## 3 In Vitro Wear Testing  
The prevalence of primary and revision total hip arthroplasty is increasing constantly. By 2030, the demand for primary total hip arthroplasties, performed in the United States, is estimated to grow by 174% to 572,000 [23]. According to studies by [PERSON] and collaborators [19], the demand for hip replacement reviews is expected to grow by 137% and 601%, respectively, between 2005 and 2030; this is linked to an increase in the demand for total hip and knee arthroplasty. The economic consequences of these demands on hospitals are burdensome [24; 25]. Since total joint replacement has been applied to younger and more active patients, current limitations are related to the rapid wear of the components. The purpose of research in tribology is, reasonably, the minimization or elimination of wear in joint tribo-systems. Wear is a phenomenon in which the worn material is expelled from the contact between two surfaces in the form of debris of particles. The expelled debris can cause unwanted issues in the tissues and can cause loss of bone mass around the implant with consequent loosening of the fixation [26]. Testing methods in science are traditionally called by their Latin names: in vitro comes from the Latin term \"in glass\" [27]. In particular, the term refers to studies of biological properties that are done in a tube rather than in a human or animal. This testing method involves experiments on biological matter (cells or tissues) outside of a living organism. The reference to glass is quite literal: in vitro experiments were historically conducted in a Petri dish. However, in vitro studies allow scientists to perform experiments that are also less expensive and can be done with fewer ethical and safety concerns. In vitro studies play an important role across all fields of medical research. In particular, research based on in vitro experiments is important in the preclinical validation of medical devices. This test must be regulated for the construction of a new medical device, which must be safe in order to be implanted in patients. The characterization of a new biomaterial can be performed according to the phases represented in the diagram in Figure 1.  
Biological and mechanical screening tests must be performed on the new biomaterials that are to be used in joint prosthetic implants in order to obtain the required quality control. Furthermore, wear tests must be performed, before clinical adoption, in order to acquire knowledge on the tribological phenomena that occur [28]. In experimental measurements, the physical laws govern the system and the simulation can represent the most realistic conditions of use. In this context, two categories of laboratory wear test equipment are common [28]:  
* The so-called quick tests, which provide information exclusively on the intrinsic features of the materials studied. These wear tests can be performed in dry and lubricated conditions and in different configurations. Measuring friction, wear, and material mechanical properties is a way for engineers to understand how materials will stand up to the rigors of biomedical application. These wear tests are fast and cheap but, unfortunately, they do not accurately reproduce the contact geometries and the kinematics of the real prostheses [29]. A schematic representation is given in Figure 2.
* Vice versa, the hip and knee joint wear simulators are machines in which real prostheses are mounted on these apparatuses, as intended in vivo, and tested in an environment that simulates physiological conditions. These simulators can reproduce a simplified level of walking, as specified in international guidelines [30; 31]. Recently, efforts are devoted to the simulation of a more demanding task that would reproduce the wear rates of daily living activities [32; 33]. A schematic representation is given in Figure 3.  
Figure 1: Flow chart showing the characterization of a new biomaterial. Figure 2: Schematic representation of the pin on disk tests.