Solid-State Fingerprint Detection

A Comparison of Capacitive and Active Array Based Human Fingerprint Readers This paper addresses relative aspects of the two currently available all-solid-state technology solutions to the digital fingerprint identification problem. In the body of this report passing mention is made of some other approaches, and a very brief summary of the whole biometric technology set can be found in APPENDIX B: Biometric Technologies. Fingerprint Acquisition What is a FingerPrint? The traction-producing skin of the palmar (hand) or plantar (foot) body surfaces, especially the fingertips, exhibit the familiar and characteristic ridges and valleys of the human fingerprint. Everyone has them, and their classification and identification are far and away the most widely-used of all of the various personal identification measurements, or “biometrics”. But the fingerprint that we can see by eye – that produces the detective’s “latent print” when we touch most surfaces, or that stamps the image when inked and pressed to paper, or that is detected when optically or capacitively scanned – is actually a structure composed mostly of dead epidermal skin. The fingerprint is initially formed in the living subdermal tissue just below the visible outer layer. In a cycle of constant rejuvenation, the newly-formed skin cells, mostly keratinocytes, slowly migrate outward through five distinct epidermal regions, deteriorating as they go, yet retaining the unique anatomical patterns originally set in the live tissue. Arriving at the surface, they replace older cells, which are then worn away by friction or abrasion, or simply sloughed off as skin detritus in a process called desquamation. These new cells are discarded from the skin surface in their turn as they are replaced by yet newer cells from below. In this way, the characteristic patterns of the epidermis are naturally and constantly renewed over time. Nevertheless, physiological processes such as dermatitis, disease, aging, and calluses can obscure them. They can be mechanically damaged – worn smooth, contaminated by dirt, etched by chemicals, or even burned or scarred – all without affecting the living cells beneath. 1 The epidermis consists of a continuous mass of cells, usually differentiated into the following regions or strata: the stratum corneum, the uppermost layer, containing dry, dead cells, flattened to form a relatively continuous thin outer membrane skin that is highly cornified or keratinized (rendered horn-like); the stratum lucidum, the region where dying cells are located, which contain an oily substance that renders them translucent; the stratum granulosum, in which the cells contain large granules; the stratum spinosum, where the cells show marked spines or bridges between each other; the stratum basale, where the cells contain living basal cells called keratinocytes and melanocytes; and the lowest region, the epidermal-dermal junction (often called the basement membrane), which consists of a thin zone of ground substance, containing no fibers, lying between the basal cells and the dermal surface. – Adapted from Gray’s Anatomy, 38th edition. Williams, Peter L., et al. Churchill Livingston.