Reliability of Single-Use Connections
The rapid expansion of single use products has allowed for significant
advances in bioprocess manufacturing. Biopharmaceutical
manufacturers are now able to perform almost all unit operations in a
single-use platform. Advantages of single use technologies are
numerous and the subject of many recent case studies by a variety of
organizations. For all the advancements that single use platforms have
provided, there are some inherit weaknesses with the current state of
technology. One such area is in connections. The move away from traditional stainless steel systems by the
biopharmaceutical industry created a move away from engineered connection and joining processes. Traditional
process systems are fabricated using well documented welding procedures and interconnected with proven
connections platforms such as the Tri-Clamp® union. Dimensions and requirements of these components and
techniques are often documented in various standards including the ASME Bioprocess Equipment (BPE) Standard.
Unfortunately, the same cannot be said for the single-use world.
The magnitude of tubing, fitting and connector combinations used in single-use systems is amazing. All of these
items are available in a variety of sizes, materials and shapes. Some component manufacturers have implemented
their own standards related to barb profiles and dimensions; however, there are no industry standards. As a result,
interconnecting all of these various components into a single-use bioprocess system is problematic. Options for
making connections range from mechanical methods such as tie wraps, Barblock® retainers or Oetiker® clamps to
over-molding techniques. Each connection method has its own advantages and disadvantages. The proper choice is
dependent on the application, sterilization method and materials.
Tie-Wraps (cable ties)
The use of tie-wraps (cable ties) is the most common method for building single
use tube sets. After pushing the tubing over a barb, a tie wrap is used to hold the
connection together by compressing the tubing into the barb. Tie wraps are an
extremely versatile, low cost option. They are available in a wide range of
lengths and widths, each of which can accommodate a variety of connection
sizes. Excess material is trimmed off after tightening. Care must be taken with
the trimmed end that remains on the assembly as it may have sharp edges that
can damage bioprocess bags or packaging.
A disadvantage to tie wraps is limited pressure resistance and leak issues. The nylon material can creep (stretch)
under load resulting in a loosening of the connection. The tubing is then able to extrude out from the area under
the tie causing a failure. Additionally with most tie wraps there is an area immediately under the hub that is not in
contact with the tubing. This area presents a weak section of the union that leaks under pressure. Both of these
issues are magnified by thermal cycling such as in an autoclave.
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Oetiker® Clamps
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Oetiker® clamps are stainless steel retainers that provide a means for securing
tubing to barb connections. The single-ear version shown is the most popular
although there are other tightening mechanisms. Oetiker® clamps provide 360°
compression of the tubing around the barb that eliminates the potential leak
path found on tie wraps. Since the retainer is stainless steel, Oetikers are not
subject to creep or expansion even at elevated temperature as in an autoclave.
Oetiker® clamps are available in a tremendous range of sizes. Each size Oetiker®
can accommodate a wide range of tubing outer diameters making them
adaptable to various barb profiles. A single Oetiker® size will typically
accommodate all possible tubing and barb combinations for a given process connection size (e.g. ½”). Oetiker®
clamps work well with most tubing materials including reinforced tubing. Typically the burst pressure of an
assembly using Oetiker® clamps is limited by the tubing or one of the other components, not the connection.
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Barblock® Retainers
Barblock® is a connection system designed by Saint Gobain Performance
Plastics. Like the other methods, they serve as a means to hold tubing in place
on a barb. Barblock employs a two part connection method that uses a retainer
and collar. The resultant connection has complete 360° compression of the
tubing around barb thereby eliminating the potential leak path common to tie
wraps. Barblock is also available as an integrated system that includes barbed
elbows, tees and crosses. When used as part of this PureFit® SIB® system,
Barblocks provide compression of the tubing along the entire length of the barb
which eliminates possible entrapment areas. Additionally, the barbs on the
PureFit SIB fittings expand the tubing outward resulting in a nearly smooth
interface between the barb and tubing. Barblock is available in both polypropylene for general applications and
PVDF which is ideal for higher temperature and pressure applications. When used with high temperature tubing like
Silicone, PVDF Barblocks can be autoclaved.
While Barblock is available in a variety of sizes, there is not necessarily a size for every possible connection. Barblock
sizes are specific to the tubing outer diameter (OD) or more precisely the OD of the tubing after installation on the
barb. Since barb profiles are not standardized across the industry, the barb profile from each source must be
reviewed to confirm compatibility with Barblock and the desired tubing. This is particularly problematic since there
is also variation in tubing wall thickness. The amount of variation that Barblock can accommodate is somewhat
determined by the type of tubing. In general, softer materials (e.g. Silicone) are more forgiving than firmer materials
(e.g. C-Flex®). In order to realize the advantages of the Barblock system, careful consideration is needed with the
overall connection design. Barblock retainers are best used in conjunction with Pure-Fit® SIB fittings.
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Over-Molded Connections
Unlike the other methods, over-molded connections are held together by a
chemical bond between the tubing and molding material. The connection is
made by assembling the tubing onto a support structure (mandrill) then placing
into a mold. Polymer material is then injected into the area around the tubing
which bonds it in place. After removal from the mold, the support structure is
removed leaving a completely unitized assembly. Well designed over-molded
connections are often as strong as the base material and have a uniform inner
diameter that eliminates areas of liquid hold up. Silicone and C-flex® are the
most common materials connected via over-molding. Typically only like
materials can be over-molded (e.g. Silicone to Silicone, C-Flex to C-Flex) although it is possible to join C-Flex to
polypropylene.
Manufacturers have developed extensive tool sets so most common connection types and sizes are available. Less
common combinations may require tooling sometimes a fee to the end user. Due to the manufacturing process,
over-molded connections are only used for interconnecting tubing. This type of connection is not appropriate for
securing tubing to other components such as filters or aseptic connectors. Often over-molding is used in
conjunction with one of the other securing methods when building a single-use bioprocess system. Since overmolding requires more set up time that other connection methods, minimum order quantities are usually higher and
lead times longer.
Type
Tie-Wrap
Oetiker®
Barblock®
Over-Mold
Common Uses
Interconnecting tubing
and process components
via barb connectors.
Interconnecting tubing
and process components
via barb connectors.
Interconnecting tubing
and process components
via barb connectors.
Interconnection of tubing
only.
Cost
Advantages
$
Versatile and available in many sizes, works
with all material types.
$$
Strong connection with 360° compression,
available in many possible sizes,
accommodates dimensional variations.
$$$
360° compression of the tubing to the barb,
no sharp edges, works with all material types.
$$$$
Fully unitized assembly with a uniform inner
diameter.
Disadvantages
Non-uniform compression of
the tube to the barb, lower
pressure and prone to leaks,
sharp edges can damage bags.
Metal construction not always
desirable.
Limited versatility on barb and
tubing dimensions, limited
sizes available.
Limited material options, only
works with like materials,
typically require minimum
order quantities.
What is the right connection to use?
The right connection method is highly dependent on the application goals and materials being used in the assembly.
Partnering with a supplier that understands the various methods provides the best chance of developing a singleuse system that both achieves the process goals and does so in a reliable, cost effective manner.
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