Originally Published MDDI September/October 2003
Needle manufacturers may find that adhesives can help them create safer devices.
Christine M. Salerni and James A. Serenson
Single-use needles make up a significant percentage of the disposable medical device market. Billions are produced annually, for use in a wide variety of devices. Hypodermic and angiographic needles, blood lancets, prepackaged medication syringes, introducer catheters, IV sets, venous-winged and port-access infusion sets, and blood collection sets all incorporate a needle or cannula.
The OSHA-driven Needlestick Safety and Prevention Act requires needle manufacturers to provide safer alternatives to existing needle designs, resulting in hundreds of new designs annually. In attempting to comply with this act while remaining competitive in the marketplace, manufacturers are exploring the uses of adhesives to facilitate needle assembly.
Traditional Methods of Needle Assembly
|Dispensing and fluorescence detection of light-cure adhesive.|
Needle assembly is a unique challenge. The joint that connects a stainless-steel cannula to a plastic hub must be well-sealed to prevent fluids, such as blood or medicine, from leaking. The joint is small and cylindrical, and only a high-strength joining method will ensure that the cannula will not move or release from the hub during use.
Manufacturers have historically used a variety of methods, such as molding, welding, interlocking, and sealing, to assemble disposable needles. However, these methods are limited. Time-consuming and expensive, insert molding the cannula into the plastic hub must be extremely accurate to ensure that leak paths are not present. Since the aesthetics of the needle assembly are critical, weld flash must be well-hidden or eliminated. Similarly, mechanically interlocking cannulae to hubs is limited due to expense and resource requirements; joint sealing is even more challenging.
Light-cure adhesives may offer a solution to assembly limitations. Adhesives generally deliver high-strength, hermetically sealed bonds; simple processing; and gap filling. Although a variety of adhesive types are suited and used for needle assembly, light-cure technology has become increasingly popular because of its fast cure and simple processing.
Light-Cure Adhesive Technology
Light curing acrylic adhesives are one-part, solvent-free liquids, with viscosities ranging from 50 cP to thixotropic gels. Upon exposure to light of the proper intensity and spectral output, the photoinitiator in the adhesive initiates cure. The resulting thermoset polymer offers excellent adhesion to a wide variety of substrates. While cure times depend on many factors, 6- to 20-second cures are typical, with cure depths in excess of 0.5 in. possible. The acrylic adhesives vary from rigid plastic materials to soft, flexible elastomers following cure.
When a conventional acrylic adhesive is cured in contact with air, polymerization can be incomplete, yielding a tacky surface. Light-cure technology offers a number of options to minimize tackiness. These include increasing irradiance levels, matching the spectral output of the light source to the absorbance spectrum of the photoinitiator, and covering the adhesive with a nitrogen blanket during cure.
These adhesives cure rapidly on demand, minimizing work in process, and offer virtually unlimited repositioning time before curing occurs. In addition, the wide range of available viscosities facilitates product selection for automated dispensing.
Light-cure cyanoacrylate technology offers manufacturers another option. Light curing cyanoacrylates are one-part, solvent-free materials with viscosities ranging from 20 to 900 cP. Upon exposure to low-intensity light, these adhesives cure rapidly to form thermoplastic polymers with superior adhesion to a wide variety of substrates. With this technology, adhesives in areas shielded from direct light will also cure to depths of 0.010 in.
Light curing cyanoacrylate adhesives are based on ethyl cyanoacrylate technology, which cures in the presence of a weak base, such as water. These adhesives also contain a photoinitiator that stimulates the secondary cure mechanism when exposed to low-intensity light. In addition, the use of specialty primers enables light-cure cyanoacrylates to develop strong bonds to polyolefins and other hard-to-bond plastics, such as fluoropolymers and acetal resins.
As illustrated in Table I, light-cure cyanoacrylates have a number of advantages over light-cure acrylics. These adhesives react rapidly to low-intensity light, allowing the end-user to select lower-cost curing systems. In addition, because light-cure cyanoacrylates are not susceptible to oxygen inhibition, surface tackiness is not a concern. The adhesives also minimize the potential for blooming or frosting, a white haze that appears around a bond line. Common with traditional cyanoacrylates, blooming occurs when a cyanoacrylate monomer reacts with moisture in the air and settles back on the part. Since any light curing cyanoacrylate material that squeezes out of a bond line or remains on a surface can be rapidly cured with light, the potential for blooming is reduced.
|Table I. Comparative benefits and considerations of light-cure acrylics and light-cure cyanoacrylates.|
Design and Process Considerations for Needle Assembly
|Highly fluorescent adhesives allow for in-process monitoring of the material's location.|
A number of key design and process variables must be considered for needle assemblies joined with light curing adhesives. Each of the variables described below can have a significant effect on the needle assembly.
Hub Material. The most commonly selected needle-hub materials include ABS, acrylic, polycarbonate, polyethylene, polypropylene, polysulfone, and polyurethane. Difficult-to-bond materials such as polyethylene and polypropylene typically require pretreatment to enhance bond strength. Corona treatment is often the pretreatment method of choice for needle manufacturers, because it can be done effectively in-line. In this process, the difficult-to-bond plastic is exposed to a corona discharge, usually in the presence of air and at atmospheric pressure. Such exposure roughens the surface and increases the surface energy of the substrate.
Another option is plasma treatment, a batch process that imparts chemical functionality to the difficult-to-bond material. Typical plasma gases include oxygen, argon, nitrogen, and ammonia. Different gases can have dramatically different effects not only on the substrate, but also on the selected adhesive.
The effects of corona discharge and plasma treatment on difficult-to-bond needle hubs are typically comparable. Table II provides typical needle pull strengths for polyethylene hubs and 22-gauge needles.
|Table II. Needle pull strength before and after plasma treatment on polyethylene hubs with 22-gauge cannulae (lb).|
Hub Well Design. The hub well, into which the cannula is inserted, has two functions. The well helps align the adhesive-dispensing tip with the cannula-to-hub bond joint, and promotes adhesive flow into the bond line. As the adhesive is introduced to the needle assembly, the well ensures that the material flows and achieves adequate bond line coverage. The larger the well diameter, the faster the adhesive typically flows into the bond region.
Core or Engagement Length. The core or engagement length is located beneath the well, where the inner diameter of the hub decreases to a slight slip-fit with the cannula. By increasing engagement length, needle pull strength will increase until the adhesive joint's strength exceeds that of the hub or cannula. However, when high-viscosity adhesives are used, the adhesive may only partially fill the engagement length.
Cannula Gauge Size. Cannula gauge size has a significant impact on both pull strength and failure mode. As gauge size increases, the circumference of the cannula decreases, resulting in reduced surface area. This reduction in bond area can decrease needle pull strength versus a needle assembly with a smaller-gauge cannula or an increased cannula circumference. Adhesive bonds of high-gauge cannulae, such as 27-gauge, often fail to the cannula, leaving the majority of the adhesive in the hub. Conversely, smaller cannula gauge sizes, such as 22-gauge, typically fail to the hub.
Diametrical Gap. The diametrical gap is the difference between the inner diameter of the hub and the outer diameter of the cannula. A 0.002-in. diametrical gap is common. As the diametrical gap decreases, the adhesive fills the annular area in the core more slowly, and is more likely to migrate out the back of the hub. This could reduce pull strength.
Annular Rings. Capable of significantly increasing adhesive strength, annular rings are recesses that are molded or machined into the well or core of the needle hub. Although the depths of annular rings can vary, recesses typically measure 0.005 to 0.008 in. When an adhesive flows into the recessed rings and cures, it bonds to the cannula and is mechanically held in the rings. Annular rings are most commonly used in hubs made of difficult-to-bond substrates such as polyethylene and polypropylene.
Dispense-Process Type. Pre- and postassembly dispensing are two primary adhesive-dispense methods used by needle manufacturers. In preassembly dispensing, manufacturers dispense high-viscosity adhesive (3000 to 30,000 cP) onto the cannula before inserting it into the well or hub. Preassembly dispensing ensures full adhesive coverage with minimal flow times.
Alternatively, manufacturers may dispense the adhesive after the cannula has been assembled to the hub. This postassembly dispensing process requires that a low-viscosity adhesive be used, typically 20 to 3000 cP. The lower the viscosity, the faster the adhesive can fully cover the joint.
|A sample needle design incorporating annular rings for increased adhesion in difficult-to-bond substrates.|
Dispense Inspection. Needle manufacturers often monitor the dispense process to ensure that it is controlled. Typical state-of-the-art monitoring systems work with adhesive viscosities ranging from water-thin liquids to thick pastes. Such systems commonly use real-time data acquisition to detect
- Plugged or broken dispense tips.
- Air bubbles.
- Missed dispense cycles.
- Low pressure (undersized drops).
- High pressure (oversized drops).
Flow or Dwell Time. Critical in postassembly dispense processes, flow or dwell time is the time required to allow dispensed adhesive to fully flow into the well and core. In general, lower-viscosity adhesives flow quickly, and achieve full coverage of the bond line sooner than high-viscosity adhesives. Table III provides a summary of the viscosity–flow time relationship for needle assemblies with a diametrical gap of 0.002 in.
TO FILL WELL
TO FILL CORE
|Data should be used as a guideline only. On-part testing should be conducted to determine actual flow times required for a specific manufacturing process.|
|Table III. Viscosity–flow time relationship with 0.002-in. diametrical gap (in min:sec).|
Cure Method. All adhesives undergo some type of curing process that converts the liquid material to a solid. Heat curing and light curing are the two most common methods for high-volume needle production, although light curing is rapidly becoming the preferred method. When outlining the assembly process, needle manufacturers must carefully consider the adhesive's performance, the required cure time, and the associated curing equipment.
Light curing adhesives require specialized light sources that emit radiant energy or light of the appropriate wavelength and intensity. Light sources vary in intensity, spectral output, cure area, size, power, and safety requirements. When compared to traditional heat-cure processes, light curing systems typically require less energy, floor space, manufacturing time, and work-in-process.
In-Line Detection. Although it is possible to audit the dispensing process in-line to ensure that the adhesive has been applied, such inspection does not guarantee that the adhesive flowed into the joint correctly. Since adhesives specifically developed for disposable medical device applications often contain fluorescent dyes, fluorescence detectors can be used to inspect the components for the presence of adhesive. Such a process can be conducted pre- or postcure.
Needle manufacturing is a precise and intricate science. Engineers attempting to design needles that are safer than those that are currently manufactured may find that adhesives offer many alternatives to traditional assembly methods. The simple and rapid curing process and the high-strength bonds that light-cure adhesives deliver make them an attractive, low-cost option to needle manufacturers.
Christine M. Salerni is medical market manager and James A. Serenson is engineering manager at Henkel Loctite (Rocky Hill, CT).
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