design 1 visit www.medicaldevicesonline.com medical device technology ❘ december 2004 The challenge to deliver There has been a surge in growth in almost all sectors of the minimally invasive medical device industry in recent years. Cardiology devices, possibly the biggest market, is expected to experience growth of more than 12% in the next year, and the neurovascular markets and peripheral vascular markets are predicted to grow in excess of 30% and 9%, respectively, in the same period. 1 To a large extent this growth has been fuelled by developments in treatment options such as new drugs, procedures and device concepts, which are being developed on an increasingly regular basis.These new treatment options require physicians to reach new areas of the anatomy with an increasing emphasis on lower profile systems. As a consequence, delivery-system designs and materials are continually being challenged to keep up with these new treatments. Many of the shaft performance characteristics of modern delivery systems are subjective.They are most commonly measured using compara- tive, company-specific tests that make absolute comparisons almost impos- sible.There also appears to be a variation between companies on the terminology used to describe many shaft performance requirements. For this reason, an explanation of the most common performance require- ments are proposed below. Definitions Pushability. The ability of the shaft to transmit energy from one end of the catheter to the other, typically from the proximal to distal end as the shaft is advanced in the patient. A shaft’s pushability can be improved by increasing its wall thickness, reducing its overall length, or increas- ing the stiffness of the material used to make the shaft. Pushability is often measured as a ratio of the force applied to the proximal end of the shaft to the corresponding force recorded at the distal end. Torqueability. The ability of the shaft to transmit a rotational displace- ment along the length of the shaft. In applications where torque is impor- tant a 1:1 torque ratio is the desired result. With this torque response, a given angular rotation of one end of the shaft will directly relate to a similar rotation of the opposing end. Torque performance is most often expressed as the ratio of the angular rotation applied to the proximal end of the shaft to the corresponding rotation measured on the distal end. Torque performance can be improved by increasing the wall thickness of the shaft, increasing the shear stiff- ness of the material used to make the shaft, or decreasing the overall length of the shaft. Kink. A measure of a shaft’s ability to maintain its cross-sectional profile during deformation. The combination of forces required to kink a tube can occur in two situations.The first occurs when the shaft is being tracked around a tight bend. Once the bend reaches a certain radius, the compressive forces on the tube cause a collapse of the wall on the inside of the bend. The second occurs when the shaft is in the user’s hands; it is possible to exert a compressive force on a section of the shaft, which will force the shaft to deform with a tight radius that can also result in kink failure. Trackability. The ability of a shaft to travel or track through tortuous anatomy.This is often measured as the force required to push a shaft through a defined path. From the point of view of the shaft, trackability is influenced by the flexibility of the shaft and can be improved by reducing the shaft’s outer diameter (o.d.) or decreasing the material’s elastic modulus. Optimising performance From the definitions above, it is evident that not all of the desired performance requirements can be optimised at once. For example, design guidelines to maximise ➔ Metal Shafts: Designs To Meet The Required Performance When designing shafts for minimally invasive devices such as catheters and guidewires, features that improve one area of performance may hinder performance in another area. This article describes how the traditional limits of metal shafts can be extended to enable their use in a variety of new applications. Liam Farrissey Creganna Medical Devices, Galway, Ireland Images: jaffa:design
Transcript
design 1
visit www.medicaldevicesonline.com medical 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 sameperiod.1 To 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 treatments.
Many of the shaft performancecharacteristics of modern deliverysystems are subjective.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 the
most common performance require-ments are proposed below.
DefinitionsPushability. The ability of the shaft
to 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 to the corresponding forcerecorded at the distal end.
Torqueability. The ability of theshaft to transmit a rotational displace-ment along the length of the shaft. 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 end.Torque performance is most oftenexpressed as the ratio of the angularrotation applied to the proximal endof the shaft to the correspondingrotation measured on the distal end.Torque performance can be improvedby increasing the wall thickness ofthe shaft, increasing the shear stiff-ness of the material used to make the
shaft, or decreasing the overall lengthof the shaft.
Kink. A 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.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 kinkfailure.
Trackability.The 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 (o.d.) or decreasing thematerial’s elastic modulus.
Optimising 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.This article describes how the traditional limits of metal shafts canbe extended to enable their use in a variety of new applications.
Liam FarrisseyCreganna Medical Devices, Galway, Ireland Im
ages
:jaffa
:des
ign
march 2004 ❘ medical device technology visit www.medicaldevicesonline.com
2 design
pushability 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 question.
Systems 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.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 pushability
to 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 biocompatible2 and chemicallyresistant. Finally, of course, costwill be a consideration in almostall designs.
MaterialsShaft materials can be divided intothree general categories.
Metal shafts. The most commonmaterial in this category is stainless
steel. 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 applications.
Polymers. Although 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 properties.
Composite shafts. Typically, theseconsist of a woven metal mesh as thematrix in a polymer composite.Theseshafts display excellent torqueabilityand high burst pressures comparedwith pure polymer tubes.
Metal-shaft designFor all shafts, stiffness is dependent onthe material’s elastic modulus, its o.d.and inner diameter (i.d.). 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 characteristics.
A 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 design.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. 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 cut
Patterns that improve flexibility or torque transmission.Figure 1:
0.02
0
% E
lon
gat
ion
un
der
5-N
load
Design type
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
Braided shaft
Slotted- metal shaft
Tight pitch spiral shaft
Loose pitch
spiral shaft
Metal shaft
The percentage of elongation of each shaft under a
5-N tensile load.
Figure 2:
design 3
visit www.medicaldevicesonline.com medical device technology ❘ december 2004
where the tube displays variousdegrees of flexibility in more thanone plane.When necessary, jacketscan be applied to the shaft to seal it,to reduce the o.d. friction or tofacilitate heat bonding of othercomponents to the shaft.
Case study The fabrication of laser-cut, flexible,stainless-steel shafts is described here.In this application, 304 stainless-steelhypotubes were processed andpassivated in the normal way.3
A laser-cut pattern was cut in thetubes using a pulse Nd-YAG laser(Rofin-Baasel4).The kerf of thecutting width of the laser beam wasless than 20 µm, which resulted in aminimal 5-µm heat-affected zone.In 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 quickly.Once 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 shaft.
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 considered.
Application one:pushabilityDelivery shafts for peripheral, self-expanding stents experience hightensile and compressive pull forces.This high force can be attributed tothe high force required to deploy theself-expanding stent from its outer
protective sheath.The outer sheath isoften polymer lined and can result inthe stent struts becoming embeddedin the polymer liner. As the stentcontinues to exert an outward radialforce, the force to move the stentfrom the outer liner can be high. Inthis case, extension of the outer orinner shafts under loading can resultin a lack of accuracy during place-ment of the stent.
To optimise the accuracy of stentpositioning, the shaft should beflexible enough to reach the carotidartery or the biliary duct, but shouldnot extend or contract under the largeforces required to deploy the stent.Until now, braided and polymershafts have been the design of choicefor this application. Pure polymertubes have the flexibility, but cannotwithstand the large deploymentforces. Solid metal shafts can sustainthe forces with minimal extension,
but have not been flexible enough tonegotiate the relevant anatomy. Forthis reason, braided shafts have beenused in the vast majority of self-expanding, stent-delivery systems.The results presented in Figure 2suggest that profiled metal shafts arean ideal solution to this designproblem. It is no surprise that thesolid metal shaft shows the leastdeformation and the braided shaftexhibits the most deformation underthe applied load. For all options, theo.d. of the tubes remained the same.Figure 3 shows the tested flexibilityof the profiled metal shafts andFigure 4 creates an impression of thereal flexibility of these shafts.
Figure 5 shows the o.d. and i.d.profile of the different shaft options.For effective comparison, the o.d.s ofall the selected tubes were as close aspossible.The o.d. of the profileddesigns was 0.001 in. (0.0254 mm) ➔
10
0
Rad
ius
at w
hic
h k
inki
ng
occ
urs
Design type
20
30
40
50
60
70
Braided shaft
Slotted- metal shaft
Tight pitch spiral shaft
Loose pitch
spiral shaft
Metal shaft
The minimum bending radius before the shaft kinks.
In this example, flexibility is measured as the ability of the tube
to resist kinking. It can be seen that the braided shaft and the
slotted, profiled metal shaft have almost identical kink properties.
Figure 3:
A laser profiled shaft negotiating a tight radius.Figure 4:
december 2004 ❘ medical device technology visit www.medicaldevicesonline.com
4 design
higher than the other designs becausea 0.0005-in. (0.0127-mm) thicksingle-wall polyester heat-shrink layerwas applied to the o.d..5 The nature ofthe braiding process meant that thebraided tube design had a signifi-cantly smaller i.d. and, therefore, asmaller working lumen.
Application two:torque performanceIn many applications, torque responseis essential for good product perfor-mance.Traditionally, polymer-braidedshafts using high-tensile wire at atight pitch have shown the bestproduct performance. Figure 6 showsthe torque response of a braided shaftand the comparative response of aprofiled metal shaft. In addition tothe superior torque performance, theprofiled metal shaft has advantages interms of profile and pushability,however, it does not have high burstpressure.
The torque-response test wasperformed using an 8Fr guidecatheter that was immersed inwater at 37 °C.The guide catheterwas tracked through an anatomymodel built to represent the humanaortic arch.The results indicated thatstainless steel shafts can be designedto give good torque response in ananatomy, where stainless steel tubingwould previously not have beenconsidered because of its highstiffness. In this instance, the stainlesssteel construction has the addedbenefit of offering a 30% reductionin the shaft wall thickness intypical applications.
Pushing performance boundariesIt is clear that there are manycompeting performance considera-tions to take into account whendesigning a medical device deliveryshaft and individual performancefeatures cannot be designed in isola-tion.With this in mind, the job of thedesigner is to obtain the best possiblecombination of features for the partic-ular application.The applicationsconsidered here allow designers todevelop the flexibility associated witha polymer shaft and the torqueresponse associated with a braidedshaft in a single product. In somecases, this allows the designer tofacilitate shaft performance outsidethe traditional constraints imposedby material properties. It moves theindustry closer to the possibility ofdesigning a shaft that will “flex likea polymer and push like steel.”
References1. Millennium Research Group 2004.2. ISO 10993-1: 2003: Requirements for
