Printable electronics present a new era of wearable electronic technologies. Detailed technologies consisting of novel ink semiconductor materials, flexible substrates, and unique processing methods can be integrated to create flexible sensors. To detect various stimuli of the human body, as well as specific environments, unique electronic devices formed by "ink-based semiconductors" onto flexible and/or stretchable substrates have become a major research trend in recent years. Materials such as inorganic, organic, and hybrid semiconductors with various structures (i.e., 1D, 2D and 3D) with printing capabilities have been considered for bio and medical applications. In this review, we report recent progress in materials and devices for future wearable sensor technologies.

Recent Progress in Materials and Devices toward Printable and Flexible Sensors

From the 1940s into the early 1960s, hundreds of scientists and engineers worldwide pursued efforts in microcircuitry-miniaturized, integrated electronic circuits By tracing the diverse activities and alternatives they explored-from early printed wiring to semiconductor integrated circuit efforts-this article provides the first comprehensive overview of the early history of microcircuitry

The Early History of Microcircuitry: An Overview

A program on microelectronics (electronic microminiaturization) has been underway for the past two years at the Diamond Ordnance Fuze Laboratories (DOFL). Fourteen-component-part, transistorized binary counters were fabricated on 1/2-inch squares of steatite ceramic, 1/50 inch in thickness; and 5-part NOR's, on 1/2-inch squares of the same thickness. The techniques that have proved most useful in the program have been 1) photolithographic procedures for the accurate placement at microscopic dimensions of physical masks and electrical insulation, 2) thin film deposition using vacuum, chemical, and screening methods, 3) the use of a conductive adhesive which yields high strength and low resistivity connections, and 4) ultrasonic drilling and air abrasion enabling substrates and devices to be formed to desired sizes and shapes. Concurrent with the physical fabrication, studies were made of detailed circuit design and system applications. Future work will include 1) refinement of the present techniques to achieve inexpensive, reliable circuit wafers capable of mass production by industry, and 2) development of advanced methods more fully utilizing thin film deposition and diffusion techniques.

The DOFLM Microelectronics Program

In the two decades since the concept of ceramic based "printed circuits" was first suggested for an Army Ordnance application, substantial changes have taken place in the techniques of electronic circuit construction in the United States. Today, such ceramic circuits, in simple and complex networks, are complemented by printed wiring assemblies in several variations. Both technologies, now well established and in mass use on production lines, represent the current plateaus in miniature circuit construction for general commercial and military usage. Other construction philosophies and technologies are now shaping in the country's industrial and military laboratories, all aimed at new orders of size reduction of electronic equipments. In the several microelectronic techniques under development, the elemental electronic part appears destined to lose its logistic identity completely, and yield its classical position as a building block to black boxes called "circuit functions." The paper provides a summary review of the evolution of current ceramic printed circuits and printed wiring practices and, in the light of today's microelectronic activities, frames the trend in equipment design in the years ahead.

Printed Circuits and Microelectronics

Printed electronics is an established industry allowing the production of electronic components such as resistors, and more complex structures such as solar cells, from functional inks. Composites, a mixture of two or more materials with different physical and/or chemical properties that combine to create a new material with properties differing from its constituent parts, have been important in areas such as the textile and automotive industries, and are significant in printed electronics as inks for printed circuit components, touch and vapour sensors. Here, the functional performance and physical behaviour of two screen printable multi-component nanocomposite inks, formulated for touch-pressure sensing applications, are investigated. They each comprise a proprietary mixture of electrically conducting and insulating nanoparticles dispersed in an insulating polymer binder, where one is opaque and the other transparent. The opaque ink has a complex surface structure consisting of a homogeneous dispersion of nanoparticles. The transparent inks structure is characterised by large aggregates of nanoparticles distributed through the printed layer. Temperature dependent electrical transport measurements under a range of compressive loadings reveal similar non-linear behaviour in both inks, with some hysteresis observed, and this behaviour is linked to the inks structures. A physical model comprising a combination of linear and non-linear conduction contributions, with the linear term attributed to direct connections between conductive particles and the non-linear term attributed to field-assisted quantum tunnelling, has been developed and used successfully to describe the underpinning physical processes behind the unique electrical functionality of the opaque ink and, to a lesser extent, the transparent ink.

Temperature dependence and touch sensitivity of electrical transport in novel nanocomposite printable inks

A comprehensive treatment of the complete field of printed circuits is presented Circuits are defined as being "printed" when they are produced on an insulated surface by any process The methods of printing circuits fall in six main classifications: (1) Painting Conductor and resistor paints are applied separately by means of a brush or a stencil bearing the electronic pattern After drying, tiny capacitors and subminiature tubes are added to complete the unit (2) Spraying Molten metal or paint is sprayed on to form the circuit conductors Resistance paints may also be sprayed Included in this classification are an abrasive spraying process and a die-casting method (3) Chemical deposition Chemical solutions are poured onto a surface originally covered with a stencil A thin metallic film is precipitated on the surface in the form of the desired electronic circuit For conductors the film is electroplated to increase its conductance (4) Vacuum processes Metallic conductors and resistors are distilled onto the surface through a suitable stencil (5) Die-stamping Conductors are punched out of metal foil by either hot or cold dies and attached to an insulated panel Resistors may also be stamped out of a specially coated plastic film (6) Dusting Conducting powders are dusted onto a surface through a stencil and fired Powders are held on either with a binder or by an electrostatic method Methods employed up to the present have been painting, spraying, and die-stamping

R.W. Curtis

Papers

Printed-Circuit Techniques