## 1 Introduction
These pathophysiological conditions can affect the inter-protein and protein-to-lipid linkages of the cell membrane [56], and alter the mechanical and structural integrity of the cell membrane, leading to reduced cell deformability. Therefore, the RBC deformability is a potential measure of cell viability subjected to morphological, structural and functional changes, and provide valuable insights into the physiology, cell biology and biorheology under pathophysiological conditions. Different techniques have been used to investigate RBC deformability, and descriptions of these techniques can be found in [17; 25; 29; 33; 42; 43; 47; 57; 58; 59; 60]. Optical tweezers is one such technique and provides a highly sensitive assessment of the cell deformability at the single cell level. With optical tweezers, it is possible to trap, manipulate and displace a living cell or a part of it without damage, either directly or using specific handles such as dielectric beads of silica [27; 36; 38; 40; 71; 72]. These investigations improve the basic understanding of RBC behaviour under healthy and pathophysiological conditions, in order to be integrated into clinical applications. However, a more detailed understanding on RBC ageing during storage, ageing-influenced changes in RBC metabolism, structural and functional aspects, is required to improve the quality, efficacy and safety of stored RBCs and thereby to prevent or reduce the adverse post-transfusion outcomes. Numerical modelling is a desirable approach to investigate the fundamental mechanics of cell deformability over the experimental investigations, which can be challenging and costly [67; 68; 73]. Accurate numerical modelling combined with experimental measurements can provide valuable insight on RBC quality and viability. This paper proposes a numerical approach inspired from the previous research for RBC morphology predictions [74; 75; 76] and for analysis of RBC deformations [39; 58] to investigate, for the first time, the deformability and morphology changes of RBCs subjected to storage lesion. A three-dimensional coarse-grained (CG)-RBC membrane model based on the bilayer-couple model (BCM) [76] was employed to predict RBC echinocytosis during 42 days of in-vitro storage at 4 \({}^{\circ}\)C in saline-adenine-glucose-manitol (SAGM) preservative solution. The evolution of RBC morphology, its membrane shear modulus and bending modulus, and cell surface area and volume based on analogous experimental measurements [8; 47], was incorporated into the model to better predict the deformability and morphological changes of stored RBCs. The deformability and morphology characteristics: cell shape factor, elongation index and membrane spicule details where applicable, of stored RBCs having discocyte, echinocyte I, echinocyte II, echinocyte III and sphero-echinocyte morphologies, were studied during the 42 days of in-vitro storage, and compared against analogous morphologies obtained via echinocytogenic shape-transforming conditions. The shape-transforming conditions have the potential to recover the morphological and deformability changes of stored RBCs, and to lower the influence of storage lesion. Therefore, the present study investigates the morphology and deformability characteristics of RBCs during in-vitro storage and compares them against echinocytic conditions. In addition, the influence of storage lesion-induced membrane structural defects on cell deformability and its recoverability of sphero-echinocyte morphology under optical tweezers stretching deformation was investigated. The following subsections briefly outline the evolution of RBC morphology and associated CG-RBC membrane model parameters for the prediction of echinocytosis, and discuss the deformability and morphology changes of RBCs during in-vitro storage. This study provides valuable predictions of the influence of storage lesion on RBC quality and viability, and is currently being executed to investigate the recoverability of deformability and morphology changes of RBCs undergoing storage lesion.