J. Soc. Cosmet. Chem., 28, 415-425 (August 1977) The physical properties of fingernails I. Apparatus for physical measurements MICHAELJ. MALONEY and ELMER G. PAQUETTE Bjorksten Research Laboratories Inc., P.O. Box 9444, Madison, WI 53715,' and ALBERT SHANSKY Bettswood Road, Norwalk, CT 0685 I. Received September 30, 1976. Synopsis Miniature APPARATUS for measuring the PHYSICAL PROPERTIES of FINGERNAILS have been developed and tested. Included are a device for cutting test specimens a template for preparing tensile bar samples apparatus for performing flexural, tensile, and tearing tests and a device for determining impact absorption. Whenever possible, methods were based on standard testing procedures so that recognized in- terpretative formulae could be used for analyzing experimental results. The range of physical properties found for the fingernails tested are as follows: flexural strength 4,928 to 17,653 psi tensile strength 4,464 to 17,081 psi tearing resistance 274.3 to 676.2 lb/in. and impact absorption (rebound ratio) 0.4632 to 0.7273. INTRODUCTION A number of investigators have reported on studies involving the human fingernail. While all of these studies have proved revealing, they have, as a group, suffered from one or more of the following limitations: (1) inadequate number of tests or subjects, (2) inadequate measuring techniques, (3) inadequate analysis or interpretation of results. Bean (1) has meticulously studied the growth of his left thumbnail over a period of twenty-five years. His observations regarding the effect of age and a case of mumps are noteworthy and his philosophy amusing. Caputo and Dadati (2) and Jarrett and Spearman (3) have made significant contributions to the understanding of nail struc- ture. Donsky (4) and Lazar (5) have sounded a warning note regarding the use of so- called nail hardeners. Dixon (6) attempted to evaluate the effect of a commercial nail food on nail splitting. While her results indicated no beneficial effect, she concluded that the trial was an educational success. Michaelson and Huntsman (7) attempted to provide a numerical answer to the ques- tion of whether or not gelatin in the diet has an effect on nail hardness. They used a Knoop indentor to evaluate hardness, and while they concluded that gelatin increases hardness, the validity of their data has been questioned by Newman and Young (8). These latter authors have proposed that flexural measurements of fingernails should 415

416 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS provide the maximum information in the most direct manner (9). Baden (10) also studied nail flexibility and developed two additional methods for measuring modulus of elasticity. Having independently arrived at the same conclusion as Baden, Newman, and Young, and having designed and built an apparatus for measuring flex strength, the authors support their conclusions regarding the value of flexural data. In addition, the authors have also devised apparatus and techniques for measuring tensile strength, impact absorption, and tearing strength of fingernails. Three of these properties are obtained by adapting a commercially available testing machine (the Instron tester). We consider this feature particularly important, since it allows other workers to perform compara- ble experiments. The fourth test (impact absorption) employs apparatus of our own design. However, the innate simplicity of the device insures that it could be easily du- plicated by others. Of the 4 tests, flexural strength and impact absorption have been demonstrated to be nondestructive. This feature allows the same nail to be tested several times and is in- valuable for experiments, where it is desirable to determine the effect of exposure to various environments. Also, since each sample can serve as its own control, the number of experiments necessary to achieve statistical significance is greatly reduced. The nail samples used for our studies were obtained from both living donors (large clippings) and cadavers. The donors ranged in age from 28 to 98 years and were about evenly divided between males and females. DESCRIPTION OF THE APPARATUS FLEXURAL TESTING APPARATUS The procedure for determining flexural properties of plastics is described in ASTM method D-790 (11). The specifications call for a specimen, in the form of a rectangular bar, which is positioned on 2 supports and a load applied at the midpoint of the span. The apparatus normally used for these tests is far too large to accommodate fingernails. To overcome this problem, therefore, a miniature fixture (Fig. 1), to be used in con- junction with an Instron tester was designed and fabricated. Even though the apparatus is much smaller than that normally used, the ratio of sample thickness to support radii is not changed, and the standard interpretive formulae may be employed. SAMPLE CUTTING APPARATUS In order to obtain reasonable accuracy when performing flexural tests, it is extremely important that the cut sides of the test specimens be as nearly parallel as possible. In our opinion, the method employed by Newman and Young (wherein the surfaces were sanded lightly) was not entirely satisfactory. Therefore, we have developed a device (shown in Fig. 2), which employs two rigidly mounted razor blades to accurately cut parallel samples for our tests. In using the apparatus, a fingernail clipping is first conditioned by soaking it in distilled water. Then the clipping is flattened by clamping it between 2 sheets of clear plastic for at least 20 min. The flattened nail is carefully positioned on the cutting edges and