The Science of Touch in Electronics Haptics, it Used to be All about Resonant Frequency

INTRODUCTION

The term “haptic” comes from a greek word haptikos, which means connect to sense of touch, and greek verb “haptesthai” which means to contact or to touch. Specifically, it describes the possibility to sense and manipulate the surrounding physical or virtual environment through touch [1]. The touching process can be performed by humans, machines, or a combination of both and the final feedback given to the user can be enhanced including additional sensory modalities like vision or hearing. The application of haptics technologies has encouraged various disciplines such as mechanics, biomedical, psychology, neurology, engineering and computer science to deeply investigate the relation between human touch and force feedback. [2].

Haptic technologies make use of actuators to apply forces to surfaces (skin in case of direct human contact) to create a tactile feedback. The actuators are able to generate, starting from an electrical stimulus, a mechanical motion. The first generation of haptic actuators made use of electromagnetic technology such as vibratory motors which displayed only a limited number of feedback sensations. Starting from these technologies, research led to various solution such as devices with touch coordinate specific responses or products with fully customizable actuation. The last generation of actuators is incorporating the most advanced haptic technology concepts, enabling pressure sensitivity and proportional response to the amount of force applied [3].

The efficiency, performance, and advancement of haptic interfaces depend on the type of feedback, manoeuvrability, manipulability, stimulation amplitude and actuator technology of the final product [4]. Factors such as feedback speed and force or interaction mode should also be considered in the design phase for effective results [5]. Considering only the feedback type, various example such as force, vibrotactile, and electrotactile feedback systems can be found, with a majority of the haptic devices based on force and vibrotactile feedback [2,6]. With many kinds of information to be defined for each end-use application and the necessity to respond quickly to market, the importance of haptic devices has skyrocketed [7].

Specifically, three different areas of haptic technologies have been defined until now: human haptics, machine haptics, and computer haptics [8]. Human haptics are based on kinesthetic information and tactile information. Machine haptics is defined as the use of machines to replace human touch autonomously or through haptic interfaces. Computer haptics has become prominent over the years and it is related to creating and rendering a tactile feedback feel of virtual objects to the user with the help of algorithms and software [3].

Even if produced by different technologies, the final haptic feedback perception of the final user belongs to one of the most complex human sensory systems which comprises the skin, our largest sensory organ, by involving signal exchange between the musculoskeletal system, the peripheral sensory nervous system of touch and the brain [9-10]. It subdivides in two main categories encompassing the kinesthetic (movements, forces and their sensory information) and tactile feedback. The tactile sensitivity to pressure, vibration and texture is the outcome of sensory neural endings and specialized receptors that are responsible for capturing mainly mechanical and thermal signals, thus providing enormous amount of information regarding our interaction with the environment as well as the state of human body.

The most sensitive areas of human skin are located at the pads of our fingertips, comprising thousands of sensory nerves per centimeter square of skin [11]. Human skin is composed by three primary layers [12] (the epidermis, the dermis and the subcutaneous) and it is divided into two main types (hairy and glabrous), each one presenting different characteristics regarding biomechanics, anatomy and sensory resources. The most part of human body is covered by hairy skin, while the glabrous skin, which has hairless and thicker characteristics, is present on the parts of the body such as hands, fingers and the soles of the feet, that are mostly used to interact with our surroundings by means of touch [13] –reasonably being these the main areas of focus for the development of haptic interfaces.

The successful re-creation of tactile haptic feedback should exploit the presence of mechanoreceptors located in the skin by mimicking the external stimulus within the appropriate spectrotemporal sensitivity and dynamic range characteristics referent to each type of the receptors present in the skin area of interest. In this manner, it would be possible to induce a natural response of the mechanoreceptors that will transduce the deformation of the skin into neural information (electrical action potentials or spikes) which are carried into the brain by the nerve fibers through the medial lemniscal pathway creating a conscious tactile perception [14], [15].

As starting point, it is vital to understand the function and the response characteristics of mechanoreceptors present in the glabrous skin. Four types of low-threshold mechanoreceptors are present in glabrous skin, each one responding in a different manner regarding skin deformation. Merkel cells and Meissner corpuscles are located under the epidermis, while Pacinian corpuscles and Ruffini endings are positioned in a deeper tissue. Merkel cells have round shape and are the most numerous receptors, reaching densities over 500 cm-2 in some areas of glabrous skin, such as fingertips, with single cell dimension of approximately 10 μm in diameter [16].

They are slowly adapting to external stimuli with receptive field size of approximately 0.5 mm, displaying large interval of frequency response, from low frequency skin vibration (few Hz) and up to frequencies over 100 Hz. These types of mechanoreceptor display good response towards static skin deformation as well as to transient skin deformation [17], [18]. The second in highest density mechanoreceptors are the oval shaped Meissner corpuscles, reaching approximatively 100 cm-2 density in most concentrated location with 50 × 150 μm in size [16], [19]–[21]. In comparison with Merkel cells, these corpuscles are rapidly adapting to external stimuli and are excited by transient or oscillating stimuli.

Their receptive field is about 3 to 4 mm with frequency response from 10 Hz to over 300 Hz and peak sensitivity around 60 Hz [17], [18]. Pacinian corpuscles, with maximum density of around 20 cm-2, present themselves with an oval appearance with typical length ranging between 0.5 and 2 mm [20], [22]. They display a high sensitivity towards transient and oscillating external stimuli by means of very large diffuse receptive field (over 20 mm). The high sensitivity (peaking around 250 Hz) can detect a transient skin displacement on nanometer-scale, covering in this way a large frequency range from 20 Hz up to over 800 Hz [23], [24].

Ruffini endings which are characterized by a 1.4 mm long spindle-shaped structures, reach a maximum density of 10 cm-2 around the nail and are as well located in deeper layers of the skin [25], [26]. They display high sensitivity toward detection of skin stretch, with lower response towards vibrations (in comparison with Pacinian corpuscles) over a large range of frequencies from just a few Hertz up to 300 Hz [27]. Depending on type, location, amplitude and frequency of a certain stimuli, it is possible to induce a response on more than one type of mechanoreceptors at the same time, creating in this way a mix of complex signals which are decoded by the brain in order to construct a conscious perception of the surrounding or interaction at play.

The complex system of human haptics creates numerous technological challenges in order to design and implement a haptic interface capable of successfully mimicking types of stimuli recognizable by the various mechanoreceptors. Merkel cells and Ruffini endings mostly respond to a low range of frequencies around few Hz, while the Meissner corpuscles have highest sensitivity to oscillation frequencies up to 100 Hz and the Pacinian corpuscles cover frequency range from tens up to 1000 Hz.

This is one of many parameters that a haptic actuator should be capable to covering in its working regime, where the other important ones are regarding temporal resolution which should be under 1 ms, maximum displacement range of 300 μm, spatial resolution of 100 μm and others [28]. Given the outstanding number of information that human body is able to analyse and respond to through tactile feedback, in the last years the haptic industry has been focusing to address some of these challenges by fabricating different types of actuators and devices in order to integrate haptic feedback and technologies the highest number of application possible.

Applications

Some of the most studied and under development application fields of haptic technologies are entertainment industry, tele-manipulation, VR/AR gaming, industrial training, specialized medical devices, to wearable gloves, military training, and surgical procedures [2]. Exploring virtual and augmented reality (VR/AR), these technologies offer interactive graphic and acoustic experiences in a virtual world or a computer-enhanced real environment and in the last years the development and market of these products has grown rapidly [29]. Considering the constant evolution of VR technologies, haptics is starting to play a significant role in providing feedback to enhance user multisensory experience. In addition to vibrotactile-based haptic actuators, new materials and technologies that can generate high-definition haptic feedback with compact, lightweight, flexible design are gaining significant attention.

Regarding medical simulation, haptic interfaces have proved to be very useful in defining fast and accurate interaction with environment, crucial for minimal invasive procedures. These include laparoscopy, interventional radiology and remote surgery. Especially in surgery, where touch is used to differentiate healthy tissue from disease infected tissue the benefit of using haptics technique is the lower fatigue and stress on the surgeons. Haptics are also used to provide a feedback from prosthetic limb to its wearer and help to the visually impaired [30].

Another interesting application field for haptic technology are teleoperations. Teleoperation describes the possibility to control machines from distance and in this technology the visual modality is the predominant source of perception, but material as well as surface characteristics can also be a very useful information, therefore the possibility to implement a haptic feedback was deeply investigated. Specifically, the implementation of haptics has led to an increase in precision of teleoperation by force and surface information feedback [31]. Given the different roles of haptic technology in each discipline and the variety of the application fields available, a deep investigation concerning design, customization and fabrication of haptic devices is of the utmost importance.

Currently, most haptic actuators are based on common electro-mechanical actuators and in particular, for the latest VR/ AR products from the gloves or hand controllers, haptic feedback mainly relies on this technology [32] since their stable performance over the years and sufficient feedback intensity. However, as downside, electro-mechanical motors are too stiff, bulky, and heavy to be used for wearable haptics or immersive VR/AR applications. Moreover, the high-power consumption which characterizes this kind of device, due to the current-driven activation mechanisms, is becoming a significant burden for haptic applications which require low dimension and small batteries.

However, the most critical issue is that it is difficult to recreate natural and complex tactile sensations. Actuators need to generate complex tactile response, such as fine textures, minuscule shapes, to possess a large force/displacement, spatial resolution, fast response time, and deliver the haptic feedback in the most natural and efficient way to the human skin. Given the difficult challenge for conventional motor-based actuators to meet all specifications [33], so far, there have been tremendous efforts to achieve those requirements, especially by utilizing emerging active materials. Specifically, many researchers focused on the investigation of suitable materials, in order to improve the efficiency of energy exchange with skin, to be skin-conformable, flexibility and stretchability.

Construction

Given these fundamental specifications, a growing number of haptic devices are designed based soft materials which include electrostatic, piezoelectric, electromagnetic, electrorheological, magnetorheological materials, each with distinct advantages and disadvantages. Among all soft material, a strongly emerging class of products are flexible actuators. They are often composed of polymers that respond to external stimulations with a size or shape change and are capable of converting electrical energy to mechanical energy and thus imparting a force and/or motion [34]. Usually, flexible actuators can be classified into two main categories, one is driven by other fields such as optical, thermal and chemical stimulus, and the other is driven by electrical field, named electro-active polymer (EAP) actuator. EAP actuators can be generally divided into two principal classes: dielectric and ionic. In ionic EAPs actuation is triggered by the displacement of ions inside the polymer; it requires only a few volts for actuation, but the ionic flow implies a high electrical current.

Examples of ionic EAPs are polyelectrolyte gels, ionic polymer-metal composites (IPMCs), conductive polymers and bucky gel actuator. Dielectric EAPs instead, are materials where the electric field-induced actuation response is initiated by the electrostatic attraction between oppositely charged conductive layers applied to the opposing surfaces of the polymer film [35]. The stress acts normal to the film surface and thus serves to compress the film along its thickness (z) and stretch the film laterally (in x and y).

This type of EAP typically requires a large actuation voltage to produce high electric fields (hundreds to thousands of volts) [36]. Examples of dielectric EAPs are electrostrictive polymers and piezoelectric elastomers, odd-numbered nylons (with an odd number of carbon atoms between amide groups) and polyurethane. One of the most studied dielectric EAP materials are piezoelectric polymers. Some example of this product are polyvinylidene fluoride (PVDF), poly(vinylidene fluorideco-trifluoroethylene) (PVDF–TrFE) (Fig.1) and polyvinyl fluoride (PVF) and each material presents specific properties based on the chemical structure. For examples, poly(vinylidene fluoride) (PVDF), polyamides, parylene-C are semicrystalline while polyimide, polyvinylidene chloride (PVDC) are amorphous piezoelectric materials.

The first type arranges positively and negatively charged ions in crystalline form when an electric field is applied. The latter type contains molecular dipoles in its molecular structure, the dipole of this polymer is aligned when electric field is applied above a glass transition temperature which results in higher coupling factors and increased dielectric constants, with a controllable mechanical flexibility.