Microwaved Media Bottle Becomes Projectile

What happened?

A lab member was melting solidified media in a two-liter glass media bottle in the microwave. Media needs for several days are prepared simultaneously on account of the limited availability of building autoclaves and inability to cold store prepared plates. The solidified media was approximately filled to the one-liter mark upon beginning the melting and the cap on the media bottle was vented.  The lab had recently purchased a new microwave after the previous unit failed and not updated procedures for the new equipment.  The new unit was smaller and unable to accommodate the height of the pre-prepared 2L bottle; the bottle was tilted and placed in the new unit at a diagonal.  The microwave was left unattended while the media was reheating but checked periodically when the timer rang.

During one of the microwaving rounds, the door of the microwave burst open, and the glass bottle flew across the lab spraying a lab member working at the computer station and another lab member passing nearby.  The bystander was sprayed with media in the face (including the eyes), neck, shoulder, and chest. No one in the lab space was wearing any PPE at the time of the accident. 

The glass bottle did not break, and the media spray was not hot enough to cause burns to the two lab members that were sprayed.  The microwave door hinges were damaged in the process and microwave became unusable. The area was cleaned up by the lab members and the PI was informed of the incident. The two sprayed lab members were attended to by lab mates and reported no injuries. 

 

What caused the accident?

While lab personnel had vented the cap, the bottle was wedged diagonally into the smaller microwave.  This likely greatly limited the ability for the bottle to vent; the use of a pour lip on the bottle increases this possibility.  The mostly closed container over pressurized while being heated, and at a critical point, the pressure was great enough to pass through the minimal venting of the bottle sending it on the path of least resistance.  The media bottle literally became a rocket spewing media in its path.  

Another possible explanation is that the media superheated.  Superheating occurs when a liquid has exceeded the boiling point without boiling due to the surface tension of the liquid.  Bubbles will violently release once the surface tension is lessened- typically from disruption.  As with microwaving other items, heat distribution throughout the solid media would be uneven.  This could cause some portions of the solid media to liquify faster than others and create superheated pockets of liquid. 

In this theory, when the bubbles did the release, the cap was not vented enough to allow that much pressure to safely release at once resulting in the projectile. Given that no burns resulted from the media contact, it is likely that this incident was not the result of a superheating event.

How can we prevent future incidences or minimize injuries?

• Always wear your PPE when anyone is working in your laboratory.  In this situation, the two lab members who could have sustained injuries were not actively conducting lab work.
• When computer workstations in laboratories are unavoidable, group them together as far away from active lab benches as possible.
• Minimize or accommodate media preparation needs for your laboratory.  The laboratory could purchase an independent autoclave and/or increase cold storage to mitigate the need to microwave pre-prepared media.  
• Vessels should always be upright, especially when pressurization is a possibility.  Had this been the case in this situation, there is a possibility this incident might have been contained to the microwave.
• Not all microwaves have the same wattage or power capabilities.  When using new equipment, compare these features to your previous equipment, and start with smaller time increments until you have identified appropriate settings for your media(s).  You may also need to modify the power output if your unit has this feature. 
• When using new equipment, evaluate existing procedures.  Update the SOP and communicate them to others.
• When microwaving media is necessary, use smaller volumes when possible. Make sure there is plenty of room in the container for the heated liquid to expand.  Agitate the media during heating cycles by gently swirling to break surface tension and prevent superheating events.  Wear thermo-protective gloves.
• Generously vent caps on containers while microwaving.
• Stay near the microwave while heating media to observe the container.  Stand on the hinge side of the door out of the door swing range as you would with an autoclave.  Avoid standing directly in front of the microwave door.
• Locate microwaves used for such purposes in a low-traffic area away from unrelated workstations. 
• Identify a different reheating method.  A bead bath or hot water bath may work, but the prolonged time for heating may compromise or alter some medias.