JINHAOQIU
MAMITANAKA
Tohoku University Sendai Japan
INTRODUCTION
Biomedical applications of smart materials can be divided into three categories: (1) implants and stents, such as bone plates and marrow needles; (2) surgical and dental instru- ments, devices, and fixtures, such as orthodontic fixtures and biopsy forceps; and (3) devices and instruments for medical checkups, such as ultrasonic devices. The appli- cations of the first category require strict biocompatibility of a material because it is implanted in the body for long periods. Among many traditional materials, including met- als, alloys, and ceramics, that are available commercially, only a limited number are currently used as prostheses or biomaterials in medicine and dentistry. The applica- tions in the second category require excellent mechanical
BIOMEDICAL APPLICATIONS 83 characteristics as well as biocompatibility. The third cate-
gory is used mainly for transducers.
Among smart materials, the Ti–Ni shape-memory al- loy (SMA) has attracted the most attention for biomedical applications in the first and second categories due to its excellent biocompatibility and mechanical characteristics.
Research on biomedical applications of the SMA started in the 1970s with animal experiments initially, followed by clinical tests. The first example of a successful biomedical application of the SMA was a bone plate, which was used to repair broken bones. Now, many medical and dental appli- cations of SMAs are available, and many new applications are being developed. On the other hand, piezoelectric ma- terials have been widely used as transducers for medical ultrasonic devices due to their sensor function that uses piezoelectricity and the actuator function that uses inverse piezoelectricity.
In this article, the properties of SMAs for biomedical applications are discussed next, followed by some clinical examples. Recent examples of biomedical applications of SMAs are summarized there after, and finally the recent examples of biomedical applications of piezoelectric mate- rials are summarized.
PROPERTIES OF SMAS FOR BIOMEDICAL APPLICATIONS The properties of SMAs that are important and have led to its wide acceptance in biomedical applications are dis- cussed in this section. Of these properties, biocompatibili- lity, which simply means the ability of a material to be accepted by the body, is the most important, especially for implants. The other important properties include super- elasticity, the shape-memory effect, hysteresis, and fatigue resistance. The properties of SMAs for biomedical applica- tions are discussed in detail in (1).
Biocompatibility
The biocompatibility of a material is its most important property if it is used as prostheses or biomaterials in medicine and dentistry. Biocompatibility means that the material is nontoxic during the implanted period. Because all materials generate a “foreign body reaction” when im- planted in the body, the degree of biocompatibility is re- lated to the extent of this reaction. Due to the rigorous demands on material properties for biocompatibilty, only these three metallic materials were qualified for use as implant materials: Fe–Cr–Ni, Co–Cr and Ti–Al–V before SMA. Investigations were carried out by many researchers on the biocompatibility of Ti–Ni (2), and an extensive re- view can be found in (1). The results of these studies show that Ti–Ni has superior corrosion resistance due to the for- mation of a passive titanium oxide layer (TiO2) similar to that found on Ti alloys. This oxide layer increases the sta- bility of the surface layers by protecting the bulk material from corrosion and creates a physical and chemical barrier to Ni oxidation.
Inin vitrodissolution studies, Bishara et al. (3) found that Ti–Ni appliances release an average of 13.05µg/day Ni in saliva, which is significantly lower than the estimated
average dietary intake of 200–300µg/day. In addition, the measured nickel blood levels of orthodontic patients who have Ni–Ti appliances show no significant increase during a 5-month period.
Shape-Memory Effect
Predeformed SMAs can remember their original shapes be- fore deformation and can recover the shape when heated if the plastic deformation takes place in the martensitic phase (4,5). Shape recovery is the result of transformation from the low-temperature martensitic phase to the high- temperature austenitic phase when it is heated. The shape- memory effect makes it easy to deploy an SMA appliance in the body and makes it possible to create a prestress after deployment, when necessary. SMA appliances are first in a compact state during deployment and then re- stored to their expanded shape by heating. If the phase transformation temperature of an SMA is below body tem- perature, shape recovery can easily be induced by the heat of the body. When the phase transformation temperature is higher than the body temperature, SMA appliances are usually heated by warm salt water or a high frequency magnetic field.
In recent studies, the shape-memory effect has also been used for actuator functions in medical applications such as a urethral valve and artificial sphincter which are dis- cussed later.
Superelasticity
SMAs exhibit superelasticity when they are in the austen- itic phase (1,5). Figure 1 shows the typical superelastic stress–strain curve (solid line) compared with the stress–
strain curve of stainless steel (dashed line). As shown in the figure, an important feature of superelastic materials is that they exhibit constant loading and unloading stresses across a wide range of strain. As shown in Fig. 1, the
Stress
A
D E
Strain Subthreshold
F B
C
εeff (TN)
εeff (SS)
Optimal force zone Excessive force zone
Figure 1. Typical stress–strain curve of superelastic materials and stainless steel. The superelastic materials exhibit constant unloading stress over a wide range of strain.
84 BIOMEDICAL APPLICATIONS
effective strain range εeff(TN) of Ti–Ni that corresponds to an optimal force zone is much larger than theεeff(SS) of stainless steel. Hence, a superelastic device can provide constant pressure even if the pressed part recedes by a lim- ited amount during the installed period. On the other hand, the pressure exerted by an appliance made from stainless steel will drop drastically if the pressed part deforms, so that performance deteriorates. The orthodontic arch wire that is presented as an example of an application in the next section was the first product to use this property.
Another example of applying this property is superelastic eyeglass frames (6). These eyeglass frames have become very popular in the United States, Europe, and Japan and are available in almost every optician’s store. These frames can be twisted a full 180◦, but more importantly the frames press against the head with a constant and comfortable stress. Not only is “fit” less important, but small bends and twists that may develop do not cause discomfort to the wearer.
The superelasticity of SMAs also makes it easy to deploy SMA stents. Stents made from stainless steel are expanded against the vessel wall by plastic deformation caused by inflating a balloon placed inside the stent. Ti–Ni stents, on the other hand, are self-expanding. More details can be found later.
Hysteresis of SMA
As shown in Fig. 1, superelastic SMA exhibits a hysteretic stress–strain relationship; the stress from A to B in the loading phase and the stress from C to D in the unloading are different. Hysteresis is usually regarded as a drawback in traditional engineering applications, but it is useful in biomedical applications. If the SMA is set at some stress–
strain state, E, for example, upon unloading during deploy- ment, it should provide a light, constant force against the organ wall, even under a certain amount of further strain release (e.g., from E to D). On the other hand, it would gen- erate a large resistive force to crushing if it is compressed in the opposite direction because it takes the loading path from E to F. Hence, the SMA material exhibits a biased stiffness at point E, which is very important in designing a SMA stent. Because the stress at the loading phase from A to B and the stress in the unloading phase from C to D depend on the material composition of the SMA, the desi- rable stress–strain curve can be obtained by optimizing the material composition.
Anti-Kinking Properties
As shown in Fig. 1, the stress of stainless steel remains nearly constant in the plastic region. This means that a small increase in stress in the plastic region could lead to a drastic increase in strain or failure of a medical appli- ance made from stainless steel (1). On the other hand, the stiffness of superelastic Ti–Ni increases drastically after point B at the end of the loading plateau. The increase in stiffness would prevent the local strain in the high strain areas from further increasing and partition the strain in the areas of lower strain. Hence, strain localization is pre- vented by creating a more uniform strain than could be realized by using a conventional material.
EXAMPLES OF BIOMEDICAL APPLICATIONS Orthopedic
Marrow Needles. Figures 2 and 3 show two types of mar- row needles that are used in the repair of a broken thigh bone (4,6–8). When a stainless steel Kunster marrow nee- dle is used, blood flow inside the bone can be blocked, and recovery can be delayed. It also has the drawback of low torsional strength. On the other hand, a Kunster marrow needle of SMA can be inserted into the bone in its initial straight shape and transformed to a curved shape by heat- ing, as shown in Fig. 2. Hence, the SMA Kunster marrow
Thigh bone
Fracture
Mallow
Before heating (a)
Shape memory alloy
Fracture closed
SMA after shape recovery
After heating (b)
Figure 2. Kunster marrow needle (6).
BIOMEDICAL APPLICATIONS 85
Heat Heat
Figure 3.Marrow needles before and after heating (5,8).
needle can avoid the disadvantages of the stainless steel needle. The SMA Kunster marrow needle can also provide a compressive force on the fracture surfaces.
The marrow needle shown in Fig. 3 has a compli- cated shape for purposes of reinforcement, which makes it difficult to insert in the broken bone. Using the shape- memory effect, insertion can be greatly improved, as shown in the figure, without loosing the reinforcing function, be- cause the needles can be inserted in a simpler shape and the necessary size and shape are recovered by heating the needle in the marrow.
Currently available joint prostheses are made of bone cement to be fixed in the bone. Stress acting on the joint prosthesis is quite intense and severe: three to six times the body weight of the patient under nominal action, and the stress is cycled up to 106times. Conventional bone cement causes several inconveniences: gradual loosening after im- plantation, resultant infection, and other complications. A prosthetic joint made of Ti–Ni SMA was developed to avoid such problems. High wear resistance is also another advan- tage of the Ti–Ni prosthetic joint.
Bone Staple and Bone Plate. The bone staple shown in Fig. 4 and the bone plate in Fig. 5 are used to fix broken bones (4–6). As shown in Fig. 4, a bone staple made of SMA can be inserted at low temperature in the holes opened in the bone, and then heated by the body temperature, it re- covers its original shape to provide a compressive force on the surfaces of the broken bone. Bone plates are attached by screws for fixing broken bones. Bone plates made of Ti–
Ni SMA are more effective in connecting broken bones than bone plates made of conventional material because SMA
Inclination angle
Open at low temperature Insert Heating Closing due to SME θ
Figure 4. Bone staple used to fix broken bones (4,6).
SMA bone plate Bone
Compressive force
Figure 5. Bone plate used to fix broken bones (4–6).
bone plates provide compressive force on the fracture sur- face of the broken bones as well as repair, as shown in Fig. 5.
Healing proceeds faster under uniform compressive force.
Dental Applications
Orthodontic Fixtures. Due to its superelasticity, Ti–Ni is used in many applications in dentistry. It is obvious that superelasticity gives the orthodontists better mechanical characteristics compared to conventional elastic materials such as stainless steel. Figure 6a,b shows a clinical ex- ample of orthodontic treatment using a superelastic Ti–Ni arch wire (5,9). When fixtures made of conventional elas- tic material such as stainless steel are used, the reforming force drops, and the fixture loosens due to movement of the teeth. Hence, the fixture must be replaced several times before the treatment is finished. An SMA fixture main- tains a constant reforming force in a wide range of teeth movement due its superelasticity, so that no replacement is required after the initial installation. Clinical results also showed faster movement of the teeth and a shorter chair time compared with stainless steel wire.
Tooth-root Prosthesis. Among several methods that re- store the masticatory function of patients missing more than one tooth, a tooth-root prosthesis is considered the method that creates the most natural masticatory func- tion. Blade-type implants made of Ti–Ni SMA, as shown in Fig. 7, have been used in Japan (5,6). The open angle of the blade is used to ensure tight initial fixation and to avoid accidental sinking during mastication. But to make the insertion operation easy, the tooth-root prosthesis is
86 BIOMEDICAL APPLICATIONS (a)
Misaligned teeth before treatment (b)
Normally aligned teeth after the first stage of treatment Figure 6. Orthodontic treatment using a superelastic arch wire:
(a) misaligned teeth before treatment; (b) normally aligned teeth after the first stage of treatment (5,9).
implanted in the jawbone as a flat shape and then the opened shape is changed by heating. Fig. 8 shows an X-ray photograph of the implanted tooth-root prosthesis. More than 5,000 clinical examples of SMA tooth-root prostheses have been reported.
Partial Denture.The key to a partial denture is the development of an attachment for connecting the partial
Figure 7. Tooth-root prosthesis (5,6).
Figure 8. X-ray photo of implanted tooth-root prosthesis (6).
denture to the retained teeth. Clasps have been conven- tionally used for about a century as the attachment for a partial denture. One of the drawbacks of clasps made of conventional elastic materials is loosening during use;
this can be improved by replacing the elastic materials by a superelastic Ti–Ni alloy (5,10). Another drawback of clasps is aesthetics because they are visible in the teeth align- ment. To solve the problem, the size of the attachment must be smaller than the width of the teeth so that it can be embedded completely in the teeth. A precision attach- ment using a small screw has recently become available, but it has to be designed and fabricated very precisely so that it lacks flexibility to follow the change in the setting condition during long-term use due to the shape change of the jawbone. Because of its flexibility, this problem can be solved by using an attachment made of SMA.
The SMA attachment consists of two parts: a fixed part that is made of a conventional dental porcelain-fusible cast alloy and is attached to the full cast crown on the anchor teeth and a movable part that is made of Ti–Ni SMA and is fixed on the side of the partial denture. Examples of movable and fixed parts are shown in Fig. 9.
Surgical Instruments
Since superelastic tubing became available in the early to mid-1990s, a variety of catheter products and other en- dovascular devices using Ti–Ni has appeared on the mar- ket. Early applications of Ti–Ni were retrieval baskets that have Ti–Ni kink-resistant shafts, as well as a superelastic basket to retrieve stones from kidneys, bladders, and bile ducts. An interesting example is the interaortic balloon pump (IABP) used in cardiac assist procedures (Fig. 10).
The use of Ni–Ti allowed a reduction in the size of the de- vice compared with the polymer tube designs and increased the flexibility and kink resistance compared with stainless steel tube designs (1).
Biopsy forceps made from stainless steel are very deli- cate instruments that can be destroyed by even very slight mishandling. Ti–Ni instruments, on the other hand, can handle serious bending without buckling, kinking, or per- manent deformation. Figure 11 shows a 1.5-mm biopsy for- ceps that consists of thin wall Ti–Ni tubing and a Ti–Ni actuator wire inside. Together they can be bent around a
BIOMEDICAL APPLICATIONS 87 (a)
Movable part (b)
Fixed part
Figure 9. Shape-memory alloy attachment for partial denture;
(a) movable part; (b) fixed part (4,5).
radius of less than 3 cm without kinking, and still allow for the opening and closing of the distal grasper jaws without increased resistance. The instrument continues to operate smoothly even while bent around tortuous paths.
Figure 10. The Arrow interaortic balloon pump uses a Nitinol™
tube to pressurize the balloon (1).
Figure 11. Ninitol™ tubing that has an internal actuating wire allows this 0.8-mm diameter grasper to operate while tied in a knot (1).
Stent
The term stent is used for devices that are used to scaffold or brace the inside circumference of tubular passages or lu- mens, such as the esophagus biliary duct, and most impor- tantly, a host of blood vessels, including coronary, carotid, iliac, aorta, and femoral arteries (Fig. 12) (1). Stenting in the cardiovascular system is most often used as a follow- up to balloon angioplasty, a procedure in which a balloon is placed in the diseased vessel and expanded to reopen a clogged lumen. Ballooning provides immediate improve- ment in blood flow, but 30% of the patients have restenosed within a year and need further treatment. The place- ment of a stent immediately after angioplasty, it has been shown, significantly decreases the propensity for resteno- sis. Stents are also used to support grafts, for example, in treating aneurysms (Fig. 13).
Most stents today are stainless steel and are expanded against a vessel wall by plastic deformation caused by inflating a balloon placed inside the stent. Ti–Ni stents, on the other hand, are self-expanding—they are shape-set to the open configuration, compressed into a catheter, then
Figure 12. A stent that maintains vessel patency and blood flow to the brain is portrayed in a cutaway view of the internal carotid artery (1).
88 BIOMEDICAL APPLICATIONS
Figure 13.Stentgrafts used to exclude aneurysms, to provide an artificial replacement for injured vessels, or prevent restenosis after angioplasty (1).
pushed out of the catheter and allowed to expand against a vessel wall. Typically, the manufactured stent’s outer dia- meter is about 10% greater than the vessel’s diameter to ensure that the stent anchors firmly in place. The flexibi- lity of Ti–Ni is about 10–20 times greater than stainless steel, and it can bear a reversible strain as high as 10%.
Ni–Ti stents are made from knitted or welded wire, laser- cut or photoetched sheet, and laser-cut tubing. The pre- ferred devices are laser-cut tubing, thus avoiding overlaps and welds (Fig. 14).
CURRENT BIOMEDICAL APPLICATIONS OF SMA Artificial Urethral Valve
Urinary incontinence is the involuntary discharge of urine caused by weakness of the urinary canal sphincter muscles due to aging and expansion of the prostate gland. How- ever, the difference in ages, the sex of patients, and the various causes of the disorder make it difficult to treat the disease simply by drugs or surgery. In this section, an arti- ficial urethral valve system driven by an SMA actuator is introduced (11).
Figure 14. Stents made from laser-cut tubing (1).
Figure 15. Artificial urethral valve.
Urethral Valve. The artificial urethral valve should be compact and should have no protrusions when it is im- planted in the lower abdominal region. In addition, it should be attachable onto various sizes of urethrae. A com- pact urethral cylindrical valve is presented in Figs. 15 and 16. The valve is 15 mm across and 20 mm long. It is composed of two semicircular stainless steel shells 0.2 mm thick and a 0.2-mm circular-arc NitinolTMplate. The shells and the NitinolTM plate are fixed together by stainless steel clamps. Further, a cylindrical sponge rubber filling is placed inside the valve to effect uniform contact between the valve and the urinary canal. In the normal state, the valve presses on the canal, so that it is choked. To free the canal, the valve is opened by actuating the SMA element.
The SMA plate, which is cylindrical at body temperature, flattens as heat is increased, and the valve, which is closed by the force of the bias spring in the normal state, is opened to release the choked urethra and allow urinary flow. To heat the SMA, a NichromeTM wire, insulated by a poly- imide membrane, was placed on the surface of the nitinol plate.
Transcutaneous Energy Transformer System. The energy to drive an in-dwelled valve should be supplied from out- side the body. A transcutaneous energy transformer system will be effective for this purpose (12,13). The system con- sists of two induction coils that transmit electrical energy wirelessly from the primary to the secondary coil. In this study, the primary coil was 70 mm in diameter and had 12 turns, and the secondary coil was 60 mm across and had eight turns. The coils are spirals formed of twisted wires that consist of 20 lengths of copper wire 0.2 mm in diameter.
Experimental Setup. Figure 17 is the schematic of an an- imal experiment that used the urethrae of male dogs of average weight 12 kg, whose thickness is similar to that of human male urethrae. The urinary canal uncovered by the cut was equipped with the SMA valve and then loaded with water at a hydrostatic pressure of 75 cmH2O, which is comparable to human abdominal muscle pressure. The temperature of the valve and the flow rate of the water passing through the canal were measured. First, the valve