glow discharge mass spectrometry (GDMS).
 
In ICP–OES and ICP–MS, the samples are usually analyzed in liquid form. Thus, solid samples are first dissolved before they can be introduced into the plasma ionization chamber. In ICP–OES, atoms excited by the plasma emit characteristic radiation. The intensity of the emitted light is proportional to the concentration of the element in solution. With the use of calibration standards, this technique provides very accurate measurement of matrix and low-level components.
 
In ICP–MS, an argon plasma is used as the excitation source for the elements of interest. However, instead of monitoring emitted radiation, ions from the plasma are extracted and analyzed in a mass spectrometer. The constituents of an unknown sample can be identified and quantified. ICP–MS offers extremely high sensitivity (often parts per billion) to a wide range of elements.
 
GDMS measures trace elemental content in inorganic solid materials such as metals and ceramics. It is a direct solid sampling technique that eliminates the need for dissolution of the sample prior to analysis. In GDMS, the sample is exposed to an argon glow discharge that sputter erodes the sample surface forming ions, which are then separated in a mass spectrometer. Detection limits for GDMS are parts per billion to parts per trillion for most elements.
 
 
Figure 6. XPS survey spectrum of nanohydroxyapatite particles. Click figure for larger image.
Crystallographic Measurements. X-ray diffraction (XRD) is a powerful nondestructive technique for characterizing crystalline materials. It provides information on crystalline structure and phase, preferred crystal orientation (texture), and other structural parameters, such as average grain size, degree of crystallinity, strain, and crystal defects. X-ray diffraction peaks are produced by constructive interference of a monochromatic beam of x-rays scattered at specific angles from each set of lattice planes in a sample.
 
Problem-Solving Examples
Figures 3–5 provide examples of the importance of choosing the correct analytical technique with both the proper depth of analysis for a particular problem and also one that provides the required type of information. 
 
A titanium aluminum vanadium (TiAlV) orthopaedic material was found to have a faint visible stain on the outer surface. Because the thickness of the contamination layer was not known, the first technique that was tried was SEM/EDS. EDS has a depth of analysis of a micron or more. Analysis on and off the stain showed only titanium, aluminum, and oxygen in both areas (see Figure 3). XPS, a much more surface-sensitive tool, was then tried. Under the conditions used, XPS analyzed the top ~75Å of the sample surface and detected primarily Si, O, and C with only 0.5% Ti from the underlying substrate observed (see Figure 4). This indicated that the stain was somewhat thinner than the 75 Å depth of analysis. The binding energy of the Si peak was consistent with silicone; however, other Si species also exist at that same binding energy. Therefore, XPS could not make a 100% positive identification that the stain was silicone. An even more surface-sensitive technique and one that provides better molecular characterization of organic materials—TOF–SIMS—was then used. TOF–SIMS obtained a secondary ion mass spectrum from the stain that positively matched a reference spectrum of polydimethylsiloxane (PDMS), aka silicone (see Figure 5). No TiAlV substrate elements were observed, as the contamination was thicker than the 15-Å depth of analysis. This clearly demonstrates the importance of choosing a technique with the appropriate analytical depth in order to get the most useful information.
 
 
Figure 7. An XPS high-resolution carbon spectrum shows the presence of (calcium) carbonate functionality on the surface of the nanohydroxyapatite particles. Click figure for larger image.
A second problem-solving example involves a multitechnique characterization of a hydroxyapatite material used for orthopaedic coatings. Coatings such as hydroxyapatite and other calcium phosphate materials are known to have beneficial biocompatible bonding properties to bone and are widely used in the orthopaedics industry. ASTM test procedures have been developed to measure the compositional and crystallographic properties of these materials. One of the standard tests in this specification is a determination of calcium-to-phosphorous (Ca:P) ratio using ICP–MS. Pure hydroxyapatite has a Ca:P ratio of 1.67. On a particular sample of nanohydroxyapatite (particle size <200 nm), ICP–MS gave a Ca:P ratio of 1.85, much larger than the expected value. Also unexpected was that the instrumental gas analysis for bulk carbon showed the presence of 1.5% carbon in the sample. To further investigate the high Ca:P ratio and the presence of C, XPS was then carried out. 
 
Figure 6 is the XPS survey spectrum that shows the presence of the expected O, Ca, and P, along with almost 12% carbon at the surface. While some hydrocarbon contamination is normal due to atmospheric exposure, the XPS high-resolution carbon spectrum shows that a relatively large percentage of the carbon is present as carbonate (see Figure 7). This explains the higher than expected Ca:P ratio. A thin CaCO3 surface layer was covering the hydroxyapatite. Small amounts of carbonate are sometimes substituted in hydroxyapatite to make them more bonelike; however, this particular sample was designed to be pure hydroxyapatite with no calcium carbonate. Only surface-sensitive techniques such as XPS can detect such a thin (~1 nm) layer. From a biocompatibility point of view, the bone cells exposed to this sample would see CaCO3 and not hydroxyapatite. 
 
Conclusion
It is important to understand the physical and mechanical properties of materials used for orthopaedic devices. However, it is the full understanding of the surface and bulk compositions a material that allows one to understand why it behaves in a particular manner in its environment, along with how the device materials can be improved on to achieve even better performance. It is essential to take advantage of the dozens of highly specialized analytical tools that are available to characterize orthopaedic materials and truly understand orthopaedic products.
 
John Newman is director of analytical services at Evans Analytical Group (Chanhassen, MN). 

 

Published in Orthotec, Fall 2010, Volume 1, No. 2


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