Optimal Halbach Configuration for Flow-through Immunomagnetic CTC Enrichment

Abstract

Due to the low frequency of circulating tumor cells (CTC), the standard CellSearch method of enumeration and isolation using a single tube of blood is insufficient to measure treatment effects consistently, or to steer personalized therapy. Using diagnostic leukapheresis this sample size can be increased; however, this also calls for a suitable new method to process larger sample inputs. In order to achieve this, we have optimized the immunomagnetic enrichment process using a flow-through magnetophoretic system. An overview of the major forces involved in magnetophoretic separation is provided and the model used for optimizing the magnetic configuration in flow through immunomagnetic enrichment is presented. The optimal Halbach array element size was calculated and both optimal and non-optimal arrays were built and tested using anti-EpCAM ferrofluid in combination with cell lines of varying EpCAM antigen expression. Experimentally measured distributions of the magnetic moment of the cell lines used for comparison were combined with predicted recoveries and fit to the experimental data. Resulting predictions agree with measured data within measurement uncertainty. The presented method can be used not only to optimize magnetophoretic separation using a variety of flow configurations but could also be adapted to optimize other (static) magnetic separation techniques.

Measuring antigen expression of cancer cell lines and circulating tumour cells

“…Using a cell line with a realistic EpCAM expression is even more relevant given the more recent insight into the existence of a subpopulation of EpCAM-low or even EpCAM-negative CTCs [9][10][11][12][13] . Several studies have already attempted to increase the sensitivity of their antigen based capture methods, either by the use of additional antigens 14,15 , or by increasing the magnetic force generated [16][17][18] , while many have also focussed on completely EpCAM independent capture methods 19 . These methods are all designed to also capture low EpCAM expressing cells; even the CellSearch system uses a specialized patented approach to bind sufficient magnetic particles to CTCs with low EpCAM expression 20 .…”

Basic Principles and Recent Advances in Magnetic Cell Separation

“…Cells will be more or less deflected from their laminar flow path according to their magnetic properties, which enables their sorting, and potentially transfer of target cells in another fluid [77]. The arrangement of several permanent magnets of different magnetization orientations expands the possibilities of this strategy, as in the Halbach configuration (Figure 11B) [71,165] strengthening the magnetic field on one side of the magnets. Alternatively, magnets arranged with opposing poles enable a large magnetic field gradient to be obtained.…”

Microfluidic‐Assisted CTC Isolation and In Situ Monitoring Using Smart Magnetic Microgels

“…CTCs can be captured using microfluidic devices with a wide variety of methods including direct positive antibody detection, [9] dielectrophoresis, [10] physical filtration, [11,12] and magnetophoresis. [13][14][15] Positive antibody capture can be implemented on a microfluidic chip by coating a micropost array with anti-epithelial cell adhesion molecule (EpCAM) or a similar antibody which will selectively bind to CTCs while other cells flow through uncaptured. [9] Dielectrophoresis can be used on a microfluidic platform to take advantage of the differences in charge between CTCs and other blood cells.…”

“…This might be due to the lack of localized force from the external magnets to the passing through microparticles. Several complex approaches have attempted to optimize the magnetic field gradient and to improve CTC capture efficiency (CE) in microfluidic devices such as using ferromagnetic wires in combination with a large permanent magnet, [ 13 ] a Halbach array of permanent magnets, [ 14 ] or fabricating the chip on a substrate containing a ferromagnetic wire array. [ 22 ] An alternating magnetic field generated by an electromagnet has also been used to selectively pass leukocytes through a filter membrane while trapping CTCs.…”