## 2 Methods  
### Design and Fabrication of the Proposed \"Universal\" Cam  
As mentioned earlier, the previously developed cam-based pulse pressure simulator incorporates a cam that is able to accurately generate the pulse pressure waveform of a specific age group. However, it requires a new cam for each of the desired age-specific pulse waveforms. It is not feasible to fabricate and replace cams each time to generate a wide range of age-specific pulse pressure waveforms. To address the limitations of the simulator, the current study proposes a universal cam that can conveniently and accurately generate pulse pressure waveforms for all age groups. This section describes the design of the proposed universal cam.  
As aging progresses, the elasticity of blood vessels changes along with the size and speed of reflected waves, resulting in changes in blood pressure waveforms with a tendency for each age [25]. Thus, the main design component of the proposed cam is to incorporate the overall trends of blood pressure with age in its profile that represents the magnitude of the pulse pressure for a range of age groups. This study uses four representative in vivo pulse pressure waveform data for teen and elderly age groups, and the curve models the mean pulse pressure of age groups by linear and polynomial regression to determine the cam profile. Figure 1a shows the normalized average pulse pressure waveforms of four age groups (15, 35, 65, and 85 years) obtained by using in vivo pulse pressure data collected from about 1000 human subjects [26]. The obtained average pulse pressure waveforms for these four age groups well represent humans' complex blood waveforms consisting of the forward wave generated from the heart and the reflected wave reflected from the branching point of the blood vessel and the heart valve [25]. Figure 1b shows the polynomial regression curve fitted to mean pulse pressure by age [27]. This curve is adopted to determine the first peak's magnitude of the pulse pressure between each of the four age groups. In order to design a continuously varying circular cam profile, the pulse pressure waveform, which is in the rectangular coordinate with pulse pressure and time axes, needs to be converted in the polar coordinate with radial and transverse axes. That is, one cycle (800 ms) of the pulse pressure waveform was converted to 360\({}^{\circ}\). Figure 1c shows the cam profiles for the four age groups obtained from the pulse pressure graphs in Figure 1a. The polynomial regression method is then employed on the four circular cam profiles in order to create the entire cam contour, as shown in Figure 1d.  
Upon designing the cam, a prototype cam was fabricated (see Figure 2). A 3D wire cutting technique was used to precisely machine the contour of the cam. A nonmagnetic and high-rigidity material (stainless-steel 304) was used. Note that four holes were cut  
Figure 1: The proposed cam design based on age-specific pulse pressure waveforms: (**a**) Normalized average pulse pressure waveforms for each age group; (**b**) Polynomial regression curves fitted to mean pulse pressure by age group (Reprinted with permission from Ref. [27]. 2010, Wolters Kluwer Health, Inc.); (**c**) A circular cam-type graph created by converting one cycle of the average pulse pressure waveform to 360 degrees; (**d**) The cam design created by regressively integrating four age-specific cam graphs.  
out inside the cam to reduce the overall weight of the cam and balance the cam during its operation with rotational motion. Note also that this cam is going to be referred to as a cylindrical or three-dimensional (or 3D) cam because its profile continuously varies along the axial direction as opposed to a conventional cam, which is a thin disk whose profile is fixed (can be considered as a 2D cam). Thus, the universal cam (from the functionality of the cam point of view) and the 3D cam (in terms of the shape of the cam) will be interchangeably used to refer to the cam proposed and fabricated in this study.