2.1. Technological and Clinical Requirements
Designing a sensing device able to detect natural grasping actions and work simultaneously within an fMRI experiment is very challenging. Furthermore, there are additional constraints set by the target population (i.e., preterm and term newborn infants), which are not fulfilled by the majority of devices currently proposed in the literature [
6,
22]. In addition, the electromagnetic field inside the MRI environment can affect the working capability of most electronic devices proposed for grasp measurements and can induce currents in metal loops leading to infant contact burns [
23,
24]. In the same way, ferromagnetic elements widely used as components of electronic graspable devices may cause artefacts on the MRI images themselves, affecting diagnostic image quality [
25].
As newborn infants are inherently uncooperative subjects, a further requirement of the device is that their natural movements can be safely measured under natural conditions [
6,
19]. To allow for handling by a newborn infant, the proposed tool should be small and lightweight, and its shape should be appropriate to improve engagement and encourage palmar grasping [
26]. Moreover, high sensitivity to a low range of loads is necessary to detect the hand closure of a newborn infant, as well as being biocompatible and easily cleaned to reduce the risk of cross-infection between subjects [
15,
27].
To meet all of these technical and clinical requirements, an FBG sensor was chosen as the sensing element. A flexible and non-toxic silicone rubber (i.e., Dragon Skin™ 10) was used as a squeezable matrix to encapsulate the FBG sensor [
28]. Lastly, a polylactic acid (PLA) structure characterized by a linkage mechanism filled with the soft silicone was designed according to the index finger dimensions of an adult human, as this is typically used to insert into the infant’s palm to elicit the grasping reflex. The proposed solution is highly sensitive, robust, safe, affordable, and infant-friendly.
2.2. Fiber Bragg Gratings Working Principles
An FBG sensor is a distributed Bragg grating inscribed into a short segment of an optical fiber produced by creating a perturbation of the effective refractive index (ηeff) of the fiber core. In its simplest form, this periodic perturbation is sinusoidal with Λ, the constant grating pitch.
Generally, an FBG works in reflection as a notch filter; when a broadband spectrum of light is guided within the core and hits on the grating segment, a smooth Gaussian-shaped narrow spectrum is reflected and represents the output of the FBG. The center of the reflected Gaussian peak is known as the Bragg wavelength (λ
B) and satisfies the Bragg condition [
29].
Strain along the fiber longitudinal axis (ε) and temperature changes (ΔT) induce variations of Λ and η
eff, which result in a λ
B shift (Δλ
B) as in
The first term of Equation (2) represents the ε effect on the grating, with Pe the effective strain-optic constant; the second term represents the ΔT effect, with α
Λ and ξ denoting the thermal expansion and the thermo-optic coefficients of the fiber, respectively. When the effects of ΔT are negligible, Equation (2) in [
29] can be rewritten as
In this work, the SGD is able to detect grasping forces applied by a newborn infant through the compression of the silicone encapsulation that allows transducing loads applied on the device into longitudinal ε experienced by the FBG.
Based on the need for a physical connection to a dedicated device (i.e., the FBG interrogator) for enlightening the gratings and reading their outputs, a patch cord can be used to connect the FBG-based device inside the MRI scanner to the interrogator placed in the control room. This connection allows separation of the SGD inside the MRI scanner room from the measuring circuitry located in the control room.
2.3. Dragon Skin™ Silicones
Dragon Skin™ materials (commercialized by Smooth On Inc., Macungie, PA, USA) are platinum care bicomponent silicone rubbers used in a variety of scenarios, including medical fields (e.g., in prosthetics as cushioning materials and in physiological monitoring for flexible sensor development [
30,
31]). They are highly compliant and highly flexible. Moreover, they are skin safe in compliance with ISO 10993-10 (Biological evaluation of medical devices—Part 10: Tests for irritation and skin sensitization) [
28,
32].
Dragon Skin™ silicones are commercialized as liquid silicone rubbers in the form of an elastomer kit containing two components (A and B). Part A contains the platinum catalyst, part B the crosslinker [
33]. The manufacturer recommends mixing Dragon Skin™ silicones in the proportion of 1A:1B by weight and thinning the liquid formulation with Silicon Thinner™ to lower the viscosity of the mix for easier pouring and vacuum degassing [
34]. Curing temperature and time are also defined in the technical bulletin. Dragon Skin™ silicones are commercialized in different hardnesses expressed in terms of Shore A Scale: 10 (Very Fast, Fast, Medium, Slow), 20, and 30, with curing time ranging from 4 min to 45 min according to the silicone hardness.
In this work, the mechanical properties of Dragon Skin™ 10 Medium, 20, and 30 were investigated in terms of stress–strain properties. To better quantify the compression behavior of Dragon Skin™ silicones, the Young modulus E (expressed in MPa) was calculated to facilitate the selection of the material that best satisfies all the requirements mentioned above. Considering the scenario of interest and target population, the application of low squeezing forces will induce a compression on the silicone rubber with ε values lower than 10% of the rubber sample (l
0). Thus, the stress–strain relationship can be described by Hooke’s law [
33]:
where σ is the stress (i.e., the external force applied to the sample per its cross-sectional area) and ε is calculated as (l − l
0)/l
0.
The standard ISO 7743:2017 (Rubber, vulcanized or thermoplastic—Determination of compression stress–strain properties) was used for defining the dimensions of the cylindrical pieces used for the compression tests. The test piece B (method C) with a diameter of 17.8 ± 0.2 mm and a height of 25.0 ± 0.2 mm was chosen [
35]. Dragon Skin™ 10 Medium, 20, and 30 were poured into a cylindrical mold designed in Solidworks (Dassault Systemes, Waltham, MA, USA) and 3D printed using PLA. As suggested by the technical bulletin, the curing process was carried out at room temperature for 5 h, 4 h, and 16 h for Dragon Skin™ 10, 20, and 30, respectively. A total of fifteen specimens were fabricated, five pieces for each hardness level.
Compression tests were carried out using a testing machine (Instron
®, Norwood, MA, USA, model 3365, load cell with a range of measurement of ±10 N, an accuracy of 0.02 N, and a resolution of 10
−5 N) to apply controlled ε values (from 0% to 25% of l
0 as suggested by the standard ISO 7743:2017) in a quasi-static condition (at a low displacement rate of 2 mm·min
−1). The static assessment of each specimen was executed by positioning the cylinder-shaped sample between the lower and the upper plates of the machine, as shown in
Figure 1. A total of five repetitive compression tests were carried out at room temperature for a total of twenty tests per sample. The loads and the displacements applied by the compression machine to the specimen were recorded at a sampling frequency of 100 Hz using Instron
® Bluehill Universal software.
The stress–strain relationships (σ vs. ε) of each Dragon Skin™ material were obtained by processing the collected data through a custom algorithm. The mean value of experimental σ (σ
exp) and the repeatability of the system response were determined by calculating the related uncertainty across the twenty tests by considering a t-Student reference distribution with 19 degrees of freedom and a level of confidence of 95% [
36]. The best fitting line of the calibration curve was obtained, and its angular coefficient was calculated to estimate E. Lastly, the linearity error was calculated by using Equation (5) in terms of the maximum linearity error (% u
Lmax).
where σ
fsexp is the full-scale output range, σ
exp(ε) the experimental stress experienced by the sample at a specific ε, and σ
th(ε) is the theoretical stress obtained by the linear model at the same ε value.
Results showed E values of 0.24 MPa, 0.47 MPa, and 0.74 MPa for Dragon Skin™ 10, 20, and 30, respectively (see
Figure 2). R-square (R
2) values higher than 0.98 were found for all the responses, and linearity errors of 5.7%, 7.8%, and 8.9% were obtained for Dragon Skin™ 10, 20, and 30, respectively.
Our results quantified the mechanical properties of Dragon Skin™ in terms of compression behavior. As expected, Dragon Skin™ 10 was found to be more flexible than Dragon Skin™ 20 and Dragon Skin™ 30. In particular, the E value of Dragon Skin™ 10 was approximately half that of Dragon Skin™ 20 (i.e., 0.24 MPa vs. 0.47 MPa) and one-third that of Dragon Skin™ 30 (i.e., 0.24 MPa vs. 0.74 MPa). The high R
2 values (for all tests R
2 > 0.98) indicated good agreement between the experimental data and the linear model. Moreover, the Dragon Skin™ 10 response showed the best linear behavior as testified by the % u
Lmax value (i.e., 5.7%), which was lower than those of Dragon Skin™ 20 (i.e., 7.8%) and Dragon Skin™ 30 (i.e., 8.9%), as shown in the respective plots in
Figure 2. Finally, Dragon Skin™ 10 showed the best results in terms of uncertainty (maximum uncertainty of 0.004 MPa) when compared to Dragon Skin™ 20 (i.e., 0.01 MPa) and Dragon Skin™ 30 (i.e., 0.007 MPa).
These findings demonstrated that Dragon Skin™ 10 is best suited to meet the technical requirements of the SGD, particularly given the expected low ranges of Fext applied by newborn infants, which would likely require high flexibility; Dragon Skin™ 10 allows the SGD to be easily squeezed by a newborn for the Fext transduction into grating ε.