## 1 Introduction  
Empowering even untrained operators to autonomously perform decentralized testing of bioliquids outside the specialized laboratory infrastructure, such as hospital labs, general practitioners' offices, in the field, or even patient self-testing at home, represents the key driver for the development of so-called \"Point-of-Care\" (PoC) devices. While immunomodnotographic devices based on lateral flow, e.g., familiar from blood glucose or pregnancy testing, have been around for several decades, aging populations, the rise of chronic and life-style related diseases correlating with increasing age, wealth, nutritional habits and sedentary lifestyles of global populations, or recent pandemic threats create a dire need for frequent, ubiquitous, simple, reliable, cost-efficient and user-friendly monitoring of biochemical indicators in various body fluids. Similar local testing capabilities are also desirable for often quasi-continuous point-of-use (\"PoU\") or at-line monitoring of industrial processes, infrastructures and the environment. Since their emergence in the early 1990s [1, 2, 3, 4, 5, 6, 7], microfluidic Lab-on-a-Chip technologies, often quite synonymously referred to as micro Total Analysis Systems (\"uTAS\"), have been touted as a highly promising candidate to provide compact and inexpensive devices, offering full sample-to-answer automation of multiplexed bioassay protocols. Over the recent decades, manifold platform technologies have been established, which might be categorized by their pumping, valving, actuation, manufacturing, assay or detection schemes. The field has substantially diversified in the meantime, and many, mostly application specific solutions, have been successfully advanced to a wealth of demonstrators and an increasing number of viable products. Due to their plain concept of flow control by a simple spindle motor and its analogies to, at the time, booming optical data storage technologies, such as CD and DVD, centrifugal microfluidic systems [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34] were one of the earliest microfluidic platforms to be developed, starting around the mid-1990s [8; 9; 10; 13; 14; 35; 36]; interestingly, these \"Lab-on-a-Disc\" (LoaD) systems, which conceptually resemble conventional centrifugal analyzers, were first commercialized before finding their way into academia around the turn of the millennium. By now, a sizable, global cohort of companies and research groups are involved in advancing and maturing LoaD technologies [10; 13; 37; 38; 39; 40; 41; 42; 43]. Note that, for the sake of simplicity, we use the term \"disc\" throughout this paper; however, a plethora of geometries, e.g., minidiscs, segments, microscope slides or tubes, which substantially deviate from the standard format of conventional optical data storage media displaying 12 cm (outer) and 1.5 cm (inner hole) diameter and 1.2 mm thickness, have been introduced. Furthermore, different mounting concepts to the axis of rotation have been implemented. Yet, centrifugal flow control only depends on the radial position and the (square of the) spin rate, while not depending on the particular shape of the carrier chip and its mode of attachment to the spindle. By virtue of the ubiquitous and unidirectional centrifugal field acting simultaneously on all liquids loaded to their potentially fast spinning device, valving concepts play a paramount role in LoaD technologies. While there have been active modes of control, e.g., by co-rotating pumps [44; 45], sacrificial barriers [46; 47; 48] opened by stimuli delivered by instrument mounted external units, or the release of stored chemical energy [49], rotationally actuated schemes have attracted distinct attention due to their smooth alignment with the low complexity concept of LoaD platforms with their rugged instrument \"playing\" a mostly single-use disc cartridge.