# A Prototype Mechatronic Device for Upper Limb Rehabilitation and Analysis of Its Functionality
## 3 Concept and Design Solution for the Rehabilitation Device
### Objects Used for Exercises
Moreover, surface texture alters the dexterity required for manipulation--[PERSON] et al. [100] found that rubber-based elements increased successful grasps by approximately 8% compared to those made of wood or other materials. All elements used in the preliminary functional testing of the newly developed BBT-based rehabilitation device were fabricated from lightweight, low-density materials. Spherical objects were produced from soft, closed-cell polyurethane foam, while block-type items were made using ABS plastic. Five distinct spherical object variants were developed (Figure 8):  
* No. 1: Smooth and uniform surface;
* No. 2: Spherical indentations;
* No. 3: Variable-shaped surface protrusions;
* No. 4: Spherical protrusions;
* No. 5: Cylindrical protrusions. The baseline object was a smooth sphere (No. 1, Figure 8b). Three variants involved the addition of surface features to smaller spheres (Nos. 3 and 4, Figure 8b), while another incorporated spherical indentations of two distinct diameters (No. 2, Figure 8a). The fifth object (No. 5) maintained the base diameter of the smooth sphere but included cylindrical surface protrusions, providing a means of evaluating grip sensitivity. Each object type was colour-coded (dark blue, light blue, yellow, pink, and red) to support task differentiation and the scaling of training difficulty. The dimensions of indentations and protrusions were selected based on the proportional geometry of human fingers [101]. A geometric overview of selected rehabilitation objects is shown in Table 1.  
In the prototype, modular cuboid blocks were used, allowing different geometric configurations through interconnection. Objects were manufactured in various colours (e.g., blue, yellow, red, green) and in both single- and two-colour variants (Figure 9a). Figure 8: Geometric variations of spherical objects (**a**) and physical object models (**b**). For pilot studies, pre-shaped coloured plastic blocks were utilised (Figure 9b, Table 2). However, in the final version, the objects may be fabricated from various materials, including heavier and textured variants. To stimulate tactile receptors and the palmar surface of the hand, certain objects feature regular or irregular surface protrusions, selected according to the individual's degree of impairment and progress in therapy. Object No. 3 was designed with two colours--one dominant--facilitating tasks involving orientation recognition and object manipulation. Users are instructed to align the object to match a visual on-screen reference. Where required, users must rotate the object using one hand only, increasing the task's complexity. Object No. 2 was engineered with two rounded edges to improve orientation within the storage compartment. Objects Nos. 4 through 7 shared identical base dimensions but differed in height, with the tallest fitting into a dedicated storage module. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Dimensions, [mm]**} & \multicolumn{5}{c}{**Object Number**} \\ \cline{2-7}  & **1** & **2** & **3** & **4** & **5** & **6** & **7** \\ \hline A & 64 & 62 & 64 & 32 & 32 & 32 \\ \hline B & 32 & 32 & 64 & 32 & 32 & 32 & 32 \\ \hline C & 54 & 64 & 34 & 19 & 43 & 62 & 100 \\ \hline R & - & 19 & - & - & - & - \\ \hline \end{tabular}
\end{table} 
Table 2: Geometry of cubes used for calibration and initial testing. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Dimensions, [mm]**} & \multicolumn{5}{c}{**Object Number**} \\ \cline{2-7}  & **1** & **2** & **3** & **4** & **5** \\ \hline D\({}_{1}\) & 62 & 70 & 62 & 64 & 72 \\ \hline D\({}_{2}\) & - & - & - & - & 40 \\ \hline d\({}_{1}\) & - & 58 & 15 & 8 & 16 \\ \hline d\({}_{2}\) & - & 29 & 8 & - & 12 \\ \hline h & - & \(0.05\times\) d\({}_{1,2}\) & - &