FIGURE 4 is a high magnification photomicrograph of the surface of a panel of AISI 304 stainless steel coated with LFC to illustrate the fineness of the microcracks that are characteristic of this coating. This compares to FIGURE 5 (see page 3) which is a photomicrograph of the surface of a conventional chromium coating. It is the fineness of these micro-cracks, which also affords LFC its improved corrosion resistance over conventional chromium by providing a more discontinuous path from the outer surface of the coating to the inner for invading fluids. This difference can be seen graphically in FIGURES 6 & 7 (see page 3) which are photo-micrographs of cross-sections of both coatings that have been subjected to a Knoop microhardness impression under a 500-gram load. The 500-gram Knoop load can be seen to have caused gross propagation of microcracks in the conventional chromium coating, while the fineness of the micrograin of the LFC coating has allowed the microcracks to remain localized under the same loading.

Internal Environment of the Human Body
The internal liquids of the human body can present a very hostile environment to medical instruments. Cellular fluids containing high concentrations of chlorides and organic acids can be very corrosive to uncoated austenitic stainless steel. While the normal pH of the body liquids is about neutral (pH 7.2-7.4), these values can rise to highly acidic (pH=4.0) at sites of bodily injury to highly basic where infections may be present. The use of invasive devices into this wide variety of aggressive chemical conditions can deem many other materials inappropriate, including chromium steels, low-grade austenitic stainless steels and sensitized stainless steels.

While it must be made clear that chromium coatings are not recommended for long-term use in the body, such as for implants, both field trials and laboratory-accredited testing have proven LFC-coated stainless steel medical instruments to be sufficiently impervious for shorter-term invasive uses, such as for surgical preparation of the receptor bone for the implant or for use in arthroscopic surgery, as results from the following tests illustrate:

USP Class VI Certification Testing
In order to evaluate the inertness of LFC coatings, panels of type 304 stainless steel were coated and subjected to the bank of tests dictated by the USP (United States Pharmacopoeia) for possible qualification for Class VI certification. TABLE I contains an itemized list of the tests performed, including all of the parameters of the individual tests. All tests were performed under the auspices of NAmSA, North American Science Associates, Inc, an FDA-registered test agency.

For a clearer understanding of the essence of the results of
these tests, they can be broken down into two basic categories, as displayed in TABLE I: (1) four standard tests for toxicity wherein the test article (304 stainless panel coated with LFC) was extracted in various solutions prior to injection into the laboratory animal, and (2) two surgical muscle implantations wherein a sample of the actual LFC-coated panel was injected or incised into the body of the animal.

Results for the twelve individual tests for toxicity after three days showed that no evidence of any systemic toxicities was observed, nor any evidence of localized animal tissue reactions, irritation, edema or erythema, as a result of having been in contact with the extract solutions.

Results shown at the bottom of TABLE I for the two implantation tests after five and nine days showed no evidence of any negative reactions in the area of the implantation, such as capsuling or irritation, as a result of having an LFC-coated stainless steel article implanted under the skin.

Meeting of all of the above mentioned test criteria qualified LFC coatings for Class VI certification.

ISO/Tripartite Biological Compatibility Tests
Various tests were chosen for testing LFC for its biocompatibility under a variety of test criteria. TABLE II is an itemized list of the tests performed, including all of the parameters of the individual tests. All tests were performed under the auspices of NAmSA, North American Science Associates, Inc.

Each of the tests listed measured the effects of an extract solution of the test article (304 stainless panel coated with LFC) under different test conditions, including application method, time length of test and end point criteria.

The last column of the table for the diversity of categories chosen shows that the solutions extracted from LFC-coated 304 stainless steel proved to be non-toxic, non-pyrogenic, non-hemolytic, non-sensitizing, and non-mutagenic.

Conclusion
Laboratory-accredited tests showed that low friction chromium has passed the rigorous test criteria necessary to achieve Class VI certification that essentially certifies it to be a viable protective coating on stainless steel medical instruments destined to be used for invasive procedures.