question: Why does touchscreens, trackpads or any other touch-based devices only work with skin contact?
comment: Because we can kinda conduct electricity (we're not great conductors but not bad ones either). So when we touch a capacitive screen, the spot where we touch gets a change in voltage. The screen can tell exactly where it's touched by the voltage change. It should also work with a sausage or any other similar material.
passages: Sensitive skin (electronic device)
  
  Sensitive skin, also known as sensate skin or electronic skin (e-skin), is an electronic sensing skin placed on the surface of a machine such as a robotic arm. The goal of the skin is to sense important environmental parameters—such as proximity to objects, heat, moisture, and direct touch sensations. Examples of a sensitive skin have been made by a group at the University of Tokyo led by Prof. Takao Someya.
   Although some standard Capacitance detection methods are projective, in the sense that they can be used to detect a finger through a non-conductive surface, they are very sensitive to fluctuations in temperature, which expand or contract the sensing plates, causing fluctuations in the capacitance of these plates. These fluctuations result in a lot of background noise, so a strong finger signal is required for accurate detection. This limits applications to those where the finger directly touches the sensing element or is sensed through a relatively thin non-conductive surface .
  
  Section::::Technologies.:Capacitive.:Projected capacitance.
   Skin conductance is not under conscious control. Instead, it is modulated autonomously by sympathetic activity which drives human behavior, cognitive and emotional states on a subconscious level. Skin conductance, therefore, offers direct insights into autonomous emotional regulation.
  
  Human extremities, including fingers, palms, and soles of feet display different bio-electrical phenomena.They can be detected with an EDA meter, a device that displays the change electrical conductance between two points over time. The two current paths are along the surface of the skin and through the body. Active measuring involves sending a small amount of current through the body.
   Due to the top layer of a PCT being glass, it is sturdier than less-expensive resistive touch technology. 
  
  Unlike traditional capacitive touch technology, it is possible for a PCT system to sense a passive stylus or gloved finger. However, moisture on the surface of the panel, high humidity, or collected dust can interfere with performance.
  
  These environmental factors, however, are not a problem with 'fine wire' based touchscreens due to the fact that wire based touchscreens have a much lower 'parasitic' capacitance, and there is greater distance between neighbouring conductors.
   Touchpads operate in one of several ways, including capacitive sensing and resistive touchscreen. The most common technology used in the 2010s senses the change of capacitance where a finger touches the pad. Capacitance-based touchpads will not sense the tip of a pencil or other similar ungrounded or non-conducting implement. Fingers insulated by a glove may also be problematic.
   Unlike a resistive touchscreen, some capacitive touchscreens cannot be used to detect a finger through electrically insulating material, such as gloves. This disadvantage especially affects usability in consumer electronics, such as touch tablet PCs and capacitive smartphones in cold weather when people may be wearing gloves. It can be overcome with a special capacitive stylus, or a special-application glove with an embroidered patch of conductive thread allowing electrical contact with the user's fingertip.
   Another form of “artificial skin” has been created out of flexible semiconductor materials that can sense touch for those with prosthetic limbs. The artificial skin is anticipated to augment robotics in conducting rudimentary jobs that would be considered delicate and require sensitive “touch”. Scientists found that by applying a layer of rubber with two parallel electrodes that stored electrical charges inside of the artificial skin, tiny amounts of pressure could be detected. When pressure is exerted, the electrical charge in the rubber is changed and the change is detected by the electrodes.
   Visual representation of applied pressure has been one area of interest in development of tactile sensors. The Bao Group at Stanford University have designed an electrochromically active electronic skin that changes color with different amounts of applied pressure. Applied pressure can also be visualized by incorporation of active-matrix organic light-emitting diode displays which emit light when pressure is applied.
  
  Section::::Sensing ability of electronic skin.:Other sensing applications.
   The history of electrovibration goes back to 1954. It was first discovered by accident and E. Mallinckrodt, A. L. Hughes and W. Sleator Jr. reported “... that dragging a dry finger over a conductive surface covered with a thin insulating layer and excited with a 110 V signal, created a characteristic rubbery feeling”. In their experiment, the finger and the metal surface create a capacitive setup. The attraction force created between the finger and the surface was too weak to perceive, but it generated a rubbery sensation when the finger was moving on the surface. This sensation was named "electrovibration" by the group. Today, Senseg and Disney Research are developing technology that could bring electrovibration to modern touchscreen devices.
   Section::::Sensing ability of electronic skin.:Tactile sensors.
  
  Applied pressure can be measured by monitoring changes in resistance or capacitance. Coplanar interdigitated electrodes embedded on single-layer graphene have been shown to provide pressure sensitivity for applied pressure as low as 0.11 kPa through measuring changes in capacitance. Piezoresistive sensors have also shown high levels of sensitivity.
   The "capacitive shunt method", described in an application note by Analog Devices (not to be confused with analog devices), senses the change in capacitance between a transmitter and receiver that are on opposite sides of the sensor. The transmitter creates an electric field which oscillates at 200–300 kHz. If a ground point, such as the finger, is placed between the transmitter and receiver, some of the field lines are shunted away, decreasing the apparent capacitance. Trackpads such as those found in some Blackberry smartphones work optically, like an optical computer mouse.
  
  Section::::Manufacturing.
  
  Major manufacturers include:
  
  BULLET::::- Alps Electric Corporation
  
  BULLET::::- Apple
   In 2015, the Apple Watch was launched. It uses skin tap sensing to deliver notifications and alerts from the mobile phone of the watch wearer.
  
  Section::::Implementation.
  
  Section::::Implementation.:Vibration.
   Wired gloves and bodysuits allow gestural interaction to occur. These send hand or body position and movement information to the computer using sensors.
  
  Section::::3D user interfaces.:Output devices.
   Resistive touchscreens typically have high resolution (4096 x 4096 DPI or higher), providing accurate touch control. Because the touchscreen responds to pressure on its surface, contact can be made with a finger or any other pointing device.
  
  Section::::Comparison with other touchscreen technology.
   Multi-touch sensing and processing occurs via an ASIC sensor that is attached to the touch surface. Usually, separate companies make the ASIC and screen that combine into a touch screen; conversely, a trackpad's surface and ASIC are usually manufactured by the same company. There have been large companies in recent years that have expanded into the growing multi-touch industry, with systems designed for everything from the casual user to multinational organizations.
   In some designs, voltage applied to this grid creates a uniform electrostatic field, which can be measured. When a conductive object, such as a finger, comes into contact with a PCT panel, it distorts the local electrostatic field at that point. This is measurable as a change in capacitance. If a finger bridges the gap between two of the "tracks", the charge field is further interrupted and detected by the controller. The capacitance can be changed and measured at every individual point on the grid. This system is able to accurately track touches.
   The group's work has been published in Nature, Science and Advanced Materials journals, as well as "Proceedings of the (United States) National Academy of Sciences". They have demonstrated pressure and temperature measurements at a resolution of a few millimeters, as well as organic transistors, organic solar cells, and organic light emitting diodes about 1 μm thick.
   Section::::Technologies.:Surface acoustic wave.
  
  Surface acoustic wave (SAW) technology uses ultrasonic waves that pass over the touchscreen panel. When the panel is touched, a portion of the wave is absorbed. The change in ultrasonic waves is processed by the controller to determine the position of the touch event. Surface acoustic wave touchscreen panels can be damaged by outside elements. Contaminants on the surface can also interfere with the functionality of the touchscreen.
  
  Section::::Technologies.:Capacitive.
   For people who must grip the active portion of the screen or must set their entire hand down on the screen, alternative touchscreen technologies are available, such as active touchscreen in which only the stylus creates input and skin touches are rejected. However, newer touchscreen technologies allow the use of multi-touch without the aforementioned vectoring issues.
   Current robotic and prosthetic hands receive far less tactile information than the human hand. Recent research has developed a tactile sensor array that mimics the mechanical properties and touch receptors of human fingertips. The sensor array is constructed as a rigid core surrounded by conductive fluid contained by an elastomeric skin. Electrodes are mounted on the surface of the rigid core and are connected to an impedance-measuring device within the core. When the artificial skin touches an object the fluid path around the electrodes is deformed, producing impedance changes that map the forces received from the object. The researchers expect that an important function of such artificial fingertips will be adjusting robotic grip on held objects.
   The first mobile phone with a capacitive touchscreen was LG Prada released in May 2007 (which was before the first iPhone).
  
  Touchscreens would not be popularly used for video games until the release of the Nintendo DS in 2004. Until recently, most consumer touchscreens could only sense one point of contact at a time, and few have had the capability to sense how hard one is touching. This has changed with the commercialization of multi-touch technology, and the Apple Watch being released with a force-sensitive display in April 2015.
   Handheld technologies use a panel that carries an electrical charge. When a finger touches the screen, the touch disrupts the panel's electrical field. The disruption is registered as a computer event (gesture) and may be sent to the software, which may then initiates a response to the gesture event.
  
  In the past few years, several companies have released products that use multi-touch. In an attempt to make the expensive technology more accessible, hobbyists have also published methods of constructing DIY touchscreens.
  
  Section::::Implementations.:Capacitive.
  
  Capacitive technologies include:
  
  BULLET::::- Surface Capacitive Technology or Near Field Imaging (NFI)
  
  BULLET::::- Projected Capacitive Touch (PCT)
   Electronic skin
  
  Electronic skin refers to flexible, stretchable and self-healing electronics that are able to mimic functionalities of human or animal skin. The broad class of materials often contain sensing abilities that are intended to reproduce the capabilities of human skin to respond to environmental factors such as changes in heat and pressure.
   An integrated circuit (IC) is located either within the printed material or within an enclosure that cradles the printed material. This IC receives a signal when a switch is innervated. The firmware located within the IC communicates via Universal Serial Bus (USB) either connected to a cable, or using a wireless protocol adapter to a reference database that can reside on media within a computer or appliance. Upon receipt of the coordinate structure from the firmware, the database correlates the position with a pre-determined link or pathway to digital content or execution command for an application. After correlating the link with the pathway, a signal is sent to retrieve and render the terminal of the path.
   Advances in electronic skin research focuses on designing materials that are stretchy, robust, and flexible. Research in the individual fields of flexible electronics and tactile sensing has progressed greatly; however, electronic skin design attempts to bring together advances in many areas of materials research without sacrificing individual benefits from each field. The successful combination of flexible and stretchable mechanical properties with sensors and the ability to self-heal would open the door to many possible applications including soft robotics, prosthetics, artificial intelligence and health monitoring.
  