Metal Shafts - Creganna

1 Design1visit device technology december 2004 The challenge to deliverThere has been a surge in growth inalmost all sectors of the minimallyinvasive medical device industry inrecent years. Cardiology devices,possibly the biggest market, isexpected to experience growth ofmore than 12% in the next year, andthe neurovascular markets andperipheral vascular markets arepredicted to grow in excess of 30%and 9%, respectively, in the a large extent this growthhas been fuelled by developments intreatment options such as new drugs,procedures and device concepts,which are being developed on anincreasingly regular basis. These newtreatment options require physiciansto reach new areas of the anatomywith an increasing emphasis on lowerprofile systems. As a consequence,delivery-system designs and materialsare continually being challenged tokeep up with these new of the shaft performancecharacteristics of modern deliverysystems are subjective.

2 They are mostcommonly measured using compara-tive, company-specific tests that makeabsolute comparisons almost impos-sible. There also appears to be avariation between companies on theterminology used to describe manyshaft performance requirements. Forthis reason, an explanation of themost common performance require-ments are proposed ability of the shaftto transmit energy from one end ofthe catheter to the other, typicallyfrom the proximal to distal end as theshaft is advanced in the patient. Ashaft s pushability can be improvedby increasing its wall thickness,reducing its overall length, or increas-ing the stiffness of the material usedto make the shaft . Pushability is oftenmeasured as a ratio of the forceapplied to the proximal end of theshaft tothe corresponding forcerecorded at the distal ability of theshaft to transmit a rotational displace-ment along the length of the shaft .

3 Inapplications where torque is impor-tant a 1:1 torque ratio is the desiredresult. With this torque response, agiven angular rotation of one end ofthe shaft will directly relate to asimilar rotation of the opposing performance is most oftenexpressed as the ratio of the angularrotation applied to the proximal endof the shaft tothe correspondingrotation measured on the distal performance can be improvedby increasing the wall thickness ofthe shaft , increasing the shear stiff-ness of the material used to make theshaft, or decreasing the overall lengthof the measure of a shaft s abilityto maintain its cross-sectional profileduring deformation. The combinationof forces required to kink a tube canoccur in two situations. The firstoccurs when the shaft is beingtracked around a tight bend. Once thebend reaches a certain radius, thecompressive forces on the tube causea collapse of the wall on the inside ofthe bend.

4 The second occurs whenthe shaft is in the user s hands; it ispossible to exert a compressive forceon a section of the shaft , which willforce the shaft to deform with a tightradius that can also result in ability of a shaftto travel or track through tortuousanatomy. This is often measured as theforce required to push a shaft througha defined path. From the point of viewof the shaft , trackability is influencedby the flexibility of the shaft and canbe improved by reducing the shaft souter diameter ( ) or decreasing thematerial s elastic performanceFrom the definitions above, it isevident that not all of the desiredperformance requirements can beoptimised at once. For example,design guidelines to maximise Metal Shafts :Designs To Meet The RequiredPerformance When designing Shafts for minimally invasive devices such as catheters andguidewires, features that improve one area of performance may hinder performancein another area.

5 This article describes how the traditional limits of Metal Shafts canbe extended to enable their use in a variety of new FarrisseyCreganna Medical Devices, Galway, IrelandImages: jaffa:designmarch 2004 medical device technologyvisit are almost the directopposite of those seeking to improvetrackability. The challenge for theshaft designer is to find the optimumcompromise for the particular appli-cation and anatomy in have been developed forthe delivery of angioplasty balloons,stents, occlusion balloons, drugs,filters, light, cryogenic energy,aneurysm coils and pressure sensorsto many different parts of the humananatomy. In all applications, productdesigners continually strive toincrease the performance of thedelivery system. Despite the diverserange of applications, the designers ofthese devices still struggle with thesame performance characteristics.

6 Theprimary function of a minimallyinvasive medical device shaft is tofacilitate the delivery of a treatmentfrom outside the body to the locallyaffected site. In most cases, theproximal end of the shaft must berelatively stiff to provide pushabilityto the device and allow pushingwithout risk of kinking. The distal endof the shaft should be flexible enoughto traverse the tortuous anatomy toreach the treatment site. Throughoutthe length of the shaft the designerseeks to minimise the outer profile ofthe device to facilitate access throughsmaller openings and thus enable theuse of smaller access sheaths or guidecatheters. At the same time, consider-ation must be given to keeping theworking inner lumen of the device aslarge as possible to facilitate moreefficient treatments. In addition,the materials used for the shaft mustbe biocompatible2and chemicallyresistant.

7 Finally, of course, costwill be a consideration in almostall materials can be divided intothree general most commonmaterial in this category is stainlesssteel. Its high elastic modulus allowsit to exhibit excellent pushability andtorque, but at the expense of flexibil-ity. Nitinol Shafts can be included inthis category, but their extremelyhigh-cost, lower elastic modulusand processing difficulties makethem a less attractive option formany there is a vastrange of compounds in this category,nylon polyurethane and polyethyleneare some of the most commonly usedmaterials for Shafts . Polymer materialshave a lower elastic modulus than istypical of Metal Shafts , but greaterflexibility. In addition, they can haveexcellent biocompatability andlubricious , theseconsist of a woven Metal mesh as thematrix in a polymer composite.

8 Theseshafts display excellent torqueabilityand high burst pressures comparedwith pure polymer designFor all Shafts , stiffness is dependent onthe material s elastic modulus, its inner diameter ( ). In thisarticle, consideration is given toextending the traditional limits ofmetal Shafts to enable their use in avariety of new applications. Thedesigns can also allow seamless transi-tions between shaft sections withdifferent performance great amount of the design workin medical devices today is devoted tooptimising the transitions betweenvarious sections of the device. Withflexible, laser-profiled, stainless-steelshafts, stiffness transitions can beeasily incorporated into the stiffness of various Shafts can bemeasured using a three-point bendtest or other deflection-based meth-ods. With this information, thestiffness of the adjoining Shafts can bematched to give an almost flawlesstransition.

9 In addition, the stiffness ofvarious sections of the shaft can beaccurately controlled and modifiedover a predefined distance. Designengineers have optimised a numberof patterns that can improve flexibilityor torque transmission (see Figure 1).Anisotropic designs (designs withdifferent performance characteristicsin varying directions) can also be cutPatterns that improve flexibility or torque 1 Elongation under 5-N loadDesign shaftSlotted- Metal shaftTight pitch spiral shaftLoose pitch spiral shaftMetal shaftThe percentage of elongation of each shaft under a5-N tensile 2:design3visit device technology december 2004where the tube displays variousdegrees of flexibility in more thanone plane. When necessary, jacketscan be applied to the shaft to seal it,to reduce the friction or tofacilitate heat bonding of othercomponents to the study The fabrication of laser-cut, flexible,stainless-steel Shafts is described this application, 304 stainless-steelhypotubes were processed andpassivated in the normal laser-cut pattern was cut in thetubes using a pulse Nd-YAG laser(Rofin-Baasel4).

10 The kerf of thecutting width of the laser beam wasless than 20 m,which resulted in aminimal 5- m heat-affected this example, the laser-cuttingsystem was originally designed tocut coronary stainless steel stents,but it had been customised to workwith long tubes. In this design, thelaser beam was held fixed while thetube was rotated and advanced underthe laser beam. When correctlyfocussed, the laser cuts through thewall of the tube. With this technique,laser-cut profiles of almost any designcan be processed relatively the tube profile has been cut,cleaned and passivated, a jacket canbe applied to the shaft . This jacket canbe achieved in a number of differentways. With current technology, themost cost-effective option is for thejacket to be extruded directly over thestainless steel tube. Other methodsinclude heat shrinking, where apolymer jacket contracts to a presetdiameter on exposure to a certaintemperature, typically in excess of100 C, and discrete bonding ofextrusions to the following is an outline of twoparticular applications where profiledmetal shaft designs have been able tooffer a performance benefit in areaswhere Metal Shafts would tradition-ally not have been one: pushabilityDelivery Shafts for peripheral, self-expanding stents experience hightensile and compressive pull high force can be attributed tothe high force required to deploy theself-expanding stent from its outerprotective sheath.