Contents I. Introduction
In recent years, many designs and fabrications of chip-based manipulator or separator using magnetic bead technology have been reported as critical issues for biomedical research [1]–[4]. Especially, a microfabricated magnetic cell separator for collection of white blood cells (WBCs) is considered to be one of many significant works in this field, which has mainly focused on miniaturized and integrated functions [5]. However, most of the applications generally have the following problems; complexity of its fabrication process, relatively lower yield in the applications, and high fabrication cost. Moreover, cell adhesion on a fluidic channel leads to a lower efficiency of cell separation [5]. In order to solve the problems including cell adhesion, we previously proposed a novel platform for cell separation—a droplet-based magnetically activated cell separator (DMACS) [6]. However, buffer injection by a micropipette for handling a microdroplet also led to an instability problem in its separation efficiency since irregular injection due to “stick & slip” phenomenon in a micropipette made a discrete flow. Therefore, we carried out experiments with advanced setup including a microsyringe pump. However, varying efficiencies in cell separation were not improved until we found that initial volume of the microdroplet is one of the major factors leading the varying efficiencies. We suppose that initial volume (the droplet volume of cell suspension before buffer injection process for dividing a hanging droplet into two fractions) decide distance from adhesion side on a coverslip to the bottom side of the hanging droplet, and is one of a main parameters that causes instability of efficiency since the magnetic flux density to attach positive cells decreases as distance from the adhesion side gets longer. Therefore, correlation between distance from the adhesion side on a coverslip to the bottom side of the hanging droplet, and initial volume was obtained through experiments. The variance of magnetic flux density under change of distance from the adhesion side was investigated based on simulation. Finally, separation experiments were performed by changing initial volumes to validate our hypothesis. In particular, positive cells in negative fraction under variation of initial volume were investigated whether the attachment of small numbers of positive cells to the adhesion side is due to weak magnetic flux density at the bottom side of the hanging droplet.
