## 1 Introduction  
A red blood cell (RBC) transfusion is a critical and lifesaving medical procedure for anaemia caused by disease leading to inadequate bone marrow production or increased RBC breakdown, or by blood loss due to trauma or surgery [1; 2; 3; 4]. Therefore, in-vitro storage is important to fulfil both constant and sudden demands of RBCs for transfusion. According to the current blood bank procedures, RBCs can be stored up to 42 days at 2-4 \({}^{\circ}\)C in a preservative solution [1; 5], and studies on alternative preservative solutions, rejuvenation procedures, anaerobic storage conditions and temperature effects [2; 6; 7] are being conducted to ensure the maximum storage time for a healthy and safe RBC supply. The stored RBCs are suitable for transfusion if the mean haemolysis is below 0.8% and more than 75% RBCs survive the first 24 h of post-transfusion period [5; 8; 9; 10; 11]. However, concerns have been raised about the quality of stored RBCs as several transfusion complications [12] are observed with respect to the shelf life of the stored RBCs. During the storage, RBCs undergo several biochemical, structural and functional changes, usually known as storage lesion [2; 4; 5; 12; 13; 14; 15; 16; 17; 18; 19; 20]. These changes include slowed metabolism with a decrease of adenosine triphosphate (ATP) concentration, oxidative damage with structural changes to band 3 protein and lipid peroxidation, functional loss of cation pumps and consequent loss of intracellular potassium and accumulation of sodium, and apoptotic changes of membrane phospholipids and membrane vesiculation [3; 19; 21; 22; 23]. Most of these changes that occur due to decreased metabolism such as depletion of ATP and 2,3-diphosphoglycerate (DPG) and cation imbalance are reversible under in vivo conditions. However, protein and lipid modifications, morphology changes, and membrane vesiculation are not reversible under in vivo conditions [15; 22]. These irreversible modifications can affect the in vivo cell survival leading to adverse post-transfusion outcomes. Therefore, it is important to measure the quality of each RBC unit prior to use, and the RBC deformability is a key indicator of RBC quality and post-transfusion viability [8; 24; 25; 26]. RBC deformability is primarily influenced by mechanical and geometrical factors of the cell such as cell surface area and volume, elasticity and viscosity of the cell membrane, and volume and viscosity of the cytosol [27; 28; 29; 30; 31; 32; 33]. Therefore, the changes in the cell membrane structure and its mechanical properties adversely affect the cell deformability, and less deformable RBCs can obstruct capillaries and require significantly higher transit time to navigate through the microvasculature leading to decreased levels of oxygen delivery to organs [34; 35; 36]. In addition, less deformable RBCs are promptly removed from the circulation at the spleen [14; 37]. Furthermore, the storage lesion gradually transforms the healthy biconcave discocyte morphology into echinocyte and sphero-echinocyte morphologies [20; 21], and the cell becomes smaller and denser [38]. In addition to the in-vitro RBC storage, certain other pathophysiological conditions such as hereditary haemolytic disorders (i.e., spherocytosis, elliptoctosis and ovalocytosis) [27; 39], sickle cell disease, malaria [40; 41; 42; 43; 44; 45; 46; 47], and shape-transforming conditions (i.e., stomatocytogenic and echinocytogenic conditions) [48; 49; 50; 51; 52; 53; 54] can alter healthy biconcave discocyte morphology. The stomatocytogenic and echinocytogenic shape-transforming conditions transform the usual biconcave discocyte shape into stomatocytes and echinocytes, respectively at constant cell surface area and volume [49; 50; 51; 52; 53]. These shape changes are reversible to the stage where sphero-stomatocyte and sphero-echinocyte are formed [55].