## 1 Introduction
The cause of the broad range of values is still not understood well, although it can most likely be attributed to the subjective perception of the load, the difference in biological material behavior and anatomical topological features, and even the time the tissue is under the load [28]. Hence, most of the authors suggested that product handles should follow the hand's shape in its optimal grasp posture for increased comfort, safety, and performance [29]. This ensures even deformation rates of the hand soft tissue, which results in more uniform contact pressure with lower peak values. However, size and shape optimization for a wide target population is impossible, due to the variability of the hand anthropometric values (hand/finger length, hand width, finger circumference, etc.) between users. Accommodation of all users with single sized handles is, thereby, not possible, which results in suboptimal grasping with higher contact pressure on the soft tissue, higher muscle activity, lower stability, lower performance and safety for many users [12]. Companies usually also do not provide their product with various handle sizes to minimize the effect of hand size and shape variation [30]. It has been shown that product handle material has an important influence on the tactile perception and, hence, comfort during use. However, most of the authors considered quasi-stiff materials (steel, plastic, wood) and ignored the possibility to improve handle ergonomics further by using deformable materials [31; 32]. Most of the handheld product handles are currently made of various plastic materials, due to manufacturing reasons [33]. Products that require high stability are also coated with rubber-like material for increased friction between the product handle and the hand. Plastic and rubber-like materials are magnitudes stiffer compared to the stiffness of soft tissue; hence, almost all deformationsduring grasping are attributed to the hand's soft tissue [32]. Sub-optimally designed handles and the distinctive non-linear mechanical behavior of rapid stiffening of the soft tissue under compression leads to a sudden rise in contact pressure on the hand's soft tissue, resulting in uneven contact pressure distribution with high peak values exceeding PDT or even PDT values. Researchers tried to mitigate this problem by using elastomeric foam materials as outer interface handle materials. Results have shown improved tactile perception with increased subjective comfort rating [34]. This can be attributed to the deformation of the foam interface material during grasping, since the foam interface material provides more uniform contact pressure distribution and lowers the peak contact pressure. However, users also reported the feeling of loss of stability during grasping, which can be explained by excessive foam deformation during grasping. Hence, improved tactile perception can be achieved with deformable material that stays undeformed at lower grasping forces and deforms when the critical contact pressure is reached. The Finite Element Method (FEM) has already been utilized in the past to model and simulate the biomechanical system human hand grasping product handles with various interface materials and to define the material parameters for improved tactile perception [31; 32]. Three-dimensional printing technology has also been used to develop a methodology for manufacturing the proposed materials obtained in the numerical analysis [35]. Results have shown the feasibility and repeatability of the methodology and manufacturing process for testing subjective responses. Due to the lack of investigation and understanding of the influence of material stiffness on the human tactile perception, in this paper, we developed a methodology to manufacture product interface materials with various stiffnesses and to evaluate the tactile perception using a subjective comfort rating questionnaire.