Product Survey: Mechanical cell disruption devices

Killing Machines
by Harald Zähringer, Labtimes 05/2014

Researchers do everything to grow their ‘workhorses’ in a happy environment. But when the cells’ time is up, they do not deter from killing the once-pampered cells with ghastly methods.

Researchers have invented a lot of different cell-killing machines (sorry, cell disruption devices): hence the cells usually lose their lives by either beating, shearing, shocking or grinding forces, exerted by these machines.

Cell beating is usually performed in bead beaters, which may be roughly divided into vortexing or shaking-type bead beaters, used for small sample volumes, and rotor-type bead beaters applied for higher volumes. Shaking bead beaters are equipped with special micro-vial holders that oscillate with a high frequency to vigorously agitate screw-cap vials mounted to it, containing small glass, ceramic or steel beads. The oscillating motion drives the beads to randomly bounce back and forth between the tube walls, crushing into pieces every cell that gets in their way.

Shaking bead beaters are useful for small probe volumes and micro beads, but they are ill-suited for larger cell preparations requiring bigger glass or steel grinding balls − the strong mechanical forces triggered by the oscillating motion of the sample holder would simply kill the whole machine. In this case, it’s better to use rotor-type bead beaters, which are based on a completely different bead impulsion concept that doesn’t require shaking. The beads are violently agitated by a teflon impeller, rotating at high speed in a clover-leaf shaped homogenisation chamber. Beads colliding with cells immediately cut through the cell wall and disrupt the cells. But whatever kind of beat beater you choose: the sustained glass bead fire produces a considerable amount of heat that affords cooling after a few minutes of cell beating.

Tear it up

Shearing forces are at the heart of many cell disruption devices. Filling a micro-tube with cells and tiny glass beads and giving it a good shake on a vortex mixer is the simplest way of cell shearing. Yeast guys use this technique, which is often confused with bead beating, to crack open the rigid cell wall of bakers yeast. Disrupting cells on a vortexer with glass beads, however, is not very effective and the throughput is pretty lousy. Not to mention that your arm will start to vibrate even faster than the vortexer after extensive rounds of cell disruption.

Homogenisers such as the Dounce and Potter-Elvehjem homogenisers are immediately recognisable as cell-shearing instruments − the cells are simply smashed between the up and down moving pestles and the inner side of the homogeniser tubes.

Rotor-stator homogenisers also known as Willems Homogenisers or Polytrons are based on a simple but very effective cell shearing technology. At first glance, a ­Polytron looks like an ordinary hand held blender used in the kitchen to prepare purées, soups or smoothies. A closer look, however, reveals that it has no blades at the end of the driving shaft in contrast to hand-held kitchen blenders. The blades are replaced by a precisely tailored shaft (rotor) that rotates inside a stationary tube (stator). But that alone won’t do the trick. Peter Willems, who built the first polytron in his laboratory in Luzern, Switzerland, back in 1957, added another brilliant feature to the rotor-stator system, which tremendously increased its homogenisation efficiency: he periodically slotted the bottoms of both rotor and stator.

Dipping the rotating system into a cell suspension generates a negative pressure that draws the cells into the rotor. Due to the centrifugal forces, the spinning cells are immediately thrown out of the rotor and leave the system through the slots. It takes only a little fantasy to imagine what happens to the cells at the slots: the shearing forces relentlessly break into pieces every cell that gets inside the slots. Polytrons do their job fast and without heating the sample but they are not cheap.

You may, alternatively, explode your cells with shockwaves, generated by an ultrasonic homogeniser. Ultrasonic homogenisers consist of three essential parts: an electronic generator, a cylindrical converter connected to the generator with a high voltage cable and a probe or horn mounted to the converter. The generator produces high frequency electrical energy, which is transformed by the converter via a piezoelectric crystal into mechanical vibration. The vibration is amplified on its way through the probe and forces the end of the probe to move extremely fast up and down. Dipping the tip of the oscillating probe into a cell suspension generates a shock wave that will destroy every cell hit by the wave.

Some cells, spores and tissues are pretty tough and may overcome even the hardest beating, shearing or shocking treatments. But they usually do not withstand the brute force of grinding. Grinding frozen cells or tissues with mortar and pestle is a simple but very effective cell disruption method. The cells are pressed by the moving pestle against the hard surface of the mortar causing tearing and ripping of the cells. But even this ancient method may be further honed.

Steamroller tactics

Being unhappy with existing cryo-grinders, the Irish neuroscientist, Liam Loftus, created his own ‘Cellcrusher’ cryo-pulveriser made of stainless steel. At first sight, the Loftus-type pulveriser looks and works like many other grinders: a pestle with a rounded tip is hammered with a mallet into a precisely tailored counter piece (mortar) of the same size to breakup frozen samples. What distinguishes Loftus’ pulveriser from other models are the different radii of pestle tip and mortar bottom. The radius of the mortar bottom is slightly wider than the radius of the rounded pestle tip. Hence, the cells are allowed to move laterally when they are sandwiched between pestle and mortar. This simple trick increases the grinding surface considerably and, as a consequence, improves grinding efficiency.

If the Cellcrusher doesn’t help, you may try Loftus’ Drill-bit, which is used together with the Cellcrusher mortar and a drill or electric screw driver. The drill-bit is basically a steel-ball having the same radius as the Cellcrusher mortar, mounted on a shaft that fits into a drill. Running the Drill-bit at appropriate speed and applying enough pressure with your arms should bring even the most stubborn cells and tissues to their knees.

First published in Labtimes 05/2014. We give no guarantee and assume no liability for article and PDF-download.

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