An Examination of Needle and Syringe force testing
Needles and syringes need to perform precisely to specification or they may seriously limit the ability of a health care professional to effectively and safely treat their patient. Force and torque testing in development and production enables the manufacturers to guarantee consistent quality and performance of their products. This article examines seven typical testing applications within this field
By David Mercer
Needle Insertion and Penetration Forces
To maximize patient comfort, it is essential that a clean insertion and extraction of the needles is achieved, resulting in minimum trauma to the epidermis and subcutaneous tissue. Malformed and insufficiently sharp needles points can cause considerable pain on insertion, easily preventable with a well-designed and consistently produced device. Needle penetration and extraction forces are measured in development and production to characterize point sharpness, the frictional forces experienced along the shaft of the needles, and the effectiveness of the bevelled tip geometry. The test typically requires a motorized force testing system, with a specialized lower fixture to hold in place a stretched foil of PBC or PU over a steel ring to simulate skin. The needle, which is mounted onto a sensitive, low capacity load cell, is brought down through the foil at a constant rate by the system’s motorized crosshead travel. A computer-controlled system will provide a real-time graphical force profile, which may be analyze for key areas of interest, including:
Needle-to-Hub Retention Strength.
Needles are held within their hub by a variety of methods, including bonding, moulding, welding, mechanical interlocking, and sealing. The retention strength of the assembly method must be sufficiently high to avoid disassembly or seal failure in use. To test retention strength, the test system is normally fitted with an aluminium mounting block, beneath which the hub is placed with the needle tip exposed vertically. In the case of smaller needles and lancets, a lever-operated pin vice may be fitted to the tip, and an axial tensile load applied at a constant rate, until the needles is completely dislodged from the hub. For larger gag needles, a small wedge grip may be required to hold the sample. This test is per-formed in conformance with international standard ISO 7864.
Surgical Needle Crimp Strength
Single-use surgical needles are supplied connected to a length of suture via a crimped end. This may take the form of a drilled-end crimp, In which a small aperture is made in the end of the needle, the suture threaded inside, and the needle passed around it, Alternatively, it may be a flange-type crimp, in which the needle end is pressed into a semi-circular formation, the suture placed into the groove, and either end of the semi-circle close around it. The pull-off force of the needle from the suture must be sufficiently high to ensure it does not come away during placement of the sutures, but sufficiently low to enable surgeons to pull the needle from the suture as required. To perform the pull-off test, the needle is held in a small wedge grip, while the suture material is wrapped around a cable cam grip. This spreads the load evenly to avoid slippage during testing. A tensile load is then applied at a slow, steady rate until the suture is dislodged from the crimp. The maximum tensile force experienced is recorded as the crimp retention strength.
Needle Materials Testing
The steel needle itself must be produced to a high standard of quality. Force testing should play an integral role in both productions (as inline checks at steel formers and supplied materials quality checks at assemblers), and in development, to identify opportunities to reduce material usage without compromising functionality.
This mechanical test enables quantification of the material properties of the needle, and identification of areas of recurring weakness. The steel needle is held at either end by pneumatic grips, which exert an adjustable pressure suited to the gage of needle, eliminating early failure at the jaw face, It is common to thread small inserts into the needle ends to avoid collapse. He pneumatic grips should have lightly serrated faces to avoid sample slippage under loading. An axial tensile load is steadily applied until a clean break occurs in a central portion of the needle, clear of the grips.
The three-point-bend test is used to assess the stiffness of the needle, and hence the likelihood of permanent deformation on insertion. The test requires the use of a specialized fixture, comprising two adjustable lower support anvils on which the sample is placed, and a single upper anvil mounted onto the motorized crosshead of an automated testing machine. The upper anvil descends to a pre-determined height, effectuating a bend in the centre of the sample. Analysis of the load at a carrying vertical displacement allows calculation of the Young’s Modulus of the needle, a useful measure for characterizing its expected behaviour under typical loading in use.
Torque Assessment of Luer Lock Connectors
Needle hubs attach to syringes (and other devices) via push/pull taper luer, or threaded luer lock connectors. The force or torque required to disassemble the two elements must be high enough to avoid failure in operation and ensure a hermetic seal in the connection, yet low enough to enable quick and easy engagement and disassembly as required by the healthcare professional. In the case of Luer locks, Torque assessment should be performed on a low-level torque testing system, with a motorized base plate and a low capacity.
Assessing the mechanical properties of needles and syringes is a crucial ingredient in the successful development and production of such devices.
Syringes must deliver and extract fluids in a smooth, controlled manner. A syringe plunger that is too east or hard to actuate, or that stalls and judders on depression will not perform reliably during injection or aspiration. The efforts required to expel liquid from the syringe, and draw liquid into the syringe, are known as the expression force and aspiration force respectively. These will be determined by a range of variables, including the viscosity of the liquid, the size of the syringe/needle aperture, the ‘fit’ of the plunger within the syringe barrel and the density of the tissue substrate. He syringe must be developed with the optimum balance of these variables to assure consitency and usability. International standard ISO 7886 describes a method for determining aspiration and expression forces using a syringe half-filled with water, vertically mounted onto a motorized force testing system. The syringe outlet is connected to a reservoir (open to atmospheric pressure, with the water level aligned to fill level in the syringe chamber. The piston is half from below by a suitable lower fixture. On initiation, the system pulls the syringe plunger down at a constant rate, drawing fluid in the chamber, and records the aspiration force. The motor is then reserved to record the expression force. Anomalous peaks and troughs in the force profile are monitored to ensure the syringe provides a smooth motion of aspiration and expression. Other common force tests performed on syringes include chamber stress cracking to assess compression resistance and disassembly force testing of the rubber plunger from the piston.
Needle and syringe packaging must ensure the device remains secure, sterile, and undamaged during transportation and storage, while remaining easily accessible on demand. Force and torque testing in design, production, and processing greatly aids the creation of user-friendly and consistently reliable packaging.
There are many useful force and torque testing applications throughout the lifecycle of needle and syringe devices; from forming the needle, joining it to its hub, and packaging it in the factory to unwrapping the needle, attaching it to a syringe, and injecting a patient at the hospital. Developers and producers alike should take advantage of these tests as a fast, inexpensive, yet accurate method of assessing the quality, usability, and conformance of their products to international standards.