Unlike the commonly known type of exoskeletons—parallel-limb exoskeletons [46,47]—series-limb exoskeletons [48,49,50] offer interesting extensions to their wearers [51]. They are designed to be connected with a user in series, extending or transforming the extremities of the user to provide new properties. PowerSkip [49] is a wearable device that provides leg extensions that help a user jump extremely high, and SpringWalker [50] is a contraption that allows faster and more economic maneuvering with longer legs. Gloves used by NASA [52] change stiffness so that their users can effortlessly hold heavy objects, and gecko-inspired climbing gloves [53] give its wearer a temporary ability to climb on vertical walls.

Supernumerary robotics (SR) [39,40,41] is a recently established field of research that studies additional robotic fingers or limbs that support physical tasks by reducing load or aiding in object manipulation. SR fingers [40,41] are designed to work in synergy with a user’s fingers, offering improved hand grasp capabilities. Shoulder-mounted SR arms [39,54] are designed to support their wearer in accomplishing assembly jobs that would normally require two men with extra robotic arms instead. Similarly, a pair of robotic legs [55] allows the user to maneuver on extreme terrain and balance better. Georgia Tech demonstrated a three-armed drumming system [56] where the high-level control of the robotic arm is performed by brain signals and software that fine-tunes the rhythm to fit into the music being played.

Seminal art works by Stelarc, such as The Third Arm [57], are muscle-controlled robotic contraptions worn on the body. Through the muscles around his abdomen, the artist was able to control the third arm, along with two biological ones, and write characters with all three arms at the same time. Horn, in her art pieces Pencil Mask (1972) and Two Hands Scratching Both Walls (1974–1975), showcases structures worn on the body that heighten the wearer’s senses.

Existing limbs may also be used and controlled through computational inputs—a computational device acquires the capability to drive muscles, e.g., independent from a user’s intention. Furthermore, the computational control of muscles can allow new synergies between individual muscles, which is not possible with our internal neuromuscular structure. Tokyo University showcased the use of functional electrical stimulation (FES) as a means of controlling the human body [44], allowing a person to control another user’s body as a surrogate for teleoperation. Inferno [58] is an art performance in which participants are asked to wear robotic exoskeletons and are controlled/forced to dance in synchronization. In other words, a user’s body is temporarily controlled by another human or a machine operator.

Sensory channels combined with motor actions allow active perception—like turning the head to see from different perspectives, therefore, better spatial cognition. An addition to new sensing capabilities to a body part may open up opportunities for new sensorimotor mappings between action and sensing. In contrast to the external morphology section, sensorimotor retargeting focusses on the data transmitted or received in accordance to motor actions.

A study by Bach-y-Rita [10], mentioned earlier in this paper, reports that people perceive visual feeds from a camera through tactile feedback on their back as coming from in front of the camera, not from their back. Subdermal implantation of a magnet has also been studied to explore sensory alterations of the fingertips [59]. FingerReader [42] is a camera worn on a finger that reads texts shown to the camera to its wearer. Effectively, the finger turns into an information probe where a user can move and point in different directions to read spatially-relevant information. Such a connection between motor behavior and sensing has been explained by Haans and IJsselsteijn: “When the mediating technology allows for these sensorimotor contingencies to be registered, or enacted, one will experience the digital content as having a specific physicality in time and space” [60].

The body can also be modified to output data. Warwick [61] implanted a microchip in his wrist that makes contact with the nerve bundles, where the nervous signals are read and transmitted to computers. Ishin-Den-Shin [43] is a wrist-worn device that conducts sound through bones, enabling one to send audio messages to another person by gently tapping on the ear.

Research in shape-changing robotics hints at how we can further extend the adaptability of our body shape and, therefore, our physical capabilities. We have developed a set of wrist-worn programmable robotic joints that offers additional action capabilities to users [62] (Figure 2). The main motivation is to create a modular system of which the functionality can be reconfigured in a more “programmatic” manner, and offer a platform through which a multitude of applications can be realized. The application includes modes where the joints act as extra actuators (or fingers), passive support structure (such as for holding a notepad for one-handed note-taking), or haptic user interfaces for VR games. Different modes of operation require different controller schemes, however, the input signal used for the control remains unchanged. In other words, software modifications alone can enable the user to have interchangeable (robotic) functions controlled by the same set of muscle movements.

Modularity in a morphology-extending prosthetic device can further improve the adaptability of such technologies. We are developing a modular assembly kit consisting of basic sensor or motor components that somewhat resemble those of human joints (Figure 3). The modules, with a universal connector design, can easily be arranged in different orders—like we arrange different programming blocks in software development. To date, we implemented servomotor modules with two different axes of rotations, as well as sensor modules so an engineer can add new sensing capabilities of choice to the module chain. Customization of end-effectors is possible through 3D printing pluggable fingertip modules, allowing the robotic device to have effectors designed for specific tasks, such as for plucking guitar strings (Figure 4).

HCI researchers have used bio-acoustic [74], capacitive [75,76], and magnetic [77] sensing to appropriate the human skin as input surface. In order to use on-skin surfaces for visual output, researchers have explored mounting small projectors [74] or displays [78] onto the body. The Vivalnk tattoo, a commercially available tattoo sticker, has near-field communication (NFC) capability [79]. For a more in-depth review of wearable designs for skin interfaces, please refer to [65].

The most critical challenge in designing on-skin interfaces is the minimization of electronics and form factor. In order to address this, several fabrication processes have been proposed. In material science, epidermal electronics integrated with soft, stretchable forms that have skin-like properties have been explored [80]. However, incorporating sensing capabilities with such form factors poses challenges in manufacturing and cost [81,82]. More widely accessible fabrication processes have been proposed as well. iSkin is a silicone-based skin overlay for touch sensing [76], however, it requires the use of material-science grade materials (PDMS with carbon) and its thickness of 700 µm is much thicker than that of epidermal electronics (0.8 µm). Skintillates prints conductive silver ink on temporary tattoo paper to create 36 µm thick on-skin electrical traces [66].

SkinSense [65] is a chemically-produced tattoo-like interface that has a layer of stretchable electrodes (Figure 5). Components, such as sensors or LEDs, can be connected, enabling designs of interactive tattoos. An example is composed of RGB LEDs and IR receivers, therefore, a facial tattoo can be controlled by a remote controller for interactive play and aesthetics purposes. DuoSkin [64] is a fabrication process that generates ultra-thin functional devices which can be attached directly onto the skin. It leverages gold metal leaf, a material that is cheap, skin-friendly, and robust for everyday wear. By overlaying multiple layers of gold metal leaf, DuoSkin devices can function as a capacitive touchpad or as an antenna for wireless communication (Figure 6). Importantly, DuoSkin incorporates aesthetic customization similar to body decorations, giving form to exposed interfaces that so far have mostly been concealed.

A chemically-produced interactive tattoo can offer minimal and specific-purpose computing powers to the skin (Figure 7). Due to its spatial affinity to the skin, such tattoos can acquire biosignals from the body or respond to environmental conditions to inform a user of potential risk. In the project Dermal Abyss, biochemical sensors are used to acquire and display information about the physiological state of the user. The biosensors developed are capable of indexing the concentration of sodium, glucose, and pH in the interstitial fluid of the skin, and output the collected data through colors on a tattoo. Currently, the experiments are conducted ex vivo using a pig skin due to clinical experimentation challenges.

EarthTones [83] is a cosmetics-inspired wearable chemical sensing powder to detect and display environmental conditions (Figure 8). It presents an alternative to current mainstream wearable displays (e.g., smart watches, LED textiles), which are digital and battery-laden. Instead, EarthTones present a direction towards soft, analog wearable displays composed of chemical-sensing powders for increased wearability, which are also built upon habitual makeup practices. The powders detect elevated levels of carbon monoxide (CO), ultraviolet (UV) rays, and ozone (O₃) and trigger corresponding color change to increase environmental awareness.

The phenomenon of gut instinct reveals a sensorial path from the inside out, where feelings are directed toward the external world. A work from Critchley and colleagues [84] suggests that activity in the right anterior insular/opercular cortex predicted subjects’ interoceptive accuracy, which correlates to awareness of emotions. For example, a person may perceive themselves to be anxious if they notice elevated heart rate. Interoceptive sensitivity and accessibility are further proven to correlate with self-awareness, wellness, and social behaviors [85].

There are very few interactive applications that utilize interoception, however, in the field of medical and psychological therapies [86,87,88] interoceptive exposures have been widely used for mitigating hypersensitivity to internal alerts. Interoceptive exposure (IE) is a procedure in which a patient goes through a set of exercises to produce a sensation that is close to what causes panic disorder [86,87], irritable bowel syndrome [88], or another problematic condition. In that way, the patient becomes desensitized to the interoceptive cues, thereby effectively removing the trigger for certain disorders.

Modulation of interoceptive signals has also been studied in VR environments. A study suggests visual stimuli that are close to one’s heartbeat signal can alter the perceived self-physiology [89]. Although the study’s aim is to affect interoception through non-interoceptive stimuli, it suggests that, in immersive environments, such techniques can be used to attribute an emotional presence beyond visual manifestation in a virtual environment.

There are well-known examples that use multisensory stimulation to alter body representation, such as the rubber hand illusion [30], and mirror-touch synesthesia [90]. An experiment by Tajadura-Jiménez [91] shows that the representation of key properties of one's body, like its length, is affected by the sound of one's actions. Recent studies have shown that synchronous interpersonal multisensory stimulation (IMS) produces quantifiable changes of mental representation of one’s self and dilutes the differentiation with others [92]. BeAnotherLab [93] created a project called The Machine To Be Another that uses virtual reality to encourage empathy by swapping the user’s body with the body of another gender.

A somatosensory intervention opens up opportunities for creating new sensations by triggering sensory receptors programmatically. Such possibility can particularly reinforce immersion with virtual avatars, where an embodiment into avatars different from the self can potentially accompany corresponding somatosensory feedback.

TreeSense [94] is a tactile VR experience of being a tree (Figure 9). Through a headset, one can see their body in a form of a tree with two main branches moving along with their arms. In addition to that, a pair of EMS devices on each arm triggers “novel” tactile sensations that are not innately possible. The triggered sensations are the result of tactile induction and actuation of multiple muscles at varying intensities, in a way that will not happen normally. Therefore, a feeling of something crawling on the body or growing from inside the body can be simulated.

The focus of this work is on somatosensory interventions through the electrical actuation of internal parts of the body. However, other sources, such as mechanical vibration, sound, or electronic implants, could expand how novel somatosensory experiences can be created. The use of programmatic induction of new somatosensory cues can be utilized for natural modalities for informing a user of their physiological state, or for virtual body ownership in VR with non-human configurations.