# Static Factors in Sitting Comfort: Seat Foam Properties, Temperature, and Contact Pressure
## 1 Introduction  
Public transportation users demand faster, safer, and more comfortable journeys. During a journey, a passenger spends most of the time seated, so it assumes a crucial impact on comfort [1]. This way, seats should fulfill their primary support function and, in addition, provide increased comfort. Perceived comfort is divided into static and dynamic categories, which are dependent on the presence or absence of vibration. Both are strongly connected, as one influences the other. Thus, they must be evaluated to properly quantify comfort levels. In a low-vibration situation, static comfort dominates the overall comfort evaluation, while in the opposite situation, the dynamics analysis is more relevant [2; 3]. When seated, passengers are in close contact with seat cushions, which are considered one of the most critical factors affecting sitting comfort [4; 5]. Therefore, evaluating the foam's mechanical properties and its influence on passengers' comfort is critical. Traditionally, seat cushions are made of polyurethane (PU) foams. Those have been universally used in multiple industries due to their versatile physical properties, such as excellent noise absorption and low thermal conductivity, comfort properties, low cost, and easy processing. PU foams are cellular polymers that are typically produced by reacting an isocyanate with polyols in the presence of blowing and crosslinking agents and additives [6; 7; 8]. There is no consensus on comfort definition. Thus, comfort quantitative evaluation is still ambiguous [9; 10; 11; 12]. Several authors suggest evaluating seat physical characteristics and their relation with sitting comfort as the first approach to overcome the aforementioned limitation. From those physical properties, the hardness, the support factor (SAG factor),and the hysteresis loss were identified as strongly affecting seating comfort, becoming the main focus of most studies. Hardness corresponds to the foam's firmness, the SAG factor indicates the supporting property, and the hysteresis loss correlates with resilience [7]. Understanding the influence of the seat's physical characteristics on passengers' sitting impressions is fundamental for designing the ideal foam, which can be accomplished by changing its mechanical properties to achieve higher comfort levels [3; 13]. According to [PERSON] et al. 's [14] comfort model, which is applied as the reference model for the present study, comfort and discomfort result from the interaction between the product, the user, and the environment and can be evaluated by a combination of psychological and physical factors. The users' feelings define psychological factors, being assessed based on questionnaires and rating scales (subjective evaluation). In contrast, the physical parameters depend on the human body weight, shape, and anthropometric dimensions, being defined by objective measurements [13; 15; 16; 17]. The passenger's pressure on the seat cushion strongly affects the seating interaction due to the biomechanical load imposed by the user's musculoskeletal structure. That load leads to skin and tissue deformation, which may affect blood flow and the musculoskeletal system [18]. The skin has thermoreceptors capable of detecting temperature changes and four types of mechanoreceptors that provide information about touch, pressure, vibration, and cutaneous tension. The Meissner corpuscles are exceptionally efficient in transducing information regarding tactile and sensitive changes. The Pacinian corpuscles, responsible for detecting vibration, have a lower response threshold than the Meissner ones. [PERSON]'s disks and [PERSON]'s corpuscles are slowly adapted mechanoreceptors. The former specialises in detecting pressure changes, and the latter can detect stretch and shear stresses. The thermoreceptors and the mechanoreceptors record the stressed surface changes and send the information to the brain, leading to comfort perception [19; 20; 21].