Team's Philip Canner was invited by EMDT to write about strategies for developing a reliable and repeatable fluid handling system.

1. Official SpOnSOr
emdt.co.uk
November/December 2011
The Magazine for Medical Product Design & Manufacturing
Extrusion
Technologies page 82
Inside Advice on
Medical Electronics
pages 24 and 80
When Is Software a
Medical Device?
page 21
Compamed Preview
page 53
What You Need to Know
about the RoHS Recast
page 42
Using Nitinol Sheet in
Medical Implants
page 28

2. design
A Fluid
Situation
Strategies for designing a reliable,
repeatable fluid handling system.
Philip Canner, Team Consulting Ltd,
Cambridge, UK
From dialysis machines and infusion phase. The article is based on the author’s much of the system as possible. Other
pumps to small handheld devices such experience of designing high-performance users—those handling point-of-care
as inhalers and pen injectors, reliable disposable fluid circuits, with flow rates in-vitro diagnostic devices, for example—
and repeatable fluid handling is a critical ranging from millilitres/hour to litres/ will expect to barely handle the fluid
process. The challenge with fluids, minute, but many of the underlying circuit at all. Above all, the project team
however, is that you almost never quite principles can be applied to the design must ensure that the product is safe
know what is going to happen until you process for all fluid handling devices. to use. Product performance, safety,
try it. An innocuous change to a fluid usability and risk management are of
circuit design, done for reasons that may Requirements capture paramount importance from the start of
have nothing to do with fluid handling, The first stage in product development is the project.
can result in an unexpected shift in fluid to define the requirements for the system. As a medical device, the fluid contact
behaviour. This can make designing Because of the challenges of ensuring that materials in the system probably will be
fluid handling devices and equipment fluid circuits do exactly what they should, required to meet certain standards such
a daunting task, and the potential for every time, it is important to verify that as USP Class VI and ISO 10993; in the
rework is huge. However, armed with the constraints have been defined fully. case of plastics, they may be required
some knowledge of the potential risks, Different users will have very different to be free of certain plasticisers such as
and where time can best be used or requirements, knowledge levels and DEHP. It is important that the material
saved, a plan can be put together that expectations, all of which will impact requirements and sterilisation process
gives the project the best possible chance the design, complexity and performance be determined early, because, as will be
Leigh Prather/Shutterstock.com
of success, both in terms of the final requirements of the system. For example, discussed later, the prototypes should be
product and the budget. perfusionists in a surgical team may made from the same materials as the final
The items outlined below are intended expect to spend time setting up a fluid device. Sterilisation often affects material
to give the project planner and team circuit and removing air bubbles from properties, and testing prototypes that
insight into what to watch out for in each it, and may want unfettered access to as are of the right material and have been

3. design
through the sterilisation process can and built to the same tolerances—and cannot be found, the alternative is the
help reduce development risk. If the will be tested with the correct fluids custom-made part. Before abandoning
design involves tubing that may be under realistic operating conditions. the off-the-shelf solution, however, it
stored for a period of time in a folded, This becomes increasingly important should be taken into consideration that
twisted or rolled state, this needs to be the smaller the scale at which you are the off-the-shelf component allows
taken into account. working, because the physics change as the team to get into the lab and test
systems shrink in size. concepts with the actual components
Conceptual design and proof-of- Therefore, it is helpful to ensure that will be in the final system—a lot of
principle that the conceptual design involves the regulatory work already may have
After the requirements for the product manufacturing processes that can be been done.
have been determined, it is time to prototyped, or to put it in reverse, When the concepts are being
generate design concepts. To ensure that the prototypes mimic as best they prepared for prototyping, it is important
that the team does not spend too much can the intended final production to know when not to use CAD.
time running down wrong tracks, plan techniques. If the manufacturer has Certainly for fluid circuits involving
on spending time in the lab as early not already been identified, then the tubing, connectors and various other
in the project as possible to test these conceptual design stage is the right time components in the line, it can be more
concepts with proof-of-principle rigs. to start thinking about manufacturing efficient to get into the lab with a
Altering fluids, contact materials, partners. By finding and talking to sketch, a few reels of tubing and all
temperatures, pressures, flow paths, manufacturers with experience in the components, and hang them off
surface finishes, sterilisation techniques the type of fluid circuit that is being scaffolding. In this way, a large number
and so forth can dramatically change designed, their insight and knowledge of concepts can be worked through, and
the system’s behaviour. This has a can be fed into the designs, saving time the learning curve can be ascended, very
significant impact on the approach and avoiding the worst-case scenario quickly.
that should be taken in the design and of designing a system unsuitable When testing prototypes in the lab,
prototyping of for volume the most important rule is to use the
such systems. production. The correct fluids. This may be the single
As helpful at As helpful at getting manufacturer most important factor in getting
getting fast insights also may have representative results. If the fluid circuit
as they may be, fast insights as they preferred is designed for handling whole blood
quickly knocked-up may be, quickly suppliers of OEM direct from the patient, testing with
rigs and prototypes components, water and red food dye is not going to
must be treated with
knocked-up rigs and and supply other help. In fact, it may well result in false
caution. The system prototypes must be services (such confidence in the design, or even lead to
that worked with as packing and a workable concept being abandoned.
the Heath Robinson
treated with caution. sterilisation) that Occasionally the first idea may work
prototype may cease can be utilised. straight away. If this does happen,
to work with a high-spec prototype. In this way, the manufacturer can be a the performance boundaries still
Only after hours of investigation do you great ally during the prototyping phase, will need to be found. More likely,
realise that one of the 0.25-mm holes enabling the project team to create however, is that the system will almost
in the original prototype was, in fact, prototypes that are near, if not identical, work, and achieving that last 20% of
blocked with swarf, making the system to the final product. the performance requirements will
function beautifully… If the conceptual design includes take 80% of the development time.
To address this problem, when it off-the-shelf components, the same Sometimes, the trickiest problems are
comes to the design process for fluid components should be used in the best solved by experimentation and by
handling devices and systems, detailed prototypes and in the final device. playing with the circuit, rather than
design of the fluid path and selection Plenty of time should be spent finding spending hours over a hot CAD station.
of the manufacturing method should and testing the parts at this stage, and Once the fluid circuit is operating
come early. Once a design that works they should be sourced from a company exactly as intended, the repeatability
has been found, the design of the that is able to supply final production and robustness of the design will need
fluid-contact path should be frozen. volumes. Time spent here finding the to be tested. Initially, this may involve
Ideally, the prototype fluid circuit will right components may well reduce the freestyle experimentation, followed by a
be made in the same way as the final overall development time. more structured system characterisation
product—i.e., using the same materials If the perfect off-the-shelf component when the design is finalised.

4. design
to manufacture and final design verification.
The most important rule during this
The Trouble with Blood and flow speeds, avoiding step changes phase is to implement the design that was
Blood is a particularly tricky fluid to in tubing diameters and carefully choos- proven in the previous phase. This requires a
handle. There are primarily four reasons ing the pumping techniques—can help thorough understanding of what is going on
for this: coagulation, haemolysis, avail- reduce haemolysis. in the fluid circuit, so that the designer knows
ability and behaviour. The challenges Availability and handling. In the UK, what can and cannot be changed. What may
associated with each of these items are human blood is available on applica- look like a harmless alteration to the fluid
highlighted below. tion from the National Blood Service path may have a significant impact on the
Coagulation. When blood is outside for the purposes of approved research characteristics of the fluid flow.
the body, it will coagulate at every and development. The blood donations Hopefully the final manufacturer will have
opportunity. Coagulation is the complex are screened, but it is still possible that been identified by this point, and a dialogue
process whereby blood forms clots; this blood-borne viruses are present, so pre- already opened. To optimise the development
can be triggered by everything from cautions should be taken to ensure safe process, agree with the manufacturer on the
exposure to a foreign solid or gaseous handling. This resource is invaluable for most appropriate point for design handover.
interface and shear forces to low flow testing systems during development, but To help with on-going risk management
rates and stagnation. Without the use again, caution needs to be taken in ana- activities, the manufacturer can be involved
of anticoagulants, such as heparin, or lysing the results of the tests. If the sys- in the design for manufacture and assembly
specialty coatings on the parts to prevent tem has to work without anticoagulants, and FMEA processes.
or reduce contact activation, blood clots bear in mind that the bagged blood will Design verification involves testing the
can form in the device and prevent it have anticoagulants present for storage design against the requirements identified
from operating properly. purposes. For example, blood parameter in the initial phase of the project, as well
Haemolysis. This term describes the sensors integrated into the fluid circuit as collating all the data required for the
rupturing of the red blood cells, caus- can behave differently depending on the fluid-contact materials used in the system.
ing the contents to spill out into the anticoagulant used. Beforehand, the manufacturer also can be
surrounding fluid. Haemolysis can be Behaviour. Blood is a multiphase, used to provide preproduction prototypes
caused by mechanical stresses, such as non-Newtonian, pseudo-plastic fluid. that are manufactured and assembled in
pressure changes, turbulence and shear As such, its flow behaviour can be the intended manner, helping to de-risk the
forces on the blood cells. The product difficult to predict and model. In our project before final product testing.
specification should clearly state the experience of developing systems that
acceptable limits of haemolysis, espe- handle various amounts of blood, this A succesful outcome
cially if the blood is to be returned to the is why we always recommend rolling When designing a fluid handling system, an
patient. Careful design of the fluid cir- up your sleeves in the lab at the earliest awareness of some of the unique challenges
cuit—choosing the correct tubing bores opportunity. involved proves invaluable in planning and
executing a successful development project.
Computational fluid dynamics Successful development often involves many
(CFD) is a powerful tool that can Computational fluid hours in the lab, experimenting in a creative
save hours of time in the lab and but scientifically rigorous way and using
can be used to identify the critical dynamics (CFD) is a the application of scientific theory, where
parameters of a fluid system. However, powerful tool that can appropriate. Project timescales need to
it has limitations and can be costly. allow for this, and project planning needs to
A decision would need to be made
save hours of time understand the great uncertainties involved
as to whether a full, or even partial, in the lab and can be in liquid handling. Expect the unexpected,
understanding of the system is critical and your project will have the greatest chance
enough to warrant spending the time,
used to identify the of success. 1
effort and money on CFD. The key is critical parameters of a
to work with a capable and trusted Philip Canner
CFD service provider, and to know
fluid system. is an Engineering Consultant at
what to expect from the CFD analysis. Detailed design, transfer to manufacture Team Consulting Ltd,
Abbey Barns, Duxford Road,
The key message from this phase is: if and design verification
Ickleton, Cambridge CB10 1SX, UK
it touches the fluid, and it works, try The concept has been chosen, the proof- tel. +44 1799 532 700
not to change it in those stages between of-principle prototypes work, and now the e-mail: pdc@team-consulting.com
prototyping and production. system can be designed in detail for transfer www.team-consulting.com
Reprinted with permission from EUROPEAN MEdicAl dEVicE TEcHNOlOGY, November/december 2011. On the web at www.emdt.co.uk
© A UBM canon Publication. All rights reserved. Foster Printing Service: 866-879-9144, www.marketingreprints.com.