3.2. Platform Design and Development

The core element of the platform is a specifically designed toy that embeds the dedicated hardware capable of sampling, digitizing and wirelessly transmitting in real-time pressure measurements to the mobile device (typically a tablet or a smartphone) on which the exergames are played (Figure 1). The toy realized here is based on Lego^TM that is a typical construction familiar to children of young age, and has the shape of a cottage (Figure 2a) that hides the sensing architecture.

The modular sensor architecture, embedded inside a toy, is represented here. (a) Force is measured at the prescribed contact point of the object through a load cell. (b) The signal is amplified, (c) sampled and filtered by a micro-controller and (d) transmitted to a remote host, where (e) it is used to animate an avatar inside a graphical game environment (cf. Figure 2a).

(a) The toy developed to measure force is shown here. A personalized Lego^TM construction hosts the sensor and the associated hardware. Notice that all the hardware components are not visible. The output of the sensing device is wirelessly connected to the tablet on which the game is run. (b) A zoom of the housing of the load cell, covered with Lego^TM bricks. (c) A Lego^TM shaft allows raising the cell such that pinch movements are also allowed. The flat tile covering the pressure point is highlighted in green. (d) The same structure can be used to accommodate different fingers apertures (depending on age, rehabilitation stage etc.). (e) The sensor and its Lego^TM case can be detached from the housing and used as a free-sensor for more complex grasping movements.

The pressure sensor adopted should be robust enough to resist possible damage when handled by children. To this aim, we have adopted a load cell (13 mm × 13 mm × 50 mm) obtained disassembling a consumer kitchen electronic scale (Zheben, SF-400) with a pressure range of 0–7 kg. The load cell has been encapsulated inside a Lego^TM structure and the sensitive part of the cell has been covered by a flat Lego^TM red tile (Figure 2c) to indicate clearly where the child has to press. The encapsulated load cell has then been positioned inside the Lego cottage in an area where the children can comfortably interact with.

The processing electronics is housed inside the cottage and connected to the load cell. The force signal is amplified by a differential operational amplifier with an adjustable gain (TI INA125P); a particular configuration has been adopted to maximize stability as shown in Appendix A. The amplifier is connected to the micro-controller of an Arduino board that samples the signal, filters it, and sends it wirelessly through a Bluetooth chip HC-06 transceiver [58] at a frequency of 250 Hertz. Bluetooth 2.0 + EDR (Enhanced Data Rate) has been adopted to limit current sink and therefore power consumption. To maximize attractiveness, children can personalize the cottage with Lego^TM characters or other constructions, provided that these do not interfere with the sensing area.

In the configuration shown in Figure 2a,b, the child can activate the sensor by pressing with one finger. To accommodate pinch grasp, a rotating mechanism, made of a Lego^TM shaft inserted into two parallel bricks, has been designed and realized (Figure 2c). In this way, the Lego^TM case of the load cell is free to rotate and emerges from its housing; the child can thus squeeze the case with two opposite fingers as required by pinching. Different pinch amplitudes can be accommodated by simply making the load cell case thicker by adding flat bricks on the top of it (Figure 2d). Finally, for power grasp (palm grasp), the cell case can be detached from the cottage and used as a free-tracker (Figure 2e). In this way, the entire repertoire of basic prehensions of the human hand is covered.

Two ranges of force have been defined: from 0 to 3 kg and from 0 to 5 kg, according to age and rehabilitation state. The prototype is built on a 30 cm × 30 cm Lego^TM base and the weight of the construction is about 0.3 kg, which makes it easily portable. The accuracy and reliability have been fully tested as reported in Appendix B. Linearity and repeatability are in the order of 0.1%, which is 3 g for the 3 kg range and 5 g for the 5 kg range.

Such a high resolution is not required when the child exercises at home where the capability of applying a force above a given threshold is sufficient for training purposes. Under this hypothesis, the tracker can be simplified and a simpler sensing mechanism, based on the same principle on which a clothespin works, has been devised.

The sensing object is constituted of a pet-toy realized through 3D printing (FABTotum Personal Fabricator printer). This has been designed to be attractive to children: a crocodile made of two parts: the upper and lower jaw pivoting around the end of the mouth (Figure 3). The child has to press the tail of the crocodile to make it open its mouth; such movement is resisted by a set of rubber bands applied to the mouth whose strength depends on their number and regulates the force that has to be exerted by the child. A smart button, constituted of a digital contact, a microcontroller, and a radio transmitter, is inserted through a frontal slot inside a lodge created in the bottom jaw of the crocodile, to make battery substitution easy. The button is used to sense when the user has exerted enough pressure to open the mouth. The smart button chosen here is a Camkix Bluetooth Remote Shutter; typically used as a remote shutter of digital cameras, it has the robustness required for use by children. Such a device transmits a digital pulse whenever the contact is open and is compatible with the blue-tooth HID (Human Interface Device) profile; it can be easily interfaced with smartphones and tablets operating system as it is recognized automatically, like regular keyboards or mice, and can therefore be used easily as an input device.

A remote shutter (a) is inserted inside the lower jaw of the crocodile (b) Force has to be exerted on the tail to open the mouth (c) When the mouth is closed a tooth in the upper jaw presses against the button trigger (grey area, red arrow, d), while when the mouth is open the tooth is raised and the button is releasedThe amount of force is regulated through a rubber band. The crocodile’s size is: 100 mm × 45 mm × 35 mm and weighs about 50 g.

For the mobility exercises, we leverage the multi-touch display of mobile terminals like smartphones or tablets. Such devices accurately detect the position of one or more fingers at 30 Hz and can therefore easily be used to measure fingers tapping, sliding, or pinching over the device surface (see videos in the Supplementary Materials). This tablet or smartphone will be the same that works as a host for the pressure-sensing device.

The sensing devices and the host constitute the platform that is given to the families to support autonomous rehabilitation of their child. It can be regarded as the client component of the typical client-server architecture used in the telerehabilitation domain [6]: the client runs the exergames, acquires the pressure data from the sensing devices, logs them and attaches a time stamp. At the end of the exercise, it transmits these data to a server, that, in turn, processes the data to compute the results and show them to the clinicians. Moreover, the server sends to the client the list of the exergames chosen by the clinicians, along with their difficulty degree (Figure 4).

The whole system architecture is shown. It consists of the following components: a mobile terminal as a host on which the exergames are run. The host is connected wireless with the smart toy used as tracker and transmits to the server the data acquired during the interaction. Such data can then be accessed through a web application form any browser by the therapist.

The server is composed itself of two main components: the data storage and a graphical application that allows clinicians to analyze rehabilitation progression and to configure the rehabilitation sessions. Such a graphical application is implemented as a web-application to allow clinicians to access the data from anywhere, even outside the hospital.

When the application is started on the mobile terminal, the user sees the exercises grouped into a suite: the user can choose among the different exergames assigned by the therapist. The exercises will be already configured at the right difficulty and to be played either with the tablet or with the pressure sensor as a tracker (Figure 5).

The exergames prescribed by the clinician are shown. They have already been configured at the correct difficulty level.