Cell Stretching Measurement Utilizing Viscoelastic Particle Focusing Sukgyun Cha, † Taeho Shin, ‡ Sung Sik Lee, § Wooyoung Shim, ∥ Gwang Lee, ∥,⊥ Seong Jae Lee, ∇ Younghun Kim,* ,† and Ju Min Kim* ,‡ † Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Republic of Korea ‡ Department of Chemical Engineering, Ajou University, Suwon 443-749, Republic of Korea § Institute of Biochemistry, ETH Zurich, Zurich, CH 8093, Switzerland ∥ Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Republic of Korea ⊥ Institute for Medical Sciences, Ajou University School of Medicine, Suwon 443-749, Republic of Korea ∇ Department of Polymer Engineering, The University of Suwon, Gyeonggi 445-743, Republic of Korea * S Supporting Information ABSTRACT: We present an efficient method for measuring cell stretching based on three-dimensional viscoelastic particle focusing. We suspended cells in a biocompatible viscoelastic medium [poly(vinylpyrrolidone) solution in phosphate-buf- fered saline]. The medium viscoelasticity significantly homo- genized the trajectories of cells along the centerline of a simple straight channel, which could not be achieved in conventional Newtonian media. More than 95% of red blood cells (RBCs) were successfully delivered to the stagnation point of a cross- slot microchannel and stretched by extensional flow. By computational simulations, we proved that this method prevents inaccuracies due to random lateral distributions of cells and, further, guarantees rotational-free cell stretching along the shear-free channel centerline. As a demonstration, we characterized the differences in RBC deformabilities among various heat treatments. Furthermore, we monitored the decrease of deformability due to nutrient starvation in human mesenchymal stem cells. We envisage that our novel method can be extended to versatile applications such as the detection of pathophysiological evolution in impaired RBCs due to malaria or diabetes and the monitoring of cell quality in stem cell therapeutics. C ell deformability is a promising label-free biomarker for the diagnosis of health. 1 For instance, the deformability of red blood cells (RBCs) in patients suffering from sickle-cell disease, malaria, or diabetes is distinguishable from that of healthy cells. 2 In addition, the deformability measurement has been suggested as an efficient platform for the quality control of stem cells. 1b Recent microfluidics-based platforms 3 for measuring cell deformability have attracted much attention due to their high throughput and potential for automated measurement. 1b,c,4 One such promising microfluidics-based approach is the visual measurement of cell stretching in various flow fields using video microscopy. 3b,5 Extensional flow fields are attractive for stretching deform- able materials such as cells, 3b,5 vesicles, 6 and DNA. 7 In this flow type, affine stretching exponentially increases as the strain experienced by the materials is accumulated, 8 and the purely extensional field is rotational-free, 9 thus, deformable materials can be highly stretched. 9 Microfluidic cross-slots have been used as platforms to generate extensional flow fields at the stagnation point (or the central region). 3b,6,7b However, this device presents limitations. First, the extensional field is not uniform inside a practical stretching device, and thus, the stretching of the deformable materials can be significantly affected by the trajectory of each material (or its initial lateral location). 10 Therefore, the seeming heterogeneity in the distribution of measured stretching may originate from the nonuniform field kinematics inside the stretching device, which can be confused with the intrinsic properties of the materials. Second, there is a low probability of finding the deformable materials near the stagnation point, where a strong and uniform extensional field is present. 7b One possible solution to these problems is to focus the materials along the channel centerline, which would simultaneously homogenize the material trajecto- ries into the cross-slot. 3b Recently, Gossett et al. demonstrated that particle focusing using inertial flows in asymmetrically curved channels could successfully homogenize the cell trajectories into the cross-slot, which was incorporated to measure the stretching of various deformable particle such as droplets and cells. 3b Received: September 23, 2012 Accepted: November 9, 2012 Published: November 19, 2012 Article pubs.acs.org/ac © 2012 American Chemical Society 10471 dx.doi.org/10.1021/ac302763n | Anal. Chem. 2012, 84, 10471−10477