Analytical and Bioanalytical Chemistry

Electronic Supplementary Material

Design and Fabrication of the MSS

Qasem Ramadan, Lau Ting Ting and Ho Shihan Bryan

As a proof of concept and practical approach, the fluidic channel in the current study has been realized using a long borosilicate glass capillary. Since the purification process using the MSS is a sequential process of several trapping and releasing actions, a long fluidic channel is required. In order to reduce the occupied space of the MSS setup, the channel has been bent in a meander shape as shown in Fig. S1. It should be noted that the fluidic channel can be realized in different methods such as injection molding of two sheets with the channel is molded on the surface of one of these sheets, or using micro fabrication technology using silicon substrate or soft lithography using Polydimethylsiloxane (PDMS). The channel shown in Fig.S1 has inner and outer diameters of 1.8 mm and 3mm, respectively and a total length of 450 mm. The channel also comprises a conical shaped bulb for the final separation and collection of the purified sample. The resultant total volumetric capacity of the tube is 1.46 ml.

Fig. S2a shows a detailed view of the MSS which comprises the following components, all made of polymethyl methacrylate (PMMA) except the magnet carriers and the gear set which are made of Delrin:

  1. Magnetic assembly (MA): This is the main part of the MSS which comprises six cylindrical-shaped magnet carriers (M1-M6) positioned directly beneath the meander glass channel. The magnetic assembly can be positioned in two different orientations with respect to the fluidic channel, i.e. the longitudinal axis of the carriers can be parallel or perpendicular to the channel. Fig. S2a shows the parallel arrangement. Each magnet carrier carries 6 cylindrical permanent magnets with diameter and length of 2 mm and 5 mm, respectively. Therefore, the magnetic assembly comprises 36 magnets with alternating polar orientation. The magnets are made of neodymium-iron-boron (NdFeB) with magnetic remanence Br and coerecivity of 0.142 Tesla and 876 KA/m, respectively. The magnets are embedded in the carriers with their longitudinal axis perpendicular to that of the carriers. Within each magnet carriers the permanent magnets are arranged in a manner such that the longitudinal axis of each magnet is perpendicular to that of the previous and the next ones. The magnet carrier are inserted in machined grooves beneath the fluidic channel which results in a total separation distance between the paramagnet magnets and the bottom surface of the fluidic channel (at j=0o mode) of 1.5 mm which ensures sufficient magnetic force on the magnetic particle suspension. The carriers can be freely slide inside these grooves and such sliding movement is necessary to release all magnetic particles residue in the channel at the end of the purification process as will be discussed later. The magnet carrier are joined together at one end by a set of gear and actuated by a stepper motor with which the rotation frequency and angle can be controlled. It should be noted that the MA can comprise variable number of magnet carriers e.g. 1, 2, etc.
  2. Fluidic channel support: This is the support body of the system which functions as the fluidic channel carriers and allows aligning the channel to the magnet array. The support piece has a set of groove that accommodates the magnet carriers and maintains steady and stable rotation of the magnetic assembly.
  3. Separation magnets (SMs): These are two relatively large and strong permanent magnets that are positioned on both sides of the glass bulb (Figs S1 and S2) to trap the magnetic particles after passing through the purification channel i.e. collecting the purified sample. These magnets are disc shaped with diameter and height of 10 mm and 5 mm, respectively which are also made of NdFeB. Similarly, these two magnets can be shifted a way with a sliding mechanism actuated by a motor (not shown) to finally release the sample at the end of the purification and separation processes.
  4. Auxiliary magnetic assembly (AMA): a set of NdFeB magnets having the same size of the magnets within the main magnetic assembly and are embedded and arranged in a PMMA sheet in a similar manner. However, these magnets are all fixed in the PMMA sheet with their longitudinal axis is perpendicular to the sheet plane. The auxiliary magnetic assembly is positioned on top of the fluidic channel with a separation distance that can be adjusted by a sliding mechanism provided using four sliders position at each corner of the support piece. This magnetic assembly is not mandatory to use in the purification process. But it assist in complete releasing of magnetic particle cluster at each potential well after the magnet rotating form j=0o to j=90o in particular when magnetic particles with high magnetic susceptibly are used. The separation distance between the channel and the auxiliary magnet is controlled using a linear motor (not shown) and can be calibrated for each type of magnetic particles.
  5. A set of Motors ( one motor is shown in Fig.S2) to provide actuation forces for the moving parts of the system:
  6. Rotational motion for the MA;
  7. Sliding motion for inserting and removing the MA;
  8. Sliding motion for the AMA;
  9. Assembly of the separation magnets.

The sliding motion mainly switches the magnetic force from maxima to minima and vice versa.

It should be noted that the magnets in the MA can be replaced by an array of miniaturized electromagnets that can be programmed to switch ON/OFF in an alternate manner to achieve the same magnetic force modulation analogous to the magnet rotation.

Figure S1: Glass fluidic channel used as a purification channel with a bulb for purified sample collection and separation. The inner and outer diameters of the channel are 1.8 mm and 3 mm, respectively.

Figure S2: (a) MSS assembly view with the magnet carrier in the inset (right); (b) Assembled view of MSS; (c) MSS image.