## 4 Discussion  
Experimental data from six individual donors, ranging from two to twelve samples each, were evaluated to generate S-N (Woehler) curves. Although scattering of the data points was observed for all groups, coefficients of determination between 0.54 and 0.88 were achieved with a combined \(R^{2}\) of 0.65. In a previous study, [PERSON] et al. used 25 Achilles tendons for cyclic testing, resulting in an S-N curve with \(R^{2}\) = 0.34 [8]. The scattering of the individual data points in this and other studies may likely be related due to natural inconsistencies of human tendons. The individual measurements of the tendon samples highly vary, even within the same individuals, due to their natural geometry. Moreover, the state of the tissue is dependent on the donor's age, sex, and health status. The preservation method and water content following the harvesting of the sample also bias the results [18; 23]. A variety of approaches to specimen clamping has been deployed to date. Methods that hold fibers of the outer layers only, including gluing or cryogenic clamping, commonly result in altered testing strength and stiffness [10]. [PERSON] et al. air-dried the ends of the specimens to be clamped instead, while keeping the remaining parts of the same sample moist to reduce the tissues' thickness and flexibility in the clamping area [10]. While this technique may facilitate the clamping, it also causes stress concentrations at the transition area between the dry and the moist parts [10; 24]. Selecting another approach, [PERSON] et al. used sandpaper-covered plates for fatigue testing of rat tendons [25]. This technique seems rather unsuitable for fatigue testing of human samples, as marked slippage was observed with relevant impacts on the accuracy of the results [15]. Equally, cryogenic clamps have been applied in studies [12]. This being a time-consuming mounting technology, the low temperatures also result in altered tissue properties and facilitate intrinsic specimen failure at the transition areas, where tissue changes from frozen to unfrozen parts. [PERSON] et al. introduced a new design of titanium clamps with lateral block boards and asymmetrical teeth jaws that does not require the samples to be frozen. However, due to the stress caused by the clamps, sample failure was predominantly observed in the clamping area and not in the intended central section, indicating significant influence of the clamps used [26]. A method called partial plastination was introduced by [PERSON] et al., aiming at minimizing the occurring measurement errors due to material slippage without involving such freezing-related issues [13; 27]. This dehydrates and impregnates the tissues' ends with resin, forming a flat and non-contractile surface to enhance mounting stability [13; 27]. [PERSON] et al. quantified the material slippage occurring even in partially plastinated  
Figure 4: Combined S–N curve for all tendon samples. Approximated graph from the data points using power regression. samples and found resulting measurement errors of up to 18%, indicating the need for digital image correlation [28]. As a consequence, fatigue testing is hampered by the existing clamping techniques. The new method presented here allows immediate testing of the sample without changing the tissue state by decreasing temperature, exposing it to chemicals and leading to denaturation, dehydration, or degreasing it or creating a sharp interface, with influence on the mechanical behavior. This may allow for more realistic in vitro tests of engineered tissues, e.g., following acellularization or chemical treatment, thus allowing conclusions to be derived on their durability before cellular ingrowth and remodeling occurs [19; 29; 30; 31; 32]. The internal pyramid structure of the here-presented clamps allows testing without chemical alterations while simultaneously reducing slippage. [PERSON] et al. introduced different sharp- and smooth-edged pyramid designs for tissues of different thicknesses [14; 15].