## 1 Introduction  
Platelet adhesion and aggregation are shear stress dependent phenomena. A repertoire of different receptors promote rolling and adhesion to extracellular matrix proteins within the vascular wall and support homotypic and heterotypic cohesion between platelets and other blood cells [1]. A useful research tool to study these interactions are microfluidic flow assays, which we refer to here as the suite of microfluidic devices that have been developed to measure platelet function under physiologic and pathologic hemodynamics [2]. The most common adhesive substrate used in these assays is type I collagen [3], but a promising alternative is collagen related peptides [4], which has advantages both in micropatterning and dissecting the different functional roles of collagen on platelet adhesion and activation. Type I collagen is found in the vascular wall and plays, at least, three roles in supporting platelet adhesion and activation [5]. First, it binds von Willebrand factor (VWF) in the plasma. VWF is necessary for platelet adhesion at high shear rates (\(>\)600 s\({}^{-1}\)) by supporting rolling mediated by the glycoprotein Ib-V-IX complex [6]. Second, it has binding sites for the integrin \(\alpha_{2}\beta_{1}\) that supports firm adhesion [7]. Third, it has binding sites for glycoprotein VI (GPVI), which serves as a key signaling receptor for platelet activation [8; 9]. Type I collagen derived from animal and human sources has been the primary collagen substrate for platelet adhesion assays in parallel plate flow chambers. These flow chambers were the primary research tool for platelet and leukocyte adhesion from the 1980s to the 1990s [10]. In the 2000s, the parallel plate flow chamber was miniaturized with the growing popularity and accessibility of microfluidics [2]. This transition to smaller channels came with several benefits; lower blood volumes, higher throughput, anatomically inspired geometries, and the ability to micropattern adhesive substrates. Yet as channel sizes became smaller, the ability to homogenously pattern type I collagen emerged as a problem. Type I collagen fibers are 10-100 \(\upmu\)m long and adsorb to substrates in a random pattern [11; 12]. In parallel flow chambers with channel widths on the order of 10 mm this was not a problem because a large enough representative area could mask the microscale heterogeneity of the fiber distribution. In microfluidic devices, as the channel width approaches the collagen fiber length, it becomes more difficult to achieve homogeneity. One solution is to mechanically degrade the fibers into smaller fibers, for example, by sonication [13]. Another approach is to deposit the collagen at a high enough concentration to completely coat the surface [14]. However, very high surface concentrations of collagen can provide such a strong platelet activation signal as to mitigate the importance of other platelet autocrine signaling mediated by adenosine diphosphate (ADP) and thromboxane A2. Moreover, large collagen fibers can extend off the surface into the flowing blood, acting as a net for platelets and other blood cells, in an unpredictable manner [4]. Alternatives to native type I collagen fibers include collagen thin films [11], acid soluble collagen [15], type III collagen [16], and collagen peptides [4; 17]. Among these alternatives, collagen peptides appear to best mimic the platelet response to type I collagen. [PERSON] et al. reported that a combination of three peptides--VWF binding peptide (VWF-III), \(\alpha_{2}\beta_{1}\) binding peptide (GFOGER), and GPVI binding peptide or collagen related peptide (CRP)--supported platelet adhesion over a physiologic range of shear rates and resulted in platelet aggregates with similar volumes and morphologies as those formed on type I collagen [4]. These peptides, in combination with other adhesive proteins, have been used in microfluidic devices to probe the relative contribution of platelet receptors on adhesion, activation, and aggregation [18].