13.3: Skin, Hair and Fiber Analysis

The skin is the largest organ in the body, consisting of water, protein, fats and minerals. Your skin is the outer layer, or integumentary system, which also includes hair, nails, oil and sweat glands, that regulates body heat and protects your body from germs and infections. There are three layers of skin: hypodermis, dermis, and epidermis. The hypodermis is the lowest layer of skin that protects the muscles and bones from superficial injuries, provides a fat barrier to help regulate heat, provides for a tributary for nerves and blood vessels, and provides the tissue needed to connect to the muscles. The dermis is the middle layer and makes up about 90% of the thickness of skin. It contains the collagen and elastin that makes the skin flexible but strong. The dermis is the layer that hosts the roots of the hair, creates oils and sweat, houses the nerves for feel and touch, and supplies the epidermis with nutrients through blood vessels. The epidermis is outer protective layer of skin. It protects the body from the environment, germs and infections by means of Langerhans cells, which are part of the immune system of the body. The epidermis also provides the pigmentation of the skin. Most important to our story is that the epidermis is constantly making new skin in a process called proliferation. These new cells replace the cells that have died off in a process called desquamation. According to Texas A&M University, the average human sheds about 600,000 skin cells every day. The Imperial London College places the number closer to 200 million. Whatever the number, those jettisoned skin cells are an excellent source of deoxyribonucleic acids; however, as you can imagine, they are very hard to find owing to their size, which is about 0.01mm to 0.03mm in diameter. In spite of this, the forensic technician should consider the possible existence of dead skin cells, especially in sexual assault crime scenes, and to make the effort to collect this evidence using tape-lifting or evidence vacuuming.

Hair is common to humans and mammals. Yes, all mammals, including those that appear to be hairless like the hippopotamus, rhinoceros, whales, dolphins, and your author. Their hair is either very fine, short, or sparse, and may only be perceptible when they are very young. Hair itself is a fibrous protein that contains a vast number of the following cross-bonds or linkages: disulphide bonds, salt linkages, and hydrogen bonds. Each hair is composed of an outer layer called the cuticle, a middle layer called the medulla, and a layer in between called the cortex. The largest mass of a hair strand is the cortex layer. The average human loses between 50 and 150 hair strands per day, which is about the same for an average dog; however, a cat can lose up to 1,000 hair strands per day. Your author’s vacuum can prove that fact.

Figure \(\PageIndex{1}\): Structure of the human hair - Wikimedia

Animal hair is significantly different in appearance under magnification from human hair. Cat hair is different in appearance from dog hair as are different breeds of cats and dogs. Hair is an excellent source of deoxyribonucleic acid. In the past, it was necessary to have the root of the hair in order to type for DNA; however, current techniques for the extraction of DNA have proven successful using just the hair strand. As you can imagine, hairs may be difficult to visualize with just the human eye. It may require the use of an Alternate Light Source or forensic light source in a process called trace evidence examination. If this process is done in the laboratory, it is essential that clothing, bedding, blankets, or carpeting is packaged in clean butcher’s paper at the crime scene. Hairs, skin cells, and fibers can be lost when an item is just stuffed into a paper bag. As with skin cells, hairs can be collected using tape lifting or evidence vacuuming techniques, but they can also be collected with sterile tweezers if care is taken not to crush the cortex and medulla. If collected in this manner, your author suggests placing the hairs into a pharmaceutical fold constructed out of clean white butcher’s paper, preferably the waxed (freezer) variety, with the waxed side facing inward.

Textile fibers or fibers from natural or man-made products like the hemp or nylon in rope can be exchanged between suspects and victims, or vice-a-versa, between those individuals and objects, or between one object to another object. These fibers can provide class and individual characteristics that may be associated with or matched to a common source. Textile fibers can be either natural or synthetic in their composition. Natural materials for textile fibers include animal hair, such as wool from sheep or alpaca, or a plant product such as cotton or hemp. Man-made or synthetic fibers such as polyester, nylon, acrylics, rayon, or other chemical derivatives. Over half of all textile fibers used in the production of textile materials are synthetic in nature. Even some natural materials are blended with synthetic materials in order to produce a more robust or resistant fabric. Forensic analysis of fibers considers the rarity or distinct proprietary components of production, the cross-sectional shape, and microscopic characteristics of each fiber strand. Color is one of the greatest variations in textiles and therefore significantly influence fiber analysis and comparison. Synthetic dyes especially can be highly discerning to the forensic analysis of the fibers owing to color variety, batch variations, and the immense array of available colors, which vary from batch and production year. Fabric type can also affect the quantity of fibers available for analysis. Tightly woven wool or knit fabrics tend to transfer fewer fibers than those that are loosely woven or knitted. Fibers that are manufactured using discrete lengths of materials, such as short, medium, or long varieties of cotton, called staple fibers, tend to shed more than fibers of a continuous length, called filament fibers, found in synthetic or blended manufacturing. Fiber transfer occurs in one of two manners: direct transfer or indirect transfer. Direct transfer occurs when the fabric comes into contact with the target by some application of force. Indirect transfer occurs when the target is placed on top of the material. This, of course, is dependent on the type of fabric involved in the contact and the nature and duration of the contact. Fibers should be documented and collected in the same manner as hairs. According to the National Center for Forensic Science, a fiber examiner must employ a stereomicroscope, a comparison microscope, and a compound light microscope equipped with a polarized light capability. Using the comparison microscope, an examiner must view questioned and known fibers side-by-side at the same magnifications in visible light and using alternative lighting such as polarized light or fluorescent lighting. The chemical composition of synthetic fibers can be determined using gas chromatography in conjunction with mass-spectrometry, which will be discussed later in this chapter. One thing to consider is the possibility of the presence of deoxyribonucleic in the form of perspiration. Therefore, it may be wise to carefully swab the fibers for DNA typing in a sterile environment prior to comparison efforts.

Figure \(\PageIndex{2}\): Fibers found in the breech of a handgun that were matched to a ski face mask worn by the suspect of an armed robbery indicated the gun had been concealed in the mask at some time. Author's collection