Tactile-STAR: A Novel Tactile STimulator And Recorder System for Evaluating and Improving Tactile Perception

Many neurological diseases impair the motor and somatosensory systems. While several different technologies are used in clinical practice to assess and improve motor functions, somatosensation is evaluated subjectively with qualitative clinical scales. Treatment of somatosensory deficits has received limited attention. To bridge the gap between the assessment and training of motor vs. somatosensory abilities, we designed, developed, and tested a novel, low-cost, two-component (bimanual) mechatronic system targeting tactile somatosensation: the Tactile-STAR—a tactile stimulator and recorder. The stimulator is an actuated pantograph structure driven by two servomotors, with an end-effector covered by a rubber material that can apply two different types of skin stimulation: brush and stretch. The stimulator has a modular design, and can be used to test the tactile perception in different parts of the body such as the hand, arm, leg, big toe, etc. The recorder is a passive pantograph that can measure hand motion using two potentiometers. The recorder can serve multiple purposes: participants can move its handle to match the direction and amplitude of the tactile stimulator, or they can use it as a master manipulator to control the tactile stimulator as a slave. Our ultimate goal is to assess and affect tactile acuity and somatosensory deficits. To demonstrate the feasibility of our novel system, we tested the Tactile-STAR with 16 healthy individuals and with three stroke survivors using the skin-brush stimulation. We verified that the system enables the mapping of tactile perception on the hand in both populations. We also tested the extent to which 30 min of training in healthy individuals led to an improvement of tactile perception. The results provide a first demonstration of the ability of this new system to characterize tactile perception in healthy individuals, as well as a quantification of the magnitude and pattern of tactile impairment in a small cohort of stroke survivors. The finding that short-term training with Tactile-STAR can improve the acuity of tactile perception in healthy individuals suggests that Tactile-STAR may have utility as a therapeutic intervention for somatosensory deficits.


Direct and inverse kinematics of the stimulator and recorder devices
The pantograph structure has two degrees of freedom. Here, we report the kinematics equations (Campion, Wang, and Hayward 2005) that we used for the direct (Figure S1A), and the inverse problem ( Figure S1B).
The horizontal position of the end effector, P3=(x3, y3) T is determined by the two angles θ1, θ5, and the dimensions of the links ai, i=1,…, 5, through the following direct kinematics equations: where P1 and P5 ( Figure S1) correspond to the positions of the two motors for the stimulator device, and the potentiometers for the recorder (a5 is the distance between them and is fixed to be equal in the two devices). P3 corresponds to the end effector of each device. Our devices use a symmetrical pantograph; this means that the length of a1 is the same of a4 and a2=a3 in both devices. The arm a1 is called the proximal arm, and the arm a2 is called the distal arm. The center of the reference frame (x, y) is fixed in P1. Note that our equation P3 ignores for both x3 and y3 the redundant non-feasible solution.
. Model of kinematics of the pantograph structure used for the direct and the inverse problem of the kinematics The inverse kinematics equations are used to find the angles θ1 and θ5 from the spatial coordinates x3, y3 of P3. Specifically, to solve the inverse kinematic problem the pantograph is divided into three triangles as in Figure S1 Panel B. The angle α1 is the angle of the triangle composed by the arms a1 and a2. The angles β1 and α5 are the angles of the middle triangle formed in the pentagon.
The angle β5 is the angle of the triangle made by the arm a4 and a3.

Development of the device through 3D printers
All parts of the pantograph structure were manufactured by a Form2 stereolithographic printer (FormLabs Inc.) with a resolution of 0.05 mm. This printer polymerizes a liquid methacrylate photopolymer resin through a laser, with a spot size of 0.140 mm. We used Tough Resin to print the pantograph structure, and it was selected for its favorable mechanical properties, including Young's modulus and tensile strength. After polymerization, the printed part was washed twice with 90% isopropyl alcohol (IPA) for ten minutes each. IPA is a solvent that removes leftover uncured resin from the printed part. After the wash, the parts were placed under a 405 nm LED light for 120 minutes at 60°C. This second treatment increases the thermal and mechanical properties of the part (e.g., tensile strength, Young's modulus, flexural strength and modulus). Young's modulus before and after the treatment are 1.7 and 2.7 GPa, and the flexural strength at 5% strain is 20.8MPa and 60.6MPa, respectively. The remaining parts were manufactured by a DeltaWASP 2040 printer (WASP project). The DeltaWASP is a fused deposition model that works on an additive principle by laying down a filament of thermoplastic material (polylactic acid, PLA). This printer can print parts with lager dimension at lower cost, without need for additional processing after printing. Figure S2. Left: CAD model of the connection between two links: a ball-bearing was fixed in one arm, and a brass axle was rigidly connected with the other arm. On the top of the axle is attached a plastic ring, which must maintain the connection fixed during the movement of the structure. Right: Drawing of a proximal arm of the pantograph structure (i.e., one of the arms attached to a motor) and its T-shaped cross-section (measurements are in mm).