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The Growing Importance of Materials Characterization in Biocompatibility Testing

Originally Published MDDI March 2002 TESTING   Before any medical materials testing program can begin in earnest, there must be a well-conceived plan for understanding the composition of a medical device material and its potential for an adverse biological effect.

Originally Published MDDI March 2002

TESTING

Before any medical materials testing program can begin in earnest, there must be a well-conceived plan for understanding the composition of a medical device material and its potential for an adverse biological effect.

David E. Albert

The number of polymers and other materials used in biomedical applications has rapidly increased in the past two decades. Although the principles behind the testing of these materials have changed little in this time, the standards, the laboratory methods, the regulatory attitudes, and the interpretation of results have all changed greatly.

At the heart of these changes is a strong emphasis on the need for ensuring biocompatibility. This heightened awareness of the safety of medical devices has taken on an international character, in large part because of the convergence of FDA regulations and International Organization for Standardization (ISO) standards.1,2As international requirements for the biological evaluation of materials continue to be developed and issued, the use of the chemical characterization of materials to establish biocompatibility will become essential.

In what follows, I will examine the principles presented in ISO 10993-1, "Biological Evaluation of Medical Devices, Part I" and in sections of the ISO 10993-18 draft document that address material characterization.3,4I will discuss chemical characterization of medical device materials as it relates to biocompatibility and the development of testing programs, with a focus on the approach set forth in the ISO 10993 standards. Finally, I will provide an overview of the significance of materials characterization and the range of applicable test methods.

RATIONALE FOR MATERIALS CHARACTERIZATION

The goal of chemical characterization is to identify and quantify the chemical constituents of a material and to help establish its biocompatibility. This concept is laid out in sections 4.1 and 4.2 of ISO 10993-1: "In the selection of materials to be used in device manufacture, the first consideration should be fitness for purpose having regard to the characteristics and properties of the material, which includes chemical, toxicological, physical, electrical, morphological and mechanical properties."

The standard specifies that, as part of the overall biological evaluation of the device, the following should be taken into consideration:

  • The material(s) of manufacture.
  • Intended additives, process contaminants, and residues.
  • Leachable substances.
  • Degradation products.
  • Other components and their interactions in the final product.
  • The properties and characteristics of the final product.

Here, the standard clearly addresses two fundamentally important issues. First, is the material safe? And, second, does it have the necessary physical and mechanical properties for its proposed function?

To initially evaluate these concerns, it is necessary to perform chemical, morphological, and physical/mechanical testing as a part of a materials characterization program. Adverse effects caused by materials are generally chemical and produced by material components, contaminants, or breakdown products that cause a biological effect in the patient. Chemical characterization will evaluate potential leachable chemicals and their bioavailability, while morphological characterization will examine the surfaces of materials. Physical/mechanical characterization will address functionality and safety.

Before making any chemical or mechanical analysis, it is important to have precise information on the synthesis of the polymer itself, including a description of the monomers used in the polymerization, the solvents used in the synthesis, and any special additives added during production of the polymer. Once this information has been retrieved, if it is available, chemical and physical testing should be considered.

The specific methods used to characterize materials will depend in part on the criticality of the medical device for which they are intended. Characterization may mean determining the physical or mechanical properties of the material, or it may mean confirming the major components with infrared analysis or other fingerprinting techniques. It may also include physicochemical tests to determine the weight of nonvolatile residues in an aqueous or alcohol extract prepared under controlled conditions, or tests to identify specific extractables and their relative quantities.

Because of the importance of materials characterization to biological evaluation, a new addition to the ISO 10993 standards, to be known as Part 18, is being drafted. It will cover requirements for providing information about the chemical composition of materials and devices, the potential for release of leachable substances, and the physical, mechanical, morphological, and predictable biological characterization of devices.

CHEMICAL CHARACTERIZATION

Over the years, a number of tests have evolved for the chemical characterization of medical device materials. All are relevant, sensitive, and rapid. Though inexpensive, they provide invaluable information for establishing material safety and biocompatibility. These tests comprise infrared analysis, aqueous and nonaqueous physicochemical tests, high-performance liquid and gas chromatography, atomic absorption spectroscopy and inductively coupled plasma spectroscopy, and a variety of mechanical/physical tests.

Infrared Analysis. Since the infrared spectrum of a chemical compound or polymer is perhaps its most characteristic physical property, infrared analysis finds extensive application in fingerprinting, or identifying, materials. Small differences in the structure and constitution of a molecule result in significant changes in the distribution of absorbance peaks in the fingerprint region of the spectrum.5By matching the infrared spectrum of an unknown with that of a known material, proof of identity is established. Reference spectra for common materials are commercially available, or may be accumulated through testing.

Physicochemical Tests. Plastic extractables include such chemicals as base polymers, fillers, lubricants, plasticizers, antioxidants, pigments, and slip agents. They may also include reaction products or degradants formed during the device manufacturing process. They can reduce the purity or potency of a drug solution; create turbidity, precipitates, and particles; and even increase toxicity. Many of these chemicals are water soluble, but others are soluble in more nonpolar environments. Thus it is prudent to design tests that characterize both aqueous and nonaqueous extractables.

Aqueous Tests. The United States Pharmacopeia (USP) describes a group of tests used to characterize the plastic components of pharmaceutical containers and medical devices.6These aqueous physicochemical tests are designed to determine the presence of water-soluble substances without regard to their identity. The aim of these tests is to ensure that the materials will not release water-soluble chemicals into the drug products or tissue fluids they contact. The tests also help detect surface contaminants that may find their way into raw materials during manufacturing.

Procedures using aqueous extracts from the sample material determine values for nonvolatile residue (chemicals extracted from the test sample or formed by degradation of the test sample during extraction), residue on ignition (inorganic residue from the sample), heavy metals, and buffering capacity (estimated strength of the buffering effect caused by extractable chemicals that alter the pH of the extract). The test results are presented as a set of four values, showing the results for test type together with the corresponding USP limits (see Table I). These aqueous extract tests are intended to serve as the basis for design specifications.

Polymer
Nonvolatile Residue
(mg)
Residue on Ignition
(mg)
Heavy Metals
(ppm)
Buffering Capacity
(ml)
ABS
1
<1
<1
<1
Polyurethane
1
≤1
≤1
<1
Polycarbonate
1
≤1
≤1
<1
Polyisoprene
9
≤1
≤1
<1
Polyvinyl chloride
1
<1
<1
1
Polyethylene
<1
<1
≤1
<1
PTFE
<1
<1
≤1
<1
Polystyrene
<1
<1
≤1
<1
Polypropylene
<1
<1
≤1
<1
Silicone
1
≤1
≤1
<1
USP Limits
15
5
1
10
Table I. Results of aqueous extraction physicochemical testing on polymers commonly used in medical devices. Typical device materials contain very few water-soluble extractables. With reference to these tests, USP limits can be used to establish specifications for raw materials.

Products that fit the classifications of USP chapter 661 are expected to be made of materials that meet the requirements of these tests initially and throughout their product lifetimes. Aqueous physicochemical tests can also be used to evaluate raw materials from lot to lot or from different vendors, thereby ensuring chemical equivalence of starting materials.

Nonaqueous Tests. The USP prescribes isopropyl alcohol (IPA) for conducting physicochemical tests of elastomeric closures used for pharmaceutical containers.7IPA is a more aggressive extraction fluid than water and can dissolve many chemicals that are insoluble in water.

The alcohol extract is analyzed for nonvolatile residue and residue on ignition. Because IPA extracts do not lend themselves to analysis of heavy metals or buffering capacity, these tests are omitted. Instead, turbidity and ultraviolet absorption tests are included as simple procedures for detecting the presence of extractables without specifically identifying their chemical makeup. Results of the alcohol physicochemical tests are presented as a set of five values for each of the end points (see Table II). USP limits do not yet exist for these tests, but they are not necessary for establishing specifications for the acceptance of materials.

Polymer
Nonvolatile Residue (mg)
Residue on Ignition (mg)
Turbidity (NTU)
Maximum Optical Density
Wavelength of Max. Optical Density (nm)
ABS
46
<1
4.18
>2.0
241
Polyurethane
119
<1
21.38
>2.0
244
Polycarbonate
<1
<1
0.04
>2.0
227
Polyisoprene
223
<1
24.38
>2.0
250
Polyvinyl chloride
123
1
0.24
>2.0
297
Polyethylene
20
<1
7.08
>2.0
241
PTFE
<1
<1
0.00
0.0
Polystyrene
66
1
8.10
1.2
290
Polypropylene
20
<1
13.10
0.1
Silicone
444
248
0.70
0.1
Table II. Results of alcohol extraction testing on polymers commonly used for medical devices. Isopropyl alcohol (IPA) is a more aggressive extraction medium than water. Most materials used for devices show measurable amounts of extractables when tested with IPA. No USP limits exist to establish acceptable levels of extractables for materials tested by this method.

Chromatographic Characterization. The various chromatographic methods, such as gas, liquid, paper, and thin-layer, have become indispensable aids in the isolation, separation, and identification of chemicals. Gas and liquid chromatography, especially high-performance liquid chromatography (HPLC), have become powerful analytical tools for the characterization of additives in polymeric medical device materials.8

In gas chromatography (GC), the sample is vaporized and injected onto the head of a chromatographic column. Elution is brought about by the flow of an inert gaseous mobile phase. In contrast to most other types of chromatography, the GC mobile phase does not interact with the analyte molecules. Its only function is to transport those molecules through the packing. Gas chromatography can be used to separate and quantitate volatile and semivolatile chemicals found in polymeric materials.

The flame ionization detector (FID) and thermal conductivity detector (TCD) methods have been the most widely used techniques in gas chromatography. However, the mass spectrometer detector (MSD) has become a powerful technique and is considered by many the ideal GC detection technology.9The MSD delivers a unique combination of universal, nonselective detection capability in its total ion chromatography mode and the highly selective and sensitive detection of a large number of compounds through postrun mass-chromatogram data analysis. A gas chromatogram/mass spectrometer (GC/MS) system provides the ultimate in sample-compound identification capability through automated library searching and a high degree of selectivity to target compounds.

HPLC is one of the fastest growing and more useful of all the analytical separation techniques. The reasons for this growth are its sensitivity, its ready adaptability to accurate quantitative determinations, and its suitability for separating nonvolatile or thermally fragile species. Liquid chromatography, because of its great flexibility and widespread applicability, can be used for the analysis of more than 80% of all known organic compounds.8

Like GC, HPLC can be performed using different types of detectors to identify and quantitate analytes. For the most part, ultraviolet/visible detectors have been the universal type for liquid chromatography, but detectors based upon refractive index, infrared absorbance, evaporative light scattering, and fluorescence, as well as on mass spectrometry, have also been widely used. Liquid chromatography using mass spectrometer detectors has seen substantial growth owing to the substantial analytical strength and sample-compound identification capabilities using automated search libraries available with the technique.

Both GC and HPLC provide excellent ways to characterize additives, residual monomers, and degradation products. Aqueous and nonaqueous extracts obtained from physicochemical tests can be subjected to chromatographic analysis to fingerprint the extractables. By using various sample-extract preparation techniques and choosing among the wide variety of detectors, nearly any polymeric material can be subjected to extractable fingerprinting by chromatography.

Figure 1. An example of an HPLC chromatogram fingerprint of plastic additives used in a medical device material. Such chromatograms can be used to evaluate devices throughout the product lifetime.

These analytical techniques help to further characterize the extractables by providing a chromatographic tracing of the chemicals present in the extract. Though often desirable, it is not necessary to identify all components of these extracts. Instead, the information may be used as the basis for establishing specifications for materials. Once established, these chromatographic specifications can be used to evaluate medical device products throughout their lifetimes. In practice, a copy of this "extractable fingerprint" is used as the reference. Future lots are compared with it to confirm that there have been no changes detectable by the method (see Figure 1).

Liquid chromatography is also used to measure the molecular weight and molecular weight distribution of polymers. By using certain columns capable of separating compounds with high molecular weights from those with low molecular weights, and detectors such as refractive index or evaporative light scattering, the molecular weights can be determined. This technique, commonly called gel permeation chromatography (GPC), can help obtain information related to hardness, brittleness, and stress-strain phenomena.

Trace-Metal Analysis. Many inorganic chemical additives can be added to polymeric materials in order to get desired physical, chemical, or mechanical properties. Additives used as stabilizers may include calcium and zinc. Pigments often contain metals such as lead, tin, arsenic, nickel, cadmium, barium, aluminum, titanium, and iron. Barium oxide is often added to plastic catheter tubing as a radiopaque agent. Still another example is the use of aluminum oxide as a polishing compound during the manufacture of intraocular lenses. This list merely illustrates how varied and common are the inorganic chemicals that may be found in polymers.

The USP has provided guidance in a physicochemical test monograph for analyzing extracts of plastics and elastomers for the presence of any heavy metal in excess of one part per million when compared to a lead standard. This is a key test in the qualification of a polymer for medical device use. However, the test is nonspecific.6,7

To fully characterize trace metals, atomic absorption spectroscopy (AAS) and inductively coupled plasma (ICP) spectroscopy are frequently used. AAS can determine the amount of specific metals present in a material or its extract, while ICP spectroscopy permits simultaneous determinations of all the periodic-table elements, with a lower limit of detectability in the parts-per-billion range.

MORPHOLOGICAL CHARACTERIZATION

Both light and scanning-electron microscopy can give valuable information about the surfaces of materials. The smoothness or roughness of surfaces can influence how materials interact with tissues and body fluids. Smoothness or roughness may also affect the binding of protein and biochemical intermediates (lymphokines and cytokines), which may also help determine a material's biocompatibility.

Electron microscopes create magnified images by using a beam of electrons as an imaging source. They therefore resolve images at much higher magnifications than can light microscopes. Microstructures can be shown in great detail, often at magnifications up to 300,000 times. Scanning electron microscopy (SEM) can yield topographic images and elemental information when used in conjunction with energy-dispersive x-ray analysis (EDX) or wavelength-dispersive x-ray spectrometry (WDS). SEM is useful in characterizing the size and morphology of microscopic specimens. Together, image and x-ray analysis are important for the identification of small particles.10Elemental analysis using SEM/EDX or SEM/WDS is useful for qualitative and semiquantitative determination of elemental content and for obtaining correlation between microstructures and elemental composition.

Atomic force microscopy (AFM) is another powerful tool for examining the topography of a surface. It works by measuring the force acting on a very fine, sharp tip that is moved over the surface. This technique can generate three-dimensional images with nanometer resolution. It can be used to evaluate the surface roughness or smoothness by measuring in nanometers the height of peaks and depth of craters on the surface. This instrument can also be used to evaluate crack formation and growth in both plastics and metals.11

PHYSICAL/MECHANICAL TESTING

Mechanical testing methods are used to evaluate materials under a variety of loading conditions (stresses and strains). Analysis of material properties and composition is useful in characterizing materials on selection and receipt, in analyzing failures and other problems, and in validating manufacturing processes.

Mechanical properties can be determined for all types of materials found in medical devices, whether metals, ceramics, polymers, or composites. A discussion of all the tests and procedures used to evaluate the mechanical properties of materials is beyond the scope of this article. Instead, I will highlight some of the more important concepts and tests available.

Hardness Testing. Hardness is generally defined as indentation resistance, scratch resistance, or rebound resilience. The instrument used to obtain the measurement is called a durometer. ISO standards cover three methods for measuring hardness: Shore hardness, ball indentation, and Rockwell hardness.12,13 Hardness values obtained from one method generally cannot be compared with those from another, although data can be empirically compared. In general, hardness is used as an end- performance property of a material.

Stress-Strain Properties and Testing. Mechanical testing that measures stress-strain relationships is used to characterize a material's behavior and performance. Stress-strain behavior greatly depends upon temperature, strain rates, and environmental conditions. The strength of materials is expressed by the ultimate values of stress, which can be found by loading the sample until fatigue. Loading can be in tension, compression, or shear. Both tensile and compression testing are based on the application of forces normal to the plane on which they act. In shear stress testing, however, the application of forces is parallel to the plane.

Figure 2. A typical stress-strain curve. The modulus is represented by the op line, the tensile strength by et, and the tensile elongation by oe.

In the stress-strain curve shown in Figure 2, op represents the modulus, et the tensile strength, and oe is the tensile elongation.14The tensile modulus of a material, also referred to as Young's modulus, is a measure of the specimen's ability to resist deformation. Some plastic materials can be weakened by gamma sterilization. Tensile-strength data collected before and after sterilization can help predict any changes in the strength of the material. Tensile-strength data should also be gathered on medical devices before and after accelerated-aging studies to help determine or predict the shelf life of a product. Finally, the tensile strength of package seals can be used to evaluate package integrity, so that sterility will not be compromised.

Compression. In compression testing, instead of being stretched, the test piece is crushed. Compressive forces are generally significantly greater than the tensile forces generated in a tensile test. The compressive strength of a material is calculated as the pounds per square inch (psi) required to rupture the specimen or deform it to a given percentage of its height. It can be expressed as psi, either at rupture or at a given percentage of deformation.15

The compressive strength of plastics is of limited design value, since most plastic materials do not fail from compressive loading alone. The data may be useful, however, in specifications distinguishing between different grades of a material or for assessing the overall strength of different materials.

Shear. Shear strength is particularly important in film and sheet products where failures from this type of load may often occur. Shear-strength testing, described in ASTM D732, is widely used.16Typically, the specimen is mounted in a punch-type shear fixture. The punch is pushed down at a rate of 0.05 in./min until the moving portion of the sample clears the stationary portion. Shear strength is calculated as the force/area sheared. The shear modulus is frequently called the modulus of rigidity.

Flexural. In the flexural or bending test, a rectangular beam is subjected to both tensile and compression stresses. A continuous change takes place from maximum tensile stress on one surface through the thickness to maximum compressive stress on the other. The top surface of the specimen is in tension while the bottom surface is in an equal compression. If the tensile and compressive moduli are equivalent, then the stress at the midpoint of thickness is zero. Typically the specimen is placed on two supports and a load is applied in the center at a specified rate. The loading at failure, in psi, is the flexural strength.

Figure 3. The effect of irradiation on the bending angle of polypropylene.

Because most thermoplastics do not break in this test, the flexural strength cannot be calculated. Instead, stress and strain is calculated. This is the loading, or force, in psi, necessary to stretch the outer surface 5%. A modulus value in flexure can also be obtained in this test. Along with characterizing a plastic's bending properties, flexure can be used to evaluate aging and sterilization effects on a product. An example is the dramatic effect irradiation can have on the bending of polypropylene syringe-barrel flanges (see Figure 3).17

Thermal Analysis.
Thermal analysis techniques measure the response of a polymer to controlled heating processes. They are widely used in the development and characterization of plastics, elastomers, and waxes. For example, characterization of polymer glass-transition temperatures, melting points, and crystallinity is important. These data are often used to help produce materials with physical properties tailored to a product's ultimate applications.18

The primary thermal analysis techniques for certifying product quality are differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Testing can be conducted to ascertain melting point, degree of crystallinity, and glass-transition temperature, or for component quantification.

For some materials, such as crystalline polymers and certain organic chemicals, DSC is used to measure melting points and degree of crystallinity. For amorphous polymers, rubbers, and cross-linked thermoset materials, DSC also provides a fast and accurate measure of the glass-transition temperature or the degree of cure. TGA is widely used to separate and quantitate the components in a mixture. Both DSC and TGA are effective in measuring decomposition of materials.

As mentioned, DSC results offer rapid measures of critical thermal characteristics such as polymer melting point or glass-transition temperature. The particularities of these transitions, in addition to identifying characteristics unique to each polymer, can provide information about a material's phase structure, thermal history, and purity.

CONCLUSION

Materials characterization forms the basis for understanding the composition of a medical device material and its potential for an adverse biological effect when the device is put into use. Comprehensive material characterization of polymers should provide sufficient information to allow evaluation of the potential success of a plastic medical device, at least with regard to its chemical and mechanical properties. It can also serve as a means to ensure standardization of materials from one lot of devices to the next. As the harmonization of ISO 10993 and FDA requirements proceeds, the methods described above will increasingly be used by the U.S. device industry to help select optimal materials and to control the uniformity of medical products.


REFERENCES

    • "Biological Evaluation of Medical Devices," ISO 10993, parts 1–13 (Geneva: International Organization for Standardization [ISO], various dates).
    • Toxicology Subgroup, Tripartite Subcommittee on Medical Devices, "Tripartite Biocompatibility Guidance for Medical Devices" (Rockville, MD: FDA, Center for Devices and Radiological Health, 1987).
    • "Biological Evaluation of Medical Devices, Part 1: Evaluation and Testing," ISO 10993-1 (Geneva: ISO, 1994).
    • DE Albert and RF Wallin, "A Practical Guide to ISO 10993-14: Materials Characterization," Medical Device & Diagnostic Industry 20, no. 2 (1998): 96–99.
    • JP Sibilia, "Molecular Spectroscopy," in A Guide to Materials Characterization and Chemical Analysis, ed. JP Sibilia (New York: VCH Publishers, 1988), 13–40.
    • "Containers," United States Pharmacopeia XXIII, (U.S. Pharmacopeial Convention) chapter 661.
    • United States Pharmacopeia XXIII, (U.S. Pharmacopeial Convention) chapter 381.
    • H Lobo, J Bonilla, and RW Riley, "Plastics Analysis—Improved Characterization of Polymer Behavior and Composition," Plastics Engineering 52, no. 11 (1996): 27–31.
    • A Amirav, "The Future of GC Detectors in the Era of Mass Spectrometer Detection," American Laboratory 33, no. 20 (2001): 28–34.
    • United States Pharmacopeia XXIV (U.S. Pharmacopeial Convention) chapter 1181.
    • M Goken, "Atomic Force Microscopy of Metallic Surfaces," Advanced Materials & Processes 155, no. 2 (1999): 35–37.
    • ASTM D2238, D2583, in Annual Book of ASTM Standards, 08.02—Plastics (II) (Philadelphia: American Society for Testing and Materials, [ASTM] 1991).
    • "Plastics and Ebonite—Determination of Indentation Hardness by Means of a Durometer (Shore Hardness)," ISO 868 (Geneva: ISO, 1985).
    • DE Albert, "Materials Characterization as an Integral Part of Global Biocompatibility," Medical Plastics and Biomaterials 4, no. 4 (1997): 16–23.
    • L Palley and AL Signorelli, "Physical Testing," in A Guide to Materials Characterization and Chemical Analysis, ed. JP Sibilia (New York: VCH Publishers, 1988), 273–284.
    • ASTM D732, in Annual Book of ASTM Standards, 08.01—Plastics (I), (Philadelphia: ASTM, 1991).
    • DM Bigg, in Polypropylene: Structure, Blends, and Composites, vol 3, ed. J Karger-Kocsis (London: Chapman & Hill, 1995), 263.
    • MP DiVito et al., "Recent Advances in Routine Thermal Analysis Instrumentation," American Laboratory 24, no. 1 (1992): 30–36.

David E. Albert is senior scientist at the NAMSA facility in Northwood, OH.

Copyright ©2002 Medical Device & Diagnostic Industry

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