###### Abstract  
Improved surgical procedures and implant developments for ligament or tendon repair require an in-depth understanding of tissue load-deformation and fatigue properties. Cyclic testing will provide crucial information on the behavior of these materials under reoccurring loads and on fatigue strength. Sparse data are available describing soft tissue behavior under cyclic loading. To examine fatigue strength, a new technology was trialed deploying 3D-printing to facilitate and standardize cyclic tests aiming to determine tendon fatigue behavior. Cadaveric flexor digitorum tendons were harvested and mounted for tensile testing with no tapering being made, using 3D-printed damps and holder arms, while ensuring a consistent testing length. Loads ranging between 200 to 510 N were applied at a frequency of 4 Hz, and cycles to failure ranged between 8 and >260,000. S-N curves (Woehler curves) were generated based on the peak stresses and cycles to failure. Power regression yielded a combined coefficient of determination of stress and cycles to failure of \(R^{2}\) = 0.65, while the individual coefficients for tissues of single donors ranged between \(R^{2}\) = 0.54 and \(R^{2}\) = 0.88. The here-presented results demonstrate that S-N curves of human tendons can be obtained using a standardized setting deploying 3D-printing technology.  
3D (three-dimensional)-printing technology; fatigue life; high cycle fatigue tests; ligament; soft tissue mechanics; tendon biomechanics; S-N test; Woehler curve +
Footnote †: journal:  
Academic Editor: [PERSON]  
25 June 2022526 July 202256 July 2022526 July 2  
tests can be further objectified by so-called S-N curves (Woehler curves), indicating a function of cyclic stress and the number of cycles to failure.  
S-N curves are commonplace in materials science of inanimate matter. In life science and especially in the biomechanics of soft tissues, however, fatigue testing of human tissues has so far only been conducted by a few groups resulting in accurate S-N curves [6; 7; 8; 9]. Due to the natural characteristics of biological soft tissues resulting in transverse contraction, clamping forms a major issue [7; 10; 11]. Even in static testing scenarios or in low-cycle tests only entailing a few hundreds of cycles or less, continued slippage may result in premature failure at the clamping interface or in a complete slip out of the sample from the clamping. Considering that biological samples are built to withstand tens of thousands of cycles under versatile conditions, standard clamping techniques [7; 12; 13] are deemed unsuitable for cyclic conditions ex vivo.  
Our group recently introduced a novel clamping technique using 3D-printing technology [14; 15]. The modular and adaptable design allows soft tissue testing to be facilitated and standardized, overcoming limitations of conventional clamping methods. Similar approaches have now been adapted by others for quasi-static testing procedures [16]. This methods-focused report builds upon recent developments in 3D-printed clamping [14; 15], modifying these techniques to provide standardized mounting options to assess tendon fatigue behavior based on cyclic testing. A secondary aim of this approach was to generate Woehler (S-N) curves from human flexor tendons ex vivo.  
The aim was to establish a method to allow for cyclic fatigue testing of biological tissues, minimizing slippage, and thereby generating valuable data for material models under dynamic conditions. It was hypothesized that using a new 3D-printed design, S-N curves of flexor tendons can be generated reflecting a moderate (\(R^{2}\geq 0.5\)) force-cycle relationship.