## 1 Introduction  
The interaction of collagen with platelets plays a pivotal role in the haemostatic response [1,2]. During disease or injury endothelial cells that normally line blood vessel walls may become damaged, exposing the underlying collagens to the blood supply and permitting them to interact with the cellular components therein. Initially, the platelet receptor complex Glycoprotein Ib/V/IX (GpIb) binds indirectly to the exposed collagen surface mediated via immobilised von Willebrand Factor (VWF) [3]. This relatively weak interaction causes previously fast flowing platelets to roll in the direction of shear flow across the VWF-collagen surface as GpIb-VWF bonds are continuously formed and ruptured in front of and behind the platelet respectively. Rolling of the platelet ceases after firm attachment is achieved via integrin \(\alpha_{2}\beta_{1}\)-collagen interaction. This firm attachment is required for the collagen to interact with the low affinity GpVI receptor which is mainly responsible for the collagen induced platelet activation. After platelet activation plasma fibrinogen and VWF may then bind to the \(\alpha_{\rm{IIb}}\beta_{3}\) integrin receptors of adjacent platelets, securing the platelet plug in place and preventing more blood loss. If any component of this sequential process is either absent or defective, there may be catastrophic consequences. For example, Bernard-Soulier syndrome is a bleeding disorder caused by a deficiency of GpIb receptors on platelets, and von Willebrand disease, characterised by a tendency to bleed after minor trauma, is caused by a lack of functional VWF. Other problems arise when arteries develop atherosclerotic plaques, which cause endothelial cell surfaces to become activated, initiating platelet attachment, platelet activation and thrombus formation. In some cases the plaque ruptures, revealing collagens which recruit and activate nearby platelets as described above. If thrombus formation occurs in vital blood vessels, such as the coronary artery or cerebral vasculature, then a heart attack or stroke may occur. It should be clear therefore that a thorough understanding of all these processes is a prerequisite for any potential treatment or prevention strategy. For the purposes of this study we focus attention on the high affinity interaction of collagen [4,5] with the platelet receptor integrin \(\alpha_{2}\beta_{1}\), and in particular the \(\alpha_{2}\) I-domain which has been shown to recapitulate the adhesion properties of the entire receptor [6]. The integrins are heterodimeric transmembrane receptors with the subunits being held together by non-covalent interactions [7]. Integrins in general have been studied extensively using conventional ensemble techniques [8]. However, only limited studies of integrin-collagen interactions at the molecular level have thus far been conducted. The technique of Single Molecule Force Spectroscopy (SMFS) [9] has been used to measure the interaction between single integrin molecules and their respective ligands using the Atomic Force Microscope (AFM) [10]. These include integrin \(\alpha_{\rm{L}}\beta_{2}\) with its cognate ligand intercellular adhesion molecule-1 [11], integrin \(\alpha_{5}\beta_{1}\) with fibronectin [12, 13, 14, 15] and integrin \(\alpha_{\text{IIb}}\beta_{3}\) with fibrinogen [16]. These experiments either employ single-cell force spectroscopy (SCFS), for which single cells expressing integrin receptors are attached to AFM cantilever tips, or else recombinant protein forms are immobilised directly onto surfaces. Recently [PERSON] _et al_. [17] utilised SCFS to study the adhesion between cells expressing \(\alpha_{2}\beta_{1}\) and a collagen type I matrix. They first demonstrated that the mean maximal detachment force for successive approach-retract cycles was significantly greater for the cells expressing the integrins as compared with the wild type cells, which did not.