Lost Batches – Nevermore: Rapping on the door of sensor technology

Although the authors’ do not have statistics or a study to show the cause of the bulk of failures in single-use bioreactors, it is their strong feeling that the largest majority are collectively due to sensor failures, contamination from leaking aseptic connectors, and over-pressure in the bags. This feeling is a result of many conversations with bioprocess engineers and listening to their detailed “Poe like” monologues and tales of woe.

In order to illustrate the potential of a bio-process engineer to slowly descend into madness over measurement inaccuracy and drift, we have liberally borrowed from the ancient Greek and Roman form called elegiac paraclausithyron, and parodied a poem that the general readership shall recognize from their high school English class. 

Once upon a midnight dreary, while I measured weak and weary,
Over many a port and sample of pure bioprocess lore,
While I plodded, nearly napping, rapidly there came a tapping,
As of something loudly rapping, rapping at my filter latch.
''Tis some built up foam,' I muttered, 'knocking at my filter latch -
Only this, and not a scratch.'

Ah, distinctly I remember it was in the scale-up transfer,
And each separate bioreactor had a perfect turn-down match.
Good control I always treasured; - vainly had I off-line measured
Both pH and DO values - values from my culture batch -
Had large drifts and variations that would kill my first fed-batch  -
And from me my titer snatch.

Thus my two-point calibration with my temperature compensation
Scared me - filled me with fantastic terrors never felt before;
So that now, to still the beating of my heart, I stood repeating
''Tis the membrane; no, my units,  or a grounding wire mismatch?
Or some bubbles, oh so noisy!  New DO probe I dispatch -
That will save my ailing batch!'

Suddenly my foam grew larger, something went wrong with my sparger,
'Cells please grow and please don't die, we'll add some antifoam' said I,
As I slowly started pumping, strong surfactants caused cell clumping,
Clogged my sensors filters, tubing, even deadbands couldn't catch,
Ever-sinking DO levels, then my process met its match -

Yes, I finally lost my batch. 

Deep into that darkness peering, long I stood there sulking, fearing,
Doubting, dreaming dreams of how my sensors I could soon dispatch;
Kleen-pak leaking was the reason, cells all died - it felt like treason,
By the sensors I had trusted, autoclaved without a scratch.
As I pondered, something murmured in my ear the word 'TruFluor!'
Only this and nothing more.

Despite these recurring issues in bioprocessing the push for testing of new technologies let alone their adoption is surprisingly slow.  Simple issues like the use of the units of "% Sat" that exist in no other industry and continually mislead operators are a mainstay.  Other simple tests - like calibrating probes multiple times to understand their repeatability and consistency are rarely performed, while SOPs dictate accuracy numbers that are difficult to achieve - even by NIST.  It is no wonder that a lost batch often seems like a late night visit from the Raven.  The only question remaining, is whether the Raven is actually new sensors, and the devil in bioprocessing really the act of clinging to dogma and process tradition that has its roots in 1950's electrochemistry?

Perhaps by adopting next generation single-use sensors that are pre-sterilized and pre-calibrated, the "new" poem could have more of a Walt-Disney like ending? Let's read on and return to the rhyming scheme of the original opus.....

Now a new bag fast inserting, all my soul within me burning,
Tubing welding, media pumping, heating, gassing as before.
Then I saw it squarely, surely, something in my sensor port?
Black and short and tightly fitting, in my port was this TruFluor -
Let my heart be still a moment and this mystery explore; -
'Tis a port plug, at the fore!

Then I saw a piece of paper, thought it was some kind of caper,
Read this was a novel sensor, USP Class VI and more -

"Has no drifts, no variations, that QA calls deviations. "
So I went and found the "reader" that was lying on the floor,

Slid it in the black plug fitting, added cable, not a chore,

Pressed the" factory cal restore".

'Prophet!' said I, 'you bedevil! Accurate with good noise level!
By that Heav'n that bends above us - reading stably that I swore!
Tell this soul with sorrow heavy how so fast you can be ready?

Why are you immune to bubbles that other sensors abhor?
Finally a DO reading that I trust and don't ignore?'
Quoth the sensor, 'Evermore.'

'Be robust in all your readings, optics rule!' I shrieked believing -
'Make thy background reading zero, just like Night's Plutonian shore!
Hold you true self-calibration, post your gamma radiation.

Help my process stay unbroken! - Keep my DO at twoscore!
Help my cells grow strong and plenty, so I make my titers soar!'
Quoth the sensor, 'Evermore.'

And the TruFluor, tightly flitting, still is sitting, still is sitting
In the bag port of my vessel in the cutout at the fore;
And its "spot" has all the seeming of an angel's that is dreaming,
LED-light in it streaming "reads" the "spot" and measures sure;
And my cells with DO plenty set to levels they adore
Shall be happy - Evermore!

To date electrochemical probes in single-use bioreactors have been like the Raven to Poe's tortured subject of the like-named poem; a source of never ending remembrance and bane of its existence. While a mainstay of bioprocessing, they are very clearly mismatched to single-use bioreactors by dint of the materials used in their construction, their method of calibration, their method of sterilization - even their principal of operation.  There are actually few things more haunting to the bioprocessing professional than to return after the weekend to find that the primary and secondary electrochemical pH probes have drifted apart and both read differently than their off-line blood gas analyzer. Subsequent study often reveals a clogged port or post-autoclave drift or even cracked or broken glass.  It is for these reasons and more that well designed and field proven single-use sensors matched to the single-use paradigm continue to make gains in the single-use marketplace.