Biomaterialsanintro-parklakes - Chap11and12

SOFT TISSUE REPLACEMENT' I: SUTURES, SKIN, AND MAXILLOFACIAL IMPLANTS In soft tissue implants as in other applications that involve engineering, the performance of an implanted device depends on both the materials used and the design of the device or implant. 7he iniiial selection of material should be based oranice. The final iudement on the suitabilitv on sound materials enanccrinr- , of the material depends on observation of the in uiw clinical performance of the implant. Such observations may require many years. This requirement of in viw observation represents one of the major problems in the selection of appropriate materials for use in the human body. Another problem is that the performance of an implant may also depend on the design rather than the materials per se. The success ofson tissue implants has primarily been due to the development of synthetic polymers. This is mainly because the-polymers can be tailor-made to match the properties of sort tissues. In addition, polymers can he made into various physical forms, such as liquid for filling spaces, fibers for suture materials, films for catheter balloons. knitted fabrics for blood vessel orostheses. and solid forms for cosmetic and weight-bearing applications. It should be rewgnized that diflerent applications require different materials with specific properties. The following are minimal requirements for all soft tissue implant materials:

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1. They should achieve a reasonably dose approximation of physical properties, especially flexibility and texture. 2. They should not deteriorate or change properties after implantation with time. 3. They should not cause adverse tissue reaction. 4. They should be noncarcinogenic, nontoxic, nonallcrgenic. and nonimmunogenic.

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5. They should be s t c r i l i l e . 6. They should be low cost. Other important factors are feasibility of mass production, aesthetic quality, etc.

11.1. SUTURES, SURGICAL TAPES. AND ADHESIVES The most wmmon soft tissue implants are sutures. In reant years, s u a c a l tapes and tissue adhesives have been added to the surpeon's amamentarium. Although their use in actual surgery is limited lo some surgical procedures, they are indispensable.

T h m are two t y p e of sutures, dassificd aa lo their long-term physical in vlw integrity: absorbable and nonabsorbable. They may also be dislinguishcd by their raw material source: natural sutured (catgut, silk, and cotton) and synthetic suture8 (nylon, polyethylene, polypropylene, stainless steel, and tantalum). Suture8 may slso be classified aaording to their physical form: monofilament and multifilament The absorbable suture, catgut, is made of collagen derived from sheep intestinal submucosa. It is usuallv t m t e d with a chromic salt to increase its strength and is cross-Hnked to retard resorption. Such treatment extends the life of catgut suture from 3-7 days up to 20-40 days. Table 11-1 gives initial strength data for catgut sutures according to Uleir sizes. The catgut sutures are preserved

SOF TISSUE REPIACEMENT I with needles in a physiological solution in order to prevent drying, which would make the sutures very brittle and thus not easily usable. It is interesting to note that the stress concentration at a surgical knot decreases the suture strength of catgut by half, no matter what kind of knotting technique is used. If is suggested that the most effectiveknotting technique is the square knot with three ties to prevent loosening. According to one study there is no measurable difference in the rate of wound healing whether the suture is tied loosely or tightly. Therefore, loosesuturingis recommended because it lessens pain and reduces ~ t t i n gsoft tissues. Catgut and other absorbable sutures (e.g., polyglycolic acid, PGA: polylactic acid, PLA) invoke tissue reactions although the effect diminishes as they are being absorbed. This is true of other natural, nonabsorbable sutures like silk and coflon, which showed more reaction than synthetic sutures like polyester, nylon, polyaqlonitrile. etc. as shown in Figure 11-1. As in the case of the wound healing process (discussed in Chapter 10). the cellular response is most intensive one day after suturing and subsides in about a week. As for the risk of infection, if the suture is contaminated even slightly the incidence of infection increases manyfold. The most significant factor in infeaion is the chemical structure, not thegeometric configuration ofthesuture. Polypropylene, nylon, and PGA sutures developed lesser degrees of infection than sutures made of stainless steel, plain and chromic catgut, and polyester. The ultimate

Tabh 11-1. Minlmum Bmskinp Loads for Brtt1.h-Mode Catgur

Mamacr (mm) Size

Minlmom

M*rimum

Minimum bratim lwd (IbO StnigM pull

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FL A G.Rmn". "Nmnnl M.meti&,~In Bunoronb.. London. 1951. pm.

M r m llnd h sunrcni Momid& L Gilb led.).

6 10 TIME idoysl Flgure 11.1. Cdluler reswnse lo a m r e d maledall. From R. W. Pastlsthwan. J. F. Schaube. M. L Dillon, and J. Mowen. 'Wound Hsalinp. II.An Evaluation ot Surgicsl Suture Matsrisl." Svrg. Gymcol. Obstst, ICU. 55Ew6, 7959.

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cause of infection is a pathogenic microorganism, not the biomaterial. The role of the suture in infection is to orovide a wnduit for i n m s s of bacteria to chemically or physically modify the body's immune response, o r to provide an environment favorable to baaerial growth.

REPLACEMENT I

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11.12. Surgical T a p s and Stapler

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Surgical tapes are intended to offer a meana of avoiding pressure necrosis, scar tissue formation.. -oroblema of stitch abscesses. and weakened tissues. The problems with surgical tapes are similar to those experienced with Band-Aids: (1) . . misaligned wound edges.. .(2). .Door adhesion caused bv moisture or dirlv wounds. (3) late separation of taper when hematoma, wound drainage, etc. occur. The wound strength and scar formation in the skin may depend on the type of incision made. If the subcutaneous muscles in the falty tissue are cut and the overlying skin is closed with tape, then the muscles retract This in turn increases the scar area, resulting in poor cosmetic appearance wmpared to a suture closun. However, with the higher strength of scar tissue, the taped wound has a higher wound strength than the sutured wounds only if the muscles were not cut. Because, of this, tapes have not enjoyed the success that WM anticipated when they were introduced. Taoes have been used successfully for aasembiina - s m .m of donor skin for skin graft, correcting nerve tissues for neural regrowth. etc. Staoln made of metals (Ta. . . stainless steel. and Ti-Ni allov) - . can be used to facilitate closure of large surgical incisions produced in procedures such as cesarean sections. intestinal surnerv. .,and surnerv for bone fractures. The tissue response to the staples is the same as that of synthetic sutures but they are not used in places where esthetic outlook is important.

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The special environment of tissues and their regenerative capacity make the develooment of an ideal tissue adhesive difficult. Past exrnrience indicates that the ideal tissue adhesive should be able to he wet and bond to tissues, be capable of rapid polymerization without producing excessive heat or toxic by-products, be resorbable as the wounds heal.not to interfere with the normal healina process. have ease of application during surgery, be sterilizsble. have adequateshelf life; and case of large-scale production. The main atrcngth of tissue adhesion comes from the wvalent bonding between amine,carboxylic acid, and hydroxyl g r o u p oftissues, and the functional groups of adheaives such as

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Flgure 11-2. Bond strength ot wounds vrith dlffsrent ~ l 0 l W amsterlal. Fmm S. Houston, J. W. Hodgs. Jr.. 0. K Ousterhour,and F. Lsonard.'The Effectof a-Cyanoaoylats on Wound Hsatlnp." J. B i o W . M s l Re*.. 3.281-289. 1m9.

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cyanoacrylate are most promising. With the addition of some plasticizers and fillers they are commercially known as Eastman 910". Crazy Cilue'. etc. An interesting wmparison is illustrated in Figure 11-2, which shdws that the bond strength of adhesive-treated wounds is about half that of sutured wounds after 10 days. Because ofthe lower strcngth and lesser predictability of in vivo performance of adhesives. the aoolication is limited to use aner trauma on fragile tissues .. such as spleen, liver, and kidney or aner an extensive surgery on soft tissues such as lung. The topical use of adhesives in plastic surgery and fractured teeth has been moderately successful. As with many other adhesives, the end results of the bond depend on many variables such as thickness, open porosity, and flexibility of the adhesive film. as well as the rate of degradation. Some workers have tried to use adhesives derived from fibrinogen. which is one of the clotting elements of blood. This material has sufficient strength (0.1 MPa) and elastic modulus (0.15 MPa) to sustain the adhesiveness for the anastomoses of nerve, microvascular surgery. dural closing, bone graft fixation, skin gran fixation. and other soft tissue fixation. This material is available wmmercially in Europe and will be in the United States pending FDA approval.

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(11.1)

N=O I

T h e n are several adhesives available of which alkyl-a-cpnoacrylate is beat known. Among the homologuea of alkyl-cyanoacrylate. the methyl- and ethyl-2-

A nylon suture was implanted in the abdominal cavify of. dog. The autum was removed sncr 10 and 20 dam and its averale tearite stren@ was measured. T h e strength decreased

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by 40 and SOX. rspcuively. How long will it take for the avendh to dcay 6W of i t l original strength7 Assume an exponential decay of alrength. Anmwr Sina the strength derrusea exponemially we can asaume

where A, B ere constants, r is time (day.). and v, is the atrnyh at time r and original 81renflh. Therefore.

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SOT TISSUE REPLACEMENT I

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Many variables and factors are involved in the development of percutaneous devices. These are: 1. End-use factors: Transmission of information (biopotentials, temperature, pressure, blood Row rate), energy (electrical stimulation, power for heart assist devices), matter (cannula for blood), and load (attachmerit of prosthesis) 2. Engineering factors a. Materials selection: polymers, ceramics, metals, and composites' b. Design variation: button, tube with and without skirt, porous or smooth surface, etc. c. Mechanicnl stresses (soft o r hard tissue interface, porous or smooth interface) 3. Rioloaical factors a. Implant host: man. dog, hog, rabbit, sheep, a c . b. lmolant location: abdominal. dorsal. forearm. a c . 4. ~ u m a factors i a. Postsurgical care b. Implantation technique c. Esthetic outlook Figure 11-3 shows a simplified cross-sectional view of a generalized percutaneous device (PD), which can be broken down into five regions:

11.2. PERCUTANEOUS AND SKIN IMPLANTS 'Ihe need for permtaneoua (tram o r through the skin) implanta haa been accelerated by the advent of artificial kidneys and hearts, and by the need for prolonged injection of drugs and nutrients. Artificial skin (or dressing) is urgently needed to maintain the body temperature of severely burned patients. Actual permanent replacement of skin by biomaterials is beyond the capability of today's technology.

The problem of obtaining a functional and 1 viable l n h r f a a between the tissue (skin) and an imolant (mrmtaneoua) device is ~rimarilydue to the following factors. First, al;hough~nitialattachment of the tissue intoihe internti= of the imolant surface occurs. it cannot be maintained for a long-. ocriod of time. since the dermal tissue cells turn over continuously and dynamically. Furthermore, downgrowth of epithelium around the implant (extrusion) or overgrowth of implant (invagination) occurs. Sewnd, any openings large enough for bacteria to penetrate may result in infection even though initially there is complete sealing between skin and implant.

A. lnterface between epidermis and PD should be completely sealed against invasion by foreign organisms. B. lnterface between dermis and PD should reinforce the sealing of (A). as well as resist mechanical stresses. Due to the relatively large thickness of the dermis, the mechanical aspect is more important at this interface. C. lnterface between hwodermis and PD should reinforce the function of .. ( 8 ) . The immobilization of the PD against piston action is a primary function of (C). D. Implant material perse should meet all of the requirements of an implant for soft tissue replacement. E. The line where epidermis, air. and PD meet is called a three-phase line which is similar to (A).

FCurs 11-3. Simp116ed cmss-sactionel view of PO-Skin intsrfscss. Fmm A. F. wn Racum end J. 8. Park, '%r. CUlIIneo~eDevices." C R C W t Rm. Bio.

ang.. 6. 37-77. 1979.

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SOFT TISSUE REPLACEMENT I uie of a pin connector with good provision Tor firm tissue attachment subcutaneously. No PDs have been completely satisfactory. Nevertheless, some researchers believe that hydroxyavatite may be a solution tothe vroblem. In one ex~erimcntal trial, hydroxiapati1e-based PDS showed very little epidermal downgrokth (1 mm aner 17 months versus 4.6 mm afler 3 months for the siliwne rubber cdntrol specimens in dona1 skin of canines. see Figure 11-6) and a high level of sucass rate (over 80% versus less than 50% for the wntroll. The amino acid wntcnts ~~~~~~ofthe tissue capsules formed overthesubcutaneous implantsofthesame mai&ials showed that the hydroxyavatite site had the same comoosition as the oeriosteum .. of the femur while the wntrol site showed a similar composition to that found in pathological tissues. Some researchers have tried to switch to subcutaneous implants that can be accessed by a needle Tor peritoneal dialysis as shown in Figure 11-7. ~~~

mechankal .tresses among st the P D l k i n Inlertsu. From A. F. van Recum end - -J. .-~ 8. Park "Percutensou. Devices:' CRC Crh. Rev. Blosng., 6, 37-77, ~

The stresses acnerated between a cylindrical PD and skin tissue can be simplified as shown in Figure 11-4. The relative motion of the skin and implant results in shear stresses. which can be avoided if the imolant floats (or moves) freely. For this reason, P D without ~ connected leads or caiheten funciion longer. ~ h c r have c hecn many different PD designs to minimize shcarstrcsrcr. All deliens have centered around creating a good skin tissue/implant attachment in order to stabilize the implant. This is done by providing felts, veloun, and other porous materials at the interface. Figure 11-5 shows a dcsinn to minimize the transfer of stresses and strains to the skin. The device includes an air chamber made of a rubber balloon (a) interposed between skin and PD. and firmer fixation o i t h e cannula by providing a large surface for tissue ingrowth (band c). Some designs have tried to minimize the trauma imposed by the external tubes and wires by

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Figure 11-6. Schematic drawlng of a Gmsss-Sieltrup PD. F m m C. Grorss-Slemrup,EnMlcWunp und Kiinische Erprobungwn HeutdurChlsitunpnVst~In~erm~dlzln, Dltsartstlon. Free Univaniiy. Berlin. 1978.

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112.2. Artfficisl Sklns

Artificial skin is another example of a percutaneous implant. and hence the problems are similar to those described in the previous section. Most needed for this application is a material that can adhere to a lame (burned) surface and thus prevent the loss of fluids, electrolytes, and other biomoleculca until the

F g u n I I 8 Hlslolo8lcal v i e w ot the csnlns dennel l1s.m adjennt m PD mads of hvdrowpatm (Isfl)and silimns rubbe. lrlght) 3 month ~ost~mpl.mmion(ca. IOOr mspnhimion). Fmm H. Aok,. U. Ahso. Y Shm. T. Tsukl. and 1 loo. ~. "Smtand v~ - - .Hrdrnrumlulil. ,. ,---...-tn. .P..nn+.--u.Dsvics end Its Clinical Applications." M.d A.og. Techno1 12. 113-220, 1887. ~

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CHAPTER 11

SOIFT TISSUE REPLLICEMENT I

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Flgvn 17-8. Schematic repmentation (not certain physicoehsmical and I mechanical raquirsmants in the design of an Wwnd bed' a P 8 sffecliva wound closure. (a) Skin grsft (hatchf ingl d w r not displace sir pockets ( a m ) efficiently fmm the greft-wound bed intsrfsca. c d ' (b) Flexure1 rlgiditq of grafl la excessive; grafl does not deform suficienHy under iU ovm W i g M to mskl) contact wilh dsprsasions in wound bad surface, and air pockets (arrow) result. (c) Shear nrssssr (arrows) cauaa buck8 ling of graft, ruptures of the grsh-wound bed bond. and formnion of air pocXst. (d) Peeling torP iifn grsh away from wound bed. (a) Excsrsivaly Kwh molstun flux mts thmugh grew causes dehydration and development of shrinkage streesss at edger (arms),which caurs lin-otf sway fmm wound bed. (1) Vary low moisture flux J causes accumulation (edema) at the graft-wound bed lntarfscs and pasling off (snows). From I. V. Ysnnas and J. F. Burke. Daslgn of an anifidal ski-I. Desic design principla. J. Biomsd. Marsr. R e . 14.0561. 1980.

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into siliwnc fluid was found to be beneficial for prevention of early fluid loss, decubitus ulcers, and reduction of pain. Rapid epithelial layer growth by culturing cells in vilm from the skin of the bum patient for covering the wound area may offer a better solution. Fwun 11-7.S u ~ m o u prhonael s dimWi8 awsim d d . Fmm C. KaMitr. 1.K ~ I P. .A Dm. R. L Stephen. and W. J. KoM. "Subsutnnwus Perhow.1 Cath.ur: ?I Yman hpal-." Anif. O w m . 3,210-117.18lS.

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wound has healed. Although a permanent skin implant is needed, it is a long way from being realized for the same reasons given in the case of percutaneous implants proper. Presently, autografting and homografting (skin transplants) are the only methods available as a permanent solution. In one study, wound closure was achieved by controlling thephysicochemical properties of the wound-covering material (membrane). Six ways were s u ~ e s t e d lo improve certain physiwchemical and mechanical requirements neassazy in the design of artificial skin. These are shown schematically in Figure 11-8. Biomechanical and chemical analysis conducted in this study led to the design of a cross-linked collagen-polysaccharide (cbondroitin 6-sulfate) composite membrane chosen for the ease in controlling porosity (5- to 15O-rrm diameter), flexibility (by varying cross-link density). and moisture flux rate. Several polymeric materials including rewnstituted mllagen have also been tried as bum dressings. Among them ate the c o.~ o.l y m mof vinyl chloride and acetate and methyl-2&anoacGlate. The methyl-2-cyanoacrylat; was found to be too brittle and histotoxic for use as a bum dressing. The ingrowth of tissue into the p o r n of sponge (lvalon", polyvinyl alcohol), and woven fabric (nylon and silicone tubber velour) was also attempted without much success. Plastic tapes have sometimes been used to hold skin g r a b during microtoming (ultrathin sectioning) and grafting procedures. For severe bums the immersion of the patient

11.3. MAXILLOFACIAL AN0

OTHER SOW-TISSUE AUGMENTATION

In the previous section we have dealt with problems associated with wound closing and wound/tissue interfacial implants. In this section we will study (cosmetic) rewnstructive implants. Although son-tissue implants can be divided into (1) space filler, (2) mechanical support, and (3) fluid carrier or storer. most have two or more combined functions. For example, breast implants fill space and provide mechanical support. 11.3.1. Maxlllofaciel lmplanta

There are two types ofmaxillofacial implant (often called prosthetics. which implies extracomreal attachment) materials: extraoral and intraoral. The latter is defined as "the a n and science of anatomic, functional or cosmetic reconstruction bv means of artificial substitutes of those renions in the maxilla. mandible. and face that are missing or defective because of surgical intervention, trauma, etC.+* There are many polymeric materials available for the extraoral implank which requires: (1) color and texture should be matched with those ofthe patient, (2) it should be mechanically and chemically stable, i.e.. it should not creep or change colors or initale %kin, and (3) it should be easily fahricaled. ~ o l y ~ i n ~ l chloride and acetale (5-20%) copolymen. -~olymelhyl methamlate. silicone. and . polyurelhane rubbers are currently~used.

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SOFT TISSUE REPLACEMENT I

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The requirements for the inhnoral implants are the same as for other implant materials, since they are in fact implanted. For maxillary. mandibular, and facial bone dcficts, metallic materials s;ch as tantalum, titanium, and C & C ~alloys. elc. are used. For son tissues like gum and chin, polymer8 such as siliwne rubber, PMMA, nc. are used for the augmentation. The use of inicctable siliwncs that . wlvmerire In mN has m v e n nartiallv successful for wrrecting facial deformities. Although this is obviously a bener approach in terms of the minimal initial surgical damage, this procedure was not accepted due to the tissue reaction and the cvenlual displacement or migration of the implant.

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11.3.2. Ear end Eys Implants The use of implants can restore the wnductivc hearing loss from otasclemsis (a heredity defect that involves a change in the bony tissueofthe ear) and chronic otitis media (the inflammation of the middle car, which may cause partial or wmpletc impairment of the ossicular chain: malleus, incus, and stapes). Many different prostheses are available to wrrect the defects. some of which are shown in Rgure 11-9. The porous polyethylene total ossicular replacement implant is used to obtain a firm fixation of the imolant bv tissue intnowth. The tilt-ton implant is designed to retard tissue ingrowth into the section of the shaR which may diminish sound conduction. Many different materials have been tried in fabricating implants: polyletrafluoroethylenc, polyethylene, siliwne rubber, stainless steel, and tantalum. More recently, polytetrafluorocthylenc-carbon wmposite (Roplam"), porous polyethylene (Plastipore"), and pyrolytic carbon (Fyrolite') have been shown to be suitable materials for cochlear (inner ear) implants. Artificial ear implants capable of processing speech have been developed and are undergoing clinical evaluation. These types of mehlear implants have electrodes that stimulate the cochlear nerve cells. The implant also has a speech processor that transforms sound waves into electrical impulses that can be conducted through coupled external and internal wils asshown in Figure 11-10. The electrical impulses can be transmitted directly by means of a PD. Eye implants are used to restore the functionalitv of the wrnca and lens when ;hey ;re damaged or diseased. Usually the corn& is transplanted from a suitable donor rather than implanted since the longevity of the cornea implant is uncertain because of fixation problems and infection. F i y r e 11-11 s h o w somc of the eve imnlants tried clinicallv. Thev are made from "transnarent" acwlin. ~< especially PMMA, which has a wmparativcly high refractive index (1.5). In cataraa, the lens of the eye becomes cloudy; the lens can then be removed surgically. The lost optical power can be restored with thick-lens spectacles, hut these cause distortion and restriction of the field of view, and somc people o b j M to their avpearance. lntraocular lensss are imvlantcd tunically .. . to replace the original eye Icns. and they restore function without the problems assosiatcd with thick spectaclo. The problems or infection and Baation of the lens to the tissues

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b u m 11-9.Proslheses for ma rscon.trunion d the ossicla. (a) PTFE "piaton" slaps. pronheeis of J. J. She.. F. Ssnabda. and 0. D. L. Smyth (Arch O I o i ~ ~ I76.61G-621. . . 1532).(b) Incus rsplammem poathsaia d Sharhy [in Hearing Loas--Robbrns in Diagnosis and Traatmsnf I.. R. Boise pd.). W. 8. Saundsn Co.. Philedslphia. 1539,p. 1411. (el J. R. Tabor prostharls for raplaesmen1 of *ole oasiculsr chain (Arch. Otolaryngoi.. 81. 141-148. 1970).(d) Pomua poWhylsna total ossicular rsplacsmml pornhesir (Smith L Nephew. Richard Medlcal Co.. Technical Publ. No. 4140. Memphis. Tenn.. 1980).

arc again the major drawbacks of intraomlar lens implants, as for the wrneal implants. The intraocular lens can damage the soft structures to which it is attached, and it can become dislodged. Nevertheless, this type of cataract surgery has become wmmonplace, and many such implantation procedures are successfully canducted. Rccentlv. somc researchen have tried to d e v e l o ~an artificial eve for ~ e o ~ l e who have lost all of the conductive functions of ;he optic nerve- The 'device orovides stimulation to the brain cells as shown in Rnure - 11-12. One of the maior problems with this type of total organ replacement is the development of suitable electrode materials that will last a long time in vivo without changing their characteristics electrochemically. Another difficulty with the artificial eye is that significant image processing goes on in the retina. Consequently, simple electrical stimulation of the visual cortex of the brain yields a very poor image.

Figure 11.12. Diagram of c ~ c s p of t aniflclal aye. Television cameras in the glsssss relay the

Flgura 11-10. Bask Eomwnents of anitrial u r implanm. From 8. J. GanU. "Coehbar Implena: An O m n i n " Acts O r o 6 t v # d Head Neck Surg.. 1,171-200.1@87.

msssags via micmcompvtan with radio warel to the array of alsctrodas on the visual cones of the brain. From W. H. Doballs, M . G. MlsdeJoraky.end J. P. Ginin. "Anificial Vision for the Blind: Elsct~clllStimulation of Virus1 Cortex Oflsn Haps for a Functional Prosthesis." Sehnca. 18). W , 1974.

11.3.3. fluid Transfer lmplanta Ruid transfer implants arc required for cases such as hydrocephalus, urinary incontinence, and chronic ear infection. Hydrocephalus. caused by abnormally high pressure of the cerebrospinal fluid in the brain, can he treated by draining the fluid (essentially an ultrafiltrate of blood) through a cannula as shown in Figure 11-13. The earlier shunt had two one-way valves at the ends while the

Ames shunt has simple slits at the discharging end. which opens when enough fluid pressure is exerted. The Ames shunt empties the fluid in the peritoneum while others drain into the bloodstream through the right internal jugular vein or right atrium of the heart. The simpler peritoneal shunt showed less incidence of infection. The use of implants for correcting the urinary system has not been s u c ~ ~ ~ s f u l because of the difficulty of joining a prosthesis to the living system to achieve fluid tightness. In addition, blockage ofthc passage by deposits from urine and constant danger of infection have been problematical. Many materials have been tried includingglass, rubber, silver, tantalum, Vitallium".polycthylene. ~ a c r o n " , ~ e f l o n polyvinyl ~, alcohol, elc. without much long-term succcss. The drainage tubes for chronic ear infection can be made from polytctrafluoroethylene (~eflon") or other inert materials. These are not permanent implants. 113.4. Space-Filling Implants

nrun tt.tt. (a) &W.I lmpiem ot 0. G. MsPhsrson and J. M. Andenan (Br. M d J.. I. 330. 1953). (b) Corm.1 Impiem of H. Cardona (Am J. Ophthslmd.. &4, 284. 11882). (el ~mnacuimr~enm.(coumw of Intra-lntsrmedics. Ino.. Pa.sdana. Calif.)

Breast implants are quite common space-filling implants. At one time, the enlargement of breasts was done with various materials such as paraffin wax. beeswax, silicone fluids, etc. by direct injection or by enclosure in a rubber balloon. There have been several problems associated with directly injected implants, including progressive instability and ultimate loss of original shape and texture, as well as infection, pain, etc. In the 1960s the FDA banned such practices by classifying injectable implants such as silicone gel, as drugs. One of the early effons in breast augmentation was to implant a sponge made of polyvinyl alcohol. However, soft tissues grew into the pores and then

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b u n 11-13, 1.) A m n d-bn hydmcsphalus shunt; (b) A m s shunt in sit". ( c ) VaIm for another shunt. The Amse shunt b made of ailicons rubber ( ~ i i a s l k @ )snd oonsins of: (A) translucent doubh-chamber flushing devlcs. (0) radiopaqua wntrlcular oathater. (C) redlopaqus cOnnaCtOr tubing, ID) radiopaqua psritonssl oathater. (El stalnlsss steel connector. (a. bl from DOW Corniw CO.. Sibstic. HospitalSurglcel Pmductn. Bulletin No. 61-061A. Midland. Mich., Dsc. 1972: (c) from F. E. Nulasn mnd E. 8. Spltr. "Trsstmanl ot Hydrocephalus by Dire3 Shunt trom Ventrlcla to Jugulsr Vein." Svrg. Forum. 2. 399403. 1951.

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SOFT TISSUE REFIACEMENT I

i n I year. Assume that the leakage is entirely by diffusion rather than by macroscopic pores. Asrume the silicone oil has a moleculsr weight o f 740 amu. Assume the membrane is Imm thick and the surface area is 400 cm'. 'The membrane has a d i f i r i o n mnstant o f D = 5 x 10.'' cm'lsec. Assume, momover. that the implant has a volume of 1000 cm' and a density o f p = I 5 $/cm'.I)iscuss other ways silicone fluid or gel could escape. Dlsmss implications.

Answer

calcified w i t h time a n d the so-called marble breast resulted. Although the enlargement o r replacement o f breasts for cosmetic reasons alone is not recommended, arostheses have been develoved for the patient who has undernone radical hastectomy or who has nonsyhmetrical deformities. he^ are probably beneficial for asvcholonical reasons. I n this case a silicone rubber bac " filled w i t h silicone gel a n d backed w i t h polywfer mesh t o permit tissue ingrowth for fixation, i s a widely accepted prosthesis as shown in f i g u r e 11-14. ~ h e a n i f i c i a penis, l testicles, and vanina f a l l i n t o the same catenorv as breast implants in that they make use o f silicones a n d are implanted f ~ r - ~ s ~ c h o l o ~reasons ical rather t h a n i o improve physical health.

.-

-

Example 11.2 Experience has shown that the silicone membrane used i n the bream implants leaked the silicone Rvid contained within to the surrounding tissue. Calculate the amount o f leakage

From AcVs fin1 law for diffusion. the flux F i s F = -D(dc/dx) i n which D is the dimsion coefficient and c i s the concentration. 'The Rux is mass per unit area per time, so that if the concentration i s initially zero i n the tissue. de massftime 5 AUX X area = FA = D-400cm2 dx

= 5 x lo-"m'fsec

1.5 g/cm3 0.1 cm

- 0 400 cm'

'The body nonnally tolerates silicones well; problem do not usually arise unless pm amounts are lost and migmtc through the tisriues. Silicone fluid or gel could also escape into surrounding tissue through porn allowed by inadequate quality control, or from cracks due to f a t i y c from repeated Rerun o f t h c mcmbme.

a2

CHAPTER 11

S?FT TISSUE REPLACEMENT I

As of 1992, there have bun some r e p o m o f allerpjc reaction8 t o goss amounts o f c s u m d silicone .el. Concern o v a s u h u s n has l e d t o revised recommendations that silimns breast i m i l a n l s not be used solelv for ammetic sunmcntation. Wcrcmarkthat the givcnvolume corresponds to a mass m = pV 1.5 k~ correspondi n g l o a wcight o f a b u t 3.3 Ib f o r each breast. I f the shape i s hemisphclial. the volume is 2 d 1 3 = 1000cmJ ao that the diameter (twice the radius) i s 15.6~~1. l l ~ area e o f !he m d ' s u r f a c e is Z d 384cm1. ~ o m m c & a l l y available implant8 are not quite hcmisphcriul. The lsrgen o m made b y one m a n u f a m r e r has a diameter o r 18 cm with a volume o f 600cm3.

-

-

-

%STRENGTH REMAINING

20 \

PROBLEMS

7 14 IMPCANT PERIOD(days)

0

(a) (b) (cl .. (d)

I n whlch dlrealon 18 the intern.1 nrm i n the skin greater7 In which direction am the mllsgm f i b m mom oriented7 How u n the bioenpinrcr obtain a cim1.r nthcr than e l l i d i a l holc f m his im~t.mt Asruming the impl8nt is nondefonnsble compared to the skin, wh.1 poblemr will .ti= M-n skin end Implant when lomd or f o r e is applied to therkin or implantby h~ndlingsrridmtmlly?

11-2 Design a b l m d adevice for kidney dl.l)nil w a h o Iwl-term llsc mnd #ire .pcdfie m.tcrlal. selcaed for cash pmn and explain why you chose esch pni-lu m.tcrld.

I1-3. h .I ~ I L .~ n . t ~ or t h t eye .nd label d i e a t 114. ~ n the v .n.tomy

-_

0

11-1. A Morngincar h Irying to o n d n m n d tkc Momehanla of a holc -led In the skin f m a tnnmotanrous implant He made I holc using. drcu1.r b i o p n drill i n the d o m l #kin of d q . l l ~ c doamcor of the drill .1 5 mm. I f lhc hole bec.mr mn cllipc with minor mnd major azis of 3 and '

.

or L. car and 1.be1

cogur: Chmmic saR:

rt.t~m.

SSI~MI

11-5. A brenn implant is msde of dllmne robber mrmbnne Rncd with fflimnc rubber f ~ mDl-u . the advanta~esmnd diudvantage ofthis daipn i n comparison with an oil-filled Implam.

11-7. Compre the &*king nrenlth of a.tyt tumm ('Tsblc 11-1) ofconcloaioru a n yow d n w ?

FDA:

h e e n 710 and 7. what

11-8. Desim a pnik implanl Ib.1 a n am 10. the m i l e furtalon for pmon r h o has l a 1 that capability due UI disease or injory. wh.1 kind o f malrri.lr wonld you need for ID mnnruaionl

Prrrvloncovr d&

11-9. lhc menllon o f tmlk bmaking thr .b.orb.bk svlllra sncr implant8llon is shown for chmmic a t ~ oand t POA sumre i n Ule =compIying figom.

~h~tporrQ: Polyglycwlic odd (FGA):

(4Ex-

Blylactic add (PLA):

the n l e of m c n p h d-ae (b) From the mathemsllul crp-ion.

m a t h m m t i ~ l l yfor bnh sum= alculnte thc r c m atrenph times.

11-10. An i d u l sumre h defined II"handles m r o n a b l y and rumnlly, minimum tissue re.aion, .dequatetensilcetrength and knotrmrily, be wl hvonble fwb.arri.1 yDvthand e.dly nrtilirablc, noneleamlytic, nampillsry, nonallergenic and nonurdnogenic" [C. C. Chu, in B ~ t i b l e P d y m e ~ Melab . and C o m p s i l q M. Szysher (4.). Chapter 22, Technomic Pub, Weslpon. Cann.. 19831. Can you ndd mom to the Iln7

21

DEF:NITIONS

rcrmm.

114. Fmpluc@is. m m p i t c or PTFE and u r b o n (pphltc). I r a h mnde o f X%by volume u e h and h" 20% porosity, what 18 hl denlily7 E.Iimnt~iw YOY~EI' mndolus.

18

(PD):

Absorbable suture m.tetial prepared fmm collagen from healthy msmmals. Chcmisl mmpound that is used lo treat mllsgcn to achieve -alinking betwen malrmlsr chains of collagen. Such treatment increases its strength, but desresrur its ficiibility. A wlymer used ar a tirrue adhesive sin- i t ran polymerize fan i n the presence of water. Polyelhylcnr rercphthslstc polyester that i s made into Abera. I f Ihc ssme polymer is madc into s film, it l a u l i e d Mylar". F m d and Drug Adminirtntion, which regulates the use of mcdiul devices i n the United States. A plasma protein of high m o l m l a r wcight that is mnverted l o fibrin throu~hthe aaion of thrombin. This material is "red lo make (abrorbrble) tissue adhesives. An implant designed to transfer msncr. information, ete. fmm the body to the outride of ,he body tnnreutancously. Pomur palycthylcne. Polymrr made from glycolic eFid and used to makc aborbsble sumre or other pmduas. Poiymcr made from Ibaic acid and u u d to make absorbable suture or other produas. A mmporitc mstcrial madc o f fibrous polrnnAuwoethylrm and carbon. 11is uauslly pornusand has low modulusend lawrtrmgth. Wmlwie carbon. Material "red in closing a wound with aitche. PolytclrafiuorDnhylcnr. Co-Cr alloy.

. .

CHAPTER I I

?(U

BIBLIOGRAPHY I. Blast, B i d e h l P%mnc+ oJMnleriab, Maml Dekkrr, New Y m f 1981. A. H. Bulbulian, Forlrrl Prosfhew Chmrln C. Tltrllornu Pub.. Sprinlfbdd. 111. 1973. M. Chvapil. "Considerations on M.oufsnurin8 Rindpls of. Synthcclc Bum hasin#: A Rcrlcv," I. Biomrd. M a f . R c r , Voi. 16.245-263. 1982. W. S. Edwards. Plasdc An&l Grslc~,Charla C. Thorn.. Pub.. Spin#fbcld. Ill.. 1WS. H. Lee and K. Ncvillc. Handbmlr oJBiomdlml h s l l s r . Chapten 4 and 13, Rudm. Tccbaolog Rtss. Plsadma. 1971. W. Lynch. Implanls: R m n # m i n r thr Human M y , Ven N U Rdnhold Co.. Rincrton, NJ., . os, .7"&.

mly

G. B. P n h "Bum Wound Corerinp: A Review." Bl-frr. Md. Ddw,AR(I: 6.1-3l.1978. A F. *on Rccvm m d I. 8. Pwk. "Rrruuararu Dsvictr," CRC MI R m h n . . 5.37-77.19l9. D. F. Williamr (ed.),F L n d a m m r a l & e ~ ~ oJBimmplibllhy, Vols. I snd 11. CRC Reu.Boe. hton. ma.. 1981. D. F. Williams (4.1. B i - ~ l I M l i v 1. CWnkcll mdk, Vd.. I sad 11. CRC h. Boe. hlon.

Fla.. 1982.

SOFT TISSUE REPLACEMENT'

II: BLOOD-INTERFACING IMPLANTS Blood-interfacing materials can be divided into two categories: short-term extracorporeal device5 such aa membranes for artificial organs (kidney and hcartlluna . - machine).~.tubes and cathclen forthe transDort ofblood. and lonn-term in siru implants such as vascular implants and implantable artificial organs. Although pacemakers for the heart are not interfaced with blood directly, they are considered here since they are devices that help to circulate blood throughout the bodv. 7he single most important requirement for blood-interfacing implants is blood wmoatibilitv (review Section 10.3). Blood maeulation is thcmost i m w n a n t . aspect of blood compatibility: the implant should not cause the blood to clot. In addition, the implant should not damage proteins. enzymes, and formed elements of blood (red blood cells, white blood cells, and platelets). The implant should not cause hcmolysis (red blood cell ~ p t u r e or ) initiation of the platelet release reaction. Blood is circulated throughout the body according to the sequence shown in Firure 12-1. Imdants arc usuallv used to e n l a c e or oatch larne arteries and veins as well as the heart and its valves. Surgical treatment without using implants is usually preferred. However, there are many unavoidable situations when it is necessary to anastornose or replace a large segment with implants

.

-

-

12.1. VASCULAR IMPLANTS lmplants have been used in various circumstances to treat vascular maladies. E x a m d a include s i m ~ l sutures e for anastomosis aRer removal of vessel seaments. vessel patches for aneurysms, as well as total replacements for large arteries. Vein It6

26n

CHAPTER 12

SOT TISSUE

REPLACEMEKT 1

2e.l

implants have encountered some difficulties because of the collapse ofan adjacent vein or clot formation, which in turn is due to low blood pressure and atagnant blood flow in veins as compared to arteries. Vein replacements have not been a major concern since autografting can be performed for the majority of cases. Nonetheless, many materials including nylon. PITE, polyester, e t c were fabricated for clinical applications. Early designs for anerial replacements were solid wall N b n made ofglaar. aluminum, gold, silver and P M M k All of the implants developed cloU and became useless. In the early 1950s. porous implants made of fabrics were introduced. which allowed tissue growth into the interstices as shown in Figure 12-2. The new tissues interface well with blood, and t h u minimize clonink Ironically, for this type of application thrombogenic materials were found to be more satisfactory. Another advantage of tissue ingrowth is the fixation of the implant by the ingrown tissues, which make a viable anchor. The initial leakage through w r e s is disadvantageous but this can be prevented by precloning the outside . surface of the implant prior to placement. Crimping of the prosthesis, sa shown in Figure 12-3, is done to prevent kinking when the implant is flexed. Also, the crimping allows expansion ofthe grafi in the longitudinal direction, which reduces strain on the arosthesis wall. Arteries can exoand circumferentiallv and lonnitudinally to accommodate the pulsatile Row of the blood.

Fipun 12-2.mefimsnsrial g r e made 4 stitching fabrlc.logatha by hend. Fmm C. A. Hufnagel. "Hintory of Vs.cuBr Grafting:' in Vsaculsr Gmfting: Clhical ~ l i c . t l mand ~ TmhnQws," C. W. Wright e l 81. Ids.]. J. Wright. Boston. 1883. pp. 1-12.

CHAPTER I1

.

SOFT TISSUE REPLACEMENT 11

ngum (2.4. Bask hsallng pmern of snwlsl prosthesis. L,lumen of msthssis: F. RMn. Y. yarn bundle; G, organiriw granvlstion ttseus: H, b a l e d fibrous o a p ~ ~ l atissue; r 0, degonsratlrs fibrous capsuiar tissue: C. cal~lnedcapsular 118aue. From S. A. Waaolmkl. C. C. Fries. A. Msnine. end J. 0. MeMahlln. "Amrial Roehatic Mstsrlals," Ann N.Y. Acad. Sci.. 148,325-344. 1988.

Although the exact sequence of tisaue formation in implants in humans is not fully documented, quite a bit is known about reactions in animals. Generally. soon aRer implantation the inner and outer surface of the implant are wvcred with fibrin and fibrous tissuc, respectively. A layer of fibroblasts r e p l a w the fibrin, bcwming neoinrima, which is sometime# called pseudointima or pseudoneointim. The long-term fate of the ncointima varies with the species of animals; in dogs it stabilizes into a constant thickness while for pigs it will grow until it occludes the vessel. In man the initial phase of the healing is the same as for animals but in later stages the inner surlace ia wvered by both fibrin and a cellular layer of fibroblasts. The sequence for heating of arterial implants in humans, dogs. and pigs is given in figure 124. The types of materials and the geomety of the implant influence the rate and nature of tissue ingrowth. A number of polymer materials a n currently used to rabricate implants, including nylon, polyester, PTFE,polypropylene, polyacrylonitrile, and silicone rubber. However. PTFE. polyester, polypropylene and silicone rubber are the most favorable materials due to the minimal deterioration of their physical properties in vim as discussed in Chapter 8. Polyester (particularly polyethylene tercphthalate, ~ a c r o n @is ) uaually preferred because of its superior handling properties. Recently a pyroiyiic carbon-wated arterial gran has been developed by the technique of ultra-low-temperature isotropic (ULTI) deposition. The nonthrombogenic properties of the pyrolytic carbon may enhance the patency of the graR

made from this material and decrease the need for postsurgical antiwagulant drugs. Another interesting arterial graft is made by pressure-injecting Silastic rubber into premachined molds made of tentacles of sea urchins. The objective is to achieve a micmporous structure for the tissues to grow into. After the Silastic rubber is cured. ;he mold is dissolved away by acid Geatment leaving replamineforms of the ~ l t r a s t ~ d uas r cshown in Rnure - 12-5. Animal exneriments showed promising results. The geometry of fabrics and porosity have a great influence on healing characteristics. The preferred porosity is such that 5000 to 10,000ml of water is passed per cm'offahricper min at a pressurcof 120 mm Hg.The Ruid permeability depends not only on the oorositr (volume fraction of pores) but also on the size. shape, and con"ectivity pore;. The lower limit wili present excessive ~eakagF of blood and the higher limit is better for tissue inerowth and healing characteric tics. Thickness of the implant is directly related to the amount of thrombus formation: the thinner the wall, the smaller or the thinner the thrombus deposited. Less thrombus results in faster organization of thcneointima. Also,smaller-caliber (c5-mm diameter) prostheses can be made more easily with thinner walls. The long-term testing of vascular prostheses is as important as it is with any other implants. A simple in vim testing machine is shown~inFigure 12-6 in which the meudoextracellular Ruid is drawn through - valve 'A' and pushed out throonh valve 'B' of the graft at 96 cycles per minute with a peak pressure at 150 mm Hg

of

-

-

-

SOP TISSUE REPIACEMEKT II

271

Figure 12-7. Percent change In tanachy for mo hlpa -9pmnhsses afIer life testing of canlns implant. From h Botrko. R. Snyder. J. L.rLin. and W. S. Ehvards.."b V i w j i n mm Ufs Tsning ol Vascular Prostheass." In Cwm#lonand Dagradstbn of Impl#nt Marsri.b A S M STP 884. B. C. Syren and A. Aohatya (ds.), ASM. Philadelphia. 1979. pp. 7W8.

Example 12-1

Fiours 12.6. Se.nnhg slectmn mlcm-pk vbw of th.npl.mm(m SIIeslii amrial prW. Fmm L F. HIReka. J. A. Gwken. R.A. Whka. end C. 8. Wright, "In V h a Compsrhon d Rspl.minetam. sllanio mnd Bbsl.odc Pohlumhsns Ansrial G n f n . " Arch. Suw.. 114. OEa702. 1078.

at 37OC. Various @ a h were tested and compared w i t h in uEoo implantation results as shown in Figure 12-7. I t can b e seen that the Teflone k n i t graft did n o t lose i t a tenacity (a measure of normalized strength). ' l h e i n i t i a l values of tenacity for Teflon" and Dacron" knit prostheses are about 1.3 and 3.0 ddenier, respectively. ' l h e Dacron'grahs showed i n i t i a l decreases and stabilized after 6 months under both in u h and in u i f m conditions.

The material properties o f arterial prostheses change followin8 i n p w t h oftissun In dm. A porous silicone rubber arterial prosthesis wss implanted i n dog9 and i t was found that one-halfofthe~oresbecamcfilled with tissueafter3 monthsofim~lantation.Thcumsthcsis has a 5-mm inside diameter. s wall thickness o f 1 mm. and a porosity of 30%. The solid silicone from which the prosthesis i s derived has a Young'a modulus of 1 0 M R . Answer the follow in^: (a) Determine the Young's modulus of the pomua silicone.

-

(b) Aasurnc that the . . Find the elastic modulus o f the ~msthcsiafollowinn tissue inrrowlh. " i n g r o m tissue is similar l o the natural sncrial wall ( E = 0.1 MPa). (c) Determine the wall tcnaion sssurning s (high) blood pressure of 200mm HI.

(a) There are sncral relationship thmt u n be used for the dnomination o f propmies o f porous materials. For example, we may consider the empirical rclstionship

i n which V i s the porosity (volume fmdion o f pores) and &,is of the solid without porosity. Thus, Flgun 1 2 d . S ~ d l . g n m o f a r t . r i d g d I l h n n n . From K. Botlko, R. Snyder. J. L.rUn.end W. S. Edrvards. "In W / i n Yltm UfeTeMlc-~of Vameulmr Pmsthesn." in CormIon and D q r s d # t h d Impl#nt M#teri.h ASTM STP BB1. 8. C. Syren and A. Acharye ( d s . ) . ASTM. RIIIadelphll, 1878, pp. 784E.

the clanic modulus

Alternatively, we may consider the model o f Gibson and Ashby, which hms bath empirical and theoretical justification:

CHAPTER

The density ratio is plp,,

-:

1

I1

- V so thmt

The actual clmtic modulus will depend on the shape oftheparea and their orientation. (b) This is n rsthcr mmplicated sptem. Obviously. the muimum elastic modulus for this problem would be 10 MPa if the wres were filled with an identical silicone rubber. we may approximate the modulus (using the scmad value above) as E = 4.90 + 0.1 x 0.3

-

12.2. HEART VALVE IMPLANTS There are lour valves in the ventricles ofthe human heart aa shown in Figure 12-8 (cf. Fig. 12-11, In the majority of cases. the len ventricular valves (mitral and aortic) become incompetent more freauently than Ule riaht ventricular valver as the result of higher left ventricular pkssurc Most imp&nt and frequently critical is the aortic valve since it is the last gate the blood haa to go through before being circulated in the body. There have been many different types ol valve implants. me early ones in the 1960s were made of flexible leaflets that mimicked the natural valves. Invariably the leaflets could not withstand fatigue for more than 3 years. In addition

.

to h;molysis, regurgitation and incompetence were major problems. Later. butt e d y leaflets and ball- or disk-in-the-cage were introduced. Some of them e n shown in Figure 12-9.The material requirements for valve implants arc the same as for vascular implants. Some additional requirements are related to the blood flow and pressure, i.e., the fonned elements of blood should not be damaged and the blood pressure should not drop below a clinically significant value. Also. valve noise should be minimal, for psychological reasons. Figure 12-10 shows a tissue valve made from collagen-rich material such as pericardial tissues. Basically the pericardium is madeup ofthree layers ofcollligen fibers oriented 60' from each layer and about 0.5 mm thick in the case of bovine pericardium. It can be cross-linked by formaldehyde. During this treatment, the cell viability is destroyed and the proteins are denatured. Therefore, the implant does not provoke immunological reactions. Also, porcine xenogran valves have been used. They are treated with a chemical process that denatures the proteins and kills any living cells. All valves have a sewing ring that is covered with various polymeric fabrics. This helps during initial fixation of the implant. Later, the ingrown tissue will render the fixation viable in a manner similar to the porous vascular implants. The cage itself is usually made of metals and covered with fabrics. to reduce noise, or with pyrolytic carbons for a nonthrombogenic surface (the disk or bail is also coated with pyrolytic carbon at the same time). The practice of covering

Kalhe -Llllrhoi F i ~ u r e114. Circuletlon of Mood in th. hean. C c m p n W h F b u n

12-1,

273

SOFT TISSUE REPLACEMEM 11

Dish typo

Figure 11.9. Schematic dlagrem of verbus m

a of hsan v s h l

SOW-TISSUE REPLACEMENT 11

276

STRESS (MPd

(b) The secondary modulus is, fmm the slope of the grsph. Figure

t2.10. lonalcu-Shll.y pericerdhl r;encwrefl hean v e b . fcouneoy d Sh1l.y. lnc. Itvine.

Calif.) the struts with fabrics has been abandoned since the fabric fatigued and broke into pieces. The ball (or disk) is usually made as a hollow structure wmposed of solid (nonporous) polymers (polypropylene, polyoxymethylenc, polychlorotrifluoroethylene, ctc.). metals (titanium, Co-Cr alloy), or pyrolytic carbon deposited on a graphite substrate. The early use o f a silicone rubber poppet was found undesirable due to lipid absorption and subsequent swelling and dimensional changes. This problem has been corrected in modern valves. Although this was an unfortunate episode (some were fatal), it helped to reinforce the realization that the in virro experiment alone is not sufficient to predict all circumstances that arise during in viw use, no matter how carefully one tries to predict. This is true of any implant research even with very simple devices.

The bovine pericardium has been tested for its mechanical pmpenies. The slrcss-strain curve of bovine perifardium is shown in the diagram opposite. Answer the following: (4CsI~lstcthe initial modulus. (b) Calsulatc the secondav mmodulu. (4 What is Ihe toughnear? Anmr

(a) The initial modulus is, from thc slope of the graph.

(e)

Thetoughness is the area under the curve up tolhc failurestrain. It canbeapproximated by a triangle. Toughness = IS MPa x

0.62

- 021 = 3.1 MPa rn m

2

MNm = 3 . r T = w m

Nm m'

12.3. HEART AND LUNG ASSIST DEVICES

The function of the heart in pumping blood can be temporarily taken over by a mechanical pump. This procedure is most uscful in cardiac surgery in which a surgical field free of blood is required. The pump must propel the blood at the correct pressure and Row rate, and its internal parts must be compatible with blood. Moreover, damage to the red blood corpuscles should be minimized. Use of artificial devices to take over the function of the lungs is also common on thoracic surgery. The human heart actually contains two pumps, one for the systemic circulation and one for the pulmonary (lung) circulation. Therefore. even if a cardiac patient has normal lung function, it is usual to assume lung function by a machine, to simplify wnnectionsbetween the pump and the patient's circulatory system. The combination of a blood pump and an oxygenator is known as the heart and lung machine.

CHAPTER I2

278

Them are basically t h m types of oxygenators as ahown in R g u n 12-11. In all cases oxygen gas is allowed to ditfuse into the blood and simultaneously waste gas (C02) is removed. In order to increase the rate of gas exchanges at the blood/gas surface of the bubble oxygenator, the gas is broken into small bubbles (about I-mm diameter; if smaller, it is hard to remove them from b l d ) to increase the surface contact area. Sometimn the blood is spread thinly as a film and exposed to the oxygen. This is called a film oxygenator. A membrane oxygenalor is similar to the membrane-type artificial kidney to be discussed later. The main difference is that the oxygenator membrane is permeable to gases only, while the kidney membrane is also prmeable to liquids. Membrane and bubble oxygenators each have advantag-. The membrane oxygenator is wnsidemd physiologically superior in view or the f a n that them is no blood-gas interface and as a result t h e n is lcss hemolysis induced by turbulence and bencr platelet function than with the bubble oxygenator. Moreover. the membrane oxygenator d o n not introduce any microbubbles or microemboli in the blood. Control of exchange of oxygen and carbon dioxide according lo the needs of :he patient is readily achieved, and t h e n is no need for antifoam agents such as with the bubble oxygenator. Membrane oxygenators a

cot

-

SOFT

TISSUE REPLACEMENT I1

277

Table 12-1. Physical Characteristics of Natural versus Artificial Lung'

Pulmonary (lor (liren/min) Held pressure (mm Hg) Pulmonary blood volume (litm) Blood lraruit lime (set) Blood film thieknrss (mm) Length of apillsry (mm) Pulmonary ventiiellsn (iitcnlmln)

I

1-4

0.1-03 0.005-0.010 0.1

3-30 0.1-3.0 20-200

~

Biaedk3l

2-10

7

50-100 40-50

Exchangc s u d a a (m') V c n ~ s l v ~0, l ~gradient r (mm H#) Vcn-alveolar CO, srsdicnt (mm HII)

' Fmn D. 0. C-y,

5 0-200

5

12

650

30-50

3-5 --

~

En*

2-10

tin+*

M.r..l

p~

Dckkn. N c r Y a k , 1976.

a n used for prolonged procedures, however; for short surgeries, bubble oxygenators a n preferred since they arc simpler to operate and consequently lcss expensive. Some of the mechanical and chemical charaneriotics of the nstural and artificial lung.(oxygenalor) arc com~arcdin Table 12-1. The surface area of the .. artificial membrane is about ten times larger than the natural lung since the amount of oxygen transfer through a membrane is proportional to the surface area, pressure, and transit time but inversely proportional to the (blood) film thickness. The blood film thickness for the artificial membrane is about 30 times larger than in the natural lung. This has to be compensated for by increased transit time (16.5 sec) and higher pressure (650mm Hg) to achieve the same amount of oxygen transfer through the artificial lung. Table 12-2. Gas Permeability of Teflon and Silicone Rubber Mernbranss..'

cot

Blood

1

Oxysra

3 4

391 306

2072

5

206 159

1112 802

7

' Fmm D. 0. COonV.

'R m e a ~ i o n

Rgure 11.1 1. Schematls diepram.

Blar6Col E l m u n ( n #

rites or oxylm. ca*on

d d l f f e ~ o ~ . n a t o n1.): rmmbr.m.

(b) bubble. and ( c )Rim.

mrmbrana of

Nisqen

Helium

184 159

105

224 187 133

81

94

-.

Silimnr rubber

08

Carbon dioxide

(mil)

l i v e n thann-.

dtoside.

1605

h k k e r . N n Y o h 1976. hcllvn ~cnnn and SIIO-

M*r.M.rc4

nilrosm. ..d

on mt,mnn-m'..tm

ISTP).

mhbo

279

SOFT T 5 S U E REPLACEMENT I1

Aorta

of course be superior to any artificial one. but that is beyond the scope of biomaterials. ~ s ~ m e n t i o n eind the introduction, most implants are designed to substitute mechanical functions,. or . ~ a s s i v ediffusive functions. The electrical functions can be taken over by some implants (pacemakers) and some primitive yet \,ital chemical functions (passive diffusion) can also be delegated to implants (kidney dialysis machine and oxygenator). Most ofthe artificial heart and heart assist devices use a simple balloon and valve system. In all cases a balloon o r membrane is used to displace blood. A simpler heart device is the intra-aonic balloon, which is placed in the descending aorta. During the diastolic phase of the heart, the balloon is inflated to prevent back flow.

Lell otvturn

12.4.1. Artificial Hearts

Rgun 11-11. Schematic dlegnm ol b s r t a ~ s t ad-8. (a)D e 8 . k ~ Mi wmrleulsr m s s . (b) Bernard-1esalst pump. From H. k and K. Nevllla. H a n d ~ o f 8 l o m s d k a t P I . s t ( c aPesadsna , Techmlw h . 8 . Paeadaa. CaIL. 1871. The membranes are usually made of siliwne rubber or mFE. The gas permeability of these materials is given in Table 12-2.Silimne rubber is 40 and 80 times more permeable to O2 and CO,. rcspeaively, than PTFE but the latter can be made about 20 times thinner due to its higher strength. Therefore, siliwne rubber is only 2 and4 times betterthan PlTEfor 0,and C 0 2transfer,respeclively. Polyurethane, natural and siliwne rubbers have been used for wnstructing balloon-type assist devices such as the lefl ventricular assist device (LVAD) as ahown in Figure 12-12 as well as for coating the inner surfaces of the total artificial heart. This in because these materials arethromboresislant. Sometimn therurfaces are coated with heparin and other nonthrombogenic molecules. The feltlike velour surface was tried-but was not successful du; to the uneven or minimal tissue attachment.

12.4. ARTIFICIAL ORGANS The ultimate triumph of biomateriala science and technology would be to make imolanta behave or function the same wav M the ornana or tissucs thev replace without affecting other tissues or organs, and without any negative effect on the patient's mental condition. True regeneration ofthe natural organ would

-

Several artificial hearts are shown in Figure 12-13. Although the design principle and material requirements are the same as those for assist devices, the power consumption (about 6 watts) is too high for the device to be completely implanted at this time. In current devices the Dower is introduced t h r o u-~ ha percutaneous device (Senion 11.2.1) in the form oTcompressed air or electricity. Such an external Dower unit was used with the first heart reolacement done for Dr. B. Clark at the University of Utah in 1982. The artificial hiart kept the patient alive for 112 days, but the external power unit restricted his movemenB. Current artificial hearts are not considered practical as permanent implants since they damage the blood and release emboli into the circulation. They may be useful for short-term use in keeping patients with end-stage heart disease alive until a transplant heart becomes available. 12.4.2. Cardiac Pacemake! A cardiac pacemaker is used to assist the regular contraction rhythm of heart muscles. The sinoatrial (SA) node of the heart originates the electrical impulses that pass through the bundle of His to the atrioventricular (AV) node. In the majority of cases, the pacemakers are used to correct the conduction problem in the bundle of His. Basically the pacemakers should deliver an cxact amount of electrical stimulation to the heart at varying heart rates. The pacemaker consists of conducting electrodes attached to a stimulator as shown in Figure 12-14. The electrodes are well insulated with rubber (usuallv silicone or polvurethanc) cxccpl for the tips. which are sutured or directly embcddcd into thecardiacwallasshown in Finure 12-I5.Thctiois usuallv madeofa noncorrosive noble metal with reasonablemechanical strength such-as h-lO%Ir alloy. The vroblems are the fatime o f the electrodes (thev are wiled like most sienificant . springs to prevent this) and the formation of collagenous scar tissue at the tip, which increases threshold electrical resistance at the point of tissue contact. The battery and electronic components are sealed hermetically by a titanium case while the electrode outlets are sealed by a polypropylene cuff.

-

.

1110

CHAPTER 11

nourn 1l-ts.Arttf1~lelheam. (a) Schumadtsr-Bunne*otmhydnuUc h w . ( b ) Sehm.Hodl.gnm and phmognph of the Jarvik Mart. (a) Fmm H. Ln end K. NsuRIe. Hendbook of Blomsdk#l Fiestic.. Pasadena Technology Ptsms, Ponadana. C.IH.. 1W1: (bJfrom W. J. Kolff, Artlficisl O w n * . J. W l s y and Sons. New Y o h 18711. and W. J. KoM, "ArtlUoI11 0lp.n. end Their Impect." In Rolymen in Msdiclns and Surgery. R. L Kronsmhsl, 2. Oler, and E. Mmrtln (sds.). Plsnum Press, N m Y o h 1876, pp. 1-28. Pacemakers are usually changed aRer 2-5 years due to the limitation of the w w e r source. A nuclear energy-powered pacemaker is commercially available. klthough this and other new p&&r p a c b (such as lithium bancry) may lenkhen the life of the w w e r source. the fatinue of the wires and diminishing- conductivity due to tissue thickening limit the maximum life of the pacemaker to lesa than 10

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floum 12-14. A typical pseamaksr c o n s l m of a pomr -cs and electronic c l n u n y sncaasd In solid plastlc. The electrical wlmr are mated with a ll@rlbleWWmsr, usually a ailicon. wbbsr. (Counssy of Msdtronle. IN.)

ZUl

CHAPTER 12

Fhum 11-18. AmundssnICPI pomue slectroda. Fmm 0. Amundsan. W. MsAhur.and M. Masherrafa. "A N a Pomus Elanmda for Endocardial SHmulation: Paw. ? . 40. 1919.

Agum 12-16. D M . m yp. ol -aka

dr.

aod.s. (a) Bmllpolnt &ctmds (Cordk). The ball has m 1-mmdlamemr.nd the eurlace8rssI.8 mm'. (b) S m - I n alsctmds (Medtmnk). (c) Details of an artsrlml electrode (Msdtmnlc 6-1). From W. Greatbmch. "Metal Electrodes in Bioenglnasdng: CRC Crit. R n . Blwng.. 5, 1-30. 1881.

yean. A porous electrode at the tip of the wins may be fixed to the cardiac muscles by tissue ingrowth as in the case of a vascular prosthesis. This may diminish the interfacial problems as shown in Figure 12-16.

12.4.3. Artificial Kidney Dlalysis Membrane The primary function of the kidney is to remove metabolic waste products. This is accomplished by passing blood through the glomerulus (Figure 12-17) under a pressure of about 75 mm Hg. The glomerulus contains up to 10 primary branches and 50 secondary loops to filter the load. The glomemli are contained

SOFT TISSUE REPLACEMENT I1

*

in Bowman's capsule. which in turn Is a part of the ncphron. the functionat unit of the kidney (see Figure 12-17). The main filtrate is urea (70 times the urea content of normal blood), sodium, chloride, bicarbonate, potassium, glucose. matinine, and umnic acid. The artificial kidney uses a synthetic semipermeable membrane to perform the filtering anion in a way similar to that of a natural kidney. m e membrane is the key component of the artificial kidney machine. In fa& the hrst attempt to filter o r dialyze blood with a machine failed due to an inadquate membrane. In addition to a membrane filter, the kidney dialyzer consists of a bath of saline fluid into which the waste produns diffuse out from the blood. and a pump to circulate blood from an artery and return the filtered blood to a vein as shown in Figure 12-18. There are basically t h m types of membranes for the kidney dialper. shown in Figure 12-19. The Rat late-type membrane was the first to be develo~ed.and can have two o r four laiers. Th; blood pluses through the s p a a s beticen the membrane lavers while the dialvsate is oasscd thmuuh the s o a a s between the membrane add the restraining bkrds. The second an2 most widely used type is the coil membrane in which two cellophane t u b a (each 9 cm in circumference and 108 cm long) are flattened and wiled with an open-mesh spacer material made of nylon. The newest type of kidney ia made of hollow fibem. Each fiber has dimensions of 255 and 28.5 p m inside and outside diameter, respeclively. and is 13.5 cm long. Each unit contains up to 11,000 hollow fibers. The blood flows through the fibers while the dialysate ia paa~edthrough the outside of the fibem. The operational charancristia of the various dialyzen are given in Table 12-3.

ZW

CHAPTER 12

Table 72-3. Comparison of the Plate and Artificial Kidneys' n.1 (2 Inyen)

coil (twin)

Mcmbnrr n r a (ma) Riming volume (liten) h m p needed? BIO& now n t c (rn~lmin) Dialysale now n t c (litcnlmin) Blood channel thisknns (mm) Treatment lime (hr)

Fmm D. 0. Coom. Bhwdhlh-

BLOOD SYSTEM

DIALYSIS SYSTEM

R * l r ( k Marcel Dckkrr. Nor York 1976.

The fiben can also be made from (soda-lime) glass. which is made porous by phase separation techniques. This type of glassfiber.has an advantage over th; organic fiber in that it can be reused after cleaning and sterilizing. Recently there have been some efforts to improve the dia&n by using charcoals. The blood can be circulated directly over the charwal or the charcoal can be made into microcapsulates incorporating enzymes or other drugs. One drawback of activated carbon filtering is its ineffective absorption ofurea, which is one of the major by-products to be eliminated by the dialysis. The majority of dialysis membranes are made from cellophane, which is derived from cellulose. Ideally, the membrane should selectively remove all of the metabolic wastes as does the normal kidney. Specifically, the membrane should not selectively sequester materials from dialyzing fluid, should be blood compatible so that an anticoagulant is not needed, and should have sufficient wet strength to pennit ultrafiltration without significant dimensional changes. It passages should allow . - waste ~ r o d u c t swhile oreventing - of low-molecular-weight passage of plasma proteins. There are two clinical-grade cellophanes available. Cupmphane' (Bemberg Co., Wuppertal, Germany) and Visking" (American Viscose Co., Fredcricksburg. Va.). The cellophane films contain 2.5-ymdiameter pores which can filter molecules smaller than 4000 g/mol. There have been many attempti to improve the allophane membrane wet strength by cross-linking, wpolyrneriration, and reinfonxment with other polym e n such as nylon fibers. Also. the surface was coated with heparin in order to prevent clotting. Other membranes such as copolymers of glycol and polyethylene terephthalate (PET)can filter selectively due to their alternate hydrophilic and hydrophobic segments. Besides improving the membrane for better dialvsis. . . the main thrust of kidney research is to make the kidnev machine more compact (portable or wearable kidney. Figure 12-2Oa) and less costly (home dialysis, reusable or disposable filters, etc.). The other important factor in dialysis is the cannula, which is used to gain access to the blood vessels. In order to minimize the repeated trauma on-the blood vessels the cannula can be implanted for a long period for chronic kidney

.__.______.___.__-----.----. ____.-'.

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Re- 12-20. (a) Dl.enm of w a m b l e aniflclal k l d w : (b) . ~ M s t k a n n g s m a t of the sinph. needle dialvais of Or. Klaue Kwp. The pump owrat- oontinu-hl or IntermittentlyMlchmnizad with the inflow end o f l a of blood. (a) From W. J. Kalff. "Artifids1 Organs and Their Impan: In P o W r s in Medicine sndSurgsw. R. L Kronsnthel. Z 0 - 7 . and E. Manin (sda.). Plenum Press. Nsw Yo*, 1975,pp. 1-28; (b) Fmm W. J. KoM. AnilicialOgsns, J. Wilsy end Sons, N w Ywk, 1978.

288

CHAPTER 12

.

SOFT n s s u E REPLACEMENT 11 '

patients. For t h e same r e a s o n a single-needle dialysis technique h a s b e e n developed as s h o w n in F i g u r e 12-ZOb. In a d d i t i o n to h e m o d i a l y s i s b y a machine. dialysis c a n also b e c a r r i e d o u t by using t h e patient's o w n peritoneum, w h i c h i s membrane. T h e b l o o d is brounht to t h e m e m b r a n e through t h e m i c r o c i r c u l a t i o n of t h e p e r i t o n e u m w h i l e dialysatc is i n t r o d u c e d i n t o t h e peritoneal cavity t h r o u g h a catheter, o n e of w h i c h i s s h o w n in F i g u r e 11-7 implanted in t h e a b d b m i n a l wall. T h e dialysate is d r a i n e d t h r o u g h t h e same catheter a f t e r solute exchange takes p l a c e a n d i s replaced by a fresh bottle. G l u c o s e i a a d d e d to t h e dialysate to increase i t s o s m o t i c oressure gradient for u l l r a f i l t r a t i o n since it i s i m w s s i b l e to o b t a i n a h i g h h y d r o s t a t i c pressure gradient.

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a-semipermeable ~

~~~~

--

Recently a s o r b e n l c a r t r i d g e t h a t regenerates t h e dialysis R u i d f o r reuse has

been a d v a n a d . T h e c a r t r i d g e was o r i g i n a l l y developed f o r hemodialysis a n d

wuld r e d u c e t h e w a t e r r e q u i r e m e n t substantially, m a k i n g dialysis m o r e portable. A schematic d i a g r a m of t h e sorbent regenerated cartridge is s h o w n in F i g u r e 12-21. I t i s interesting to n o t e t h a t i n o r d e r t o remove urea, enzymolysis i s c a r r i e d out b y urcase since c a r b o n cannot absorb u r e a effectively as m e n t i o n e d before.

Cslcuiate the number of ions released i n a year i m m a platinum pacemaker tip. Assume that the average cumm flow is 10 p A and thst Ule surface arcs is 1 em'.

Answer

R + Rf+ 2c-. so that the number o f atoms per year is

REGENERATED DIALYSATE TO DIALYZER

ACTIVATED CARBON CREATININE, URICACID, OROANIC WASTE

HYDRATED ZIRCONIUM OXIDE PHOSPHATE. FLUORIDE

289

10 x

lod-

wul

3.15 x

lo7sccfyr

sec 1.6 x 1 0 - ' 9 c o u l / c l c a m

= 1.97 x 10" c l a m n s l y r

PROBLEMS 12-1. What will be the b l w d urea nitrogen m n a n t n t i o n a n n 5 and 1 0 h n of dialysis i f the initis1 concentration is lODmg% (m#% is mn~cnrrationi n mg per 100ml)lThe concentration aner dialysis can be exexponentially.

i n which C. is the original dialpate mnenastion. p, is the blood now rate, r is time, v is the volume of body Ruid (60% ofbody weight). and b b a conrtonl delmnimcdby mass tnnsfer-ffidcnt (XI, and sufiea a n ( A ) of the membrane by cxp(KA/Q.) aomrding to Cooncy (BiomrdImI &n.fimnhg Rimcipln. M-i Detkcr. New Y o 4 1976, p.332).

a.

12-2. S c l m the mosl rrlatcd match. (a) low m r f f i d m t of M d o n

PURIFICATION LAYER HEAVY METALS ICu.Pb,.ls.l OXlD121N0 SUBSTANCES

-----

-

DIALYSATE WITH WASTE PRODUCTS

Flgure 12-21. SehaaHe dlwrarn of a aarbsnt mgemntion cartridge. Fmm R. A Wnd, "lnvsstigalion of the Risk and Hazards vrlth Dsvicas Assoelated vrith Pornonam1 D l s b i s and Sorbent Regsnerated Oislysata Oetivey Sy.1.m~:' FDA Contract NO.22Ml-WOt. rsviwd draft repon. 1882.

(b) lmsile force lnnsmission (4 Laplaa equalion (dl Havemian sytem (el elastin ( 0 Langcr's line

1. b o r n ( ) 2. skin ( ) 3. lmdon ( ) 4. .Rev ( ) 5. iigammmm nochae ( ) 6. carlilese ( )

A124 (b) hydmryapatile (c) pymlytic carbon (d) calcium phorpbalc (4Biog1ass0 ( n barium titanate )I( gnpbite

I.R u l d i i d bed ( ) 2. morb.blc arsmic ( ) 3. upphim ( ) 4. minenl phua of bone ( ) 5. picrorlmrlc ( ) 6. Si0,-C.0-Na,O-P,O, ( ) 7. rubnntr o f heart valve disk ( )

(9)