paratus rosthetie Victor

John

for the mechanical adhesives

Tam, BSqa F. Wolfaardt,

M. Gary Faulkner, PhD, BDS, MDent, PhDC

testin PEng,b

maxillofaci

and

University of Alberta, Edmonton,Alta., Canada The use of adhesives to retain facial prostheses remains controversial. This controversy arises in part from a lack of information on the hiomechanical performance of this group of materials. No standard exists for maxillofacial prosthetic adhesives. This study describes the rationale for and design of an apparatus for testing maxillofacial and other skin adhesives. The apparatus provides for tensile, torsion, and combined tensile-torsion tests in a hard machine test. The results of the calibration studies of the apparatus are discussed. (J PROSTHET DENT 1992;67:230-5.)

acial prosthesesare usually attached to the facial tissueswith adhesives.l>2 Although controversy exists regarding the useof adhesivesto retain facial prostheses,3-6 their usecontinuesto increase2,5despite a lack of information on available products. Information on the nature, behavior and biocompatibility of adhesivesin current use is inadequate and incomplete. The study of adhesionto skin is complex becauseof the physiologic, biochemical, and histologic conditions that occur at the interface. Facial prosthetic adhesivesare commonly used on skin surfaces compromised by surgery, chemotherapy, and radiotherapy. The useof adhesiveson skin surfacesof patients who have had adjunctive therapy presentsa particular problem becausethe skin is subject to insult and change.The interaction of adhesivematerials with skin tissueis not clearly understood. With the rapid development of adhesive technology, it would be expected that highly specializedadhesiveswould have been developed for applications to compromisedtissues.However, specific methodsand materials required for adhesionto skin have not attracted the scientific attention deserved. Instead, the adhesive products reaching the market are materials that have been adopted along with their inherent inadequacies from nonmedical applications.7 It is therefore necessaryto subject these materials to scientific testing to better understand their mechanical behavior and biologic performance. Presentedat the 14thScientificandTechnicalConference, Institute of Maxillofacial Technology,Chichester,England. Supportedby CentralResearchFund grant No. 55-40696 of the University of Alberta. Y2andidate,Master of Science in Mechanical Engineering, Faculty of Engineering. bChairman, Department of Mechanical Engineering, Faculty of Engineering. CChairman, Division of RemovableProsthodontics,Faculty of Dentistry. l&W25663

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1. Factors affecting adhesivejoint strength in useof maxillofacial prosthetic adhesives. Fig.

To more clearly define the mechanicalbehavior of some commonly used, commercially available facial prosthetic skin adhesives,an apparatus wasdesignedto test the materials in both in vitro and in vivo situations. Mechanical testing of adhesivesis typically done by usingtensile, peel, and sheartest& s,gthat have beenadapted from industrial standardstests, even though these may not represent the usual situation for facial prostheses.Most often this testing is done by using a standard testing machine in which two surfaces,with adhesiveinterposed, are pulled apart at predetermined rates. The specimensare mounted to apply a tensile, stripping (peel) or shear load to the interface. The adhesivejoint strength at the skin surface may be influenced by numerousfactors (Fig. I). Althougb the interaction of all of these factors on the joint strength is unknown, several investigators have studied someof the parameters. Most investigations have used “hard” testing apparatus,which impliesthat the surfacesare pulled apart by rigid motion of the machine heads while the load is

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Fig.

Fig.

2. Custom-designed

BJECTIVES

The apparatus is designed to allow tensile, torsion, and combined tensile-torsion tests in both in vitro and in vivo situations. The design includes a standardized test specimen molded from silicone prosthetic material. The appa-

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diagram

of test apparatus.

test apparatus.

monitored by a load cell. An alternate procedure was used by Andrews et al.* who developed a “soft” machine that included a flexible link between the specimen and the load cell. For a peel test this system determined a lower bond for the peeling load. In several previous studies, both the peeling and the shear tests that were done had alignment problems because the direction of loading changed as the test progressed. Tensile, peel, and shear test techniques apply in certain clinical situations; however, for the complex dynamics of varying prosthesis flexibility and underlying skin movements, they may not present a complete picture of adhesive performance on the face. For this reason a specific testing apparatus that more closely approximates the clinical situation was constructed. This report describes the rationale for and design of a custom adhesive testing apparatus. The results of a pilot study applying the apparatus are described in a subsequent report. SIG

3. Schematic

DENTISTRY

Fig. 4. Plan view of motor platform. millimeters.

All measurements

in

ratus applies tensile and torsional loads at variable rates while continuously monitoring the loads applied to the adhesive surface. The essential design features include (1) accommodation of the head and the limbs for in vivo tests as well as to allow in vitro evaluation of various materials samples; (2) provision for monitoring the bonding load before testing; (3) provision of tensile testing at variable loading rates in the order of 25.4 mm/minute; (4) provision of torsional testing at variable angular rates in the order of 1 rpm (84.8 mm/minute); (5) provision of tensile stresses up to 50 kPa; and (6) recording of the tensile and shearing stresses on an XY plotter.

DESCRIPTION

OF APPAR

The testing apparatus consists of a test cluster that applies and measures the tensile/compressive and torsional loads (Figs. 2 and 3). The test cluster is mounted on an adjustable platform and stand to allow movement in all three dimensions. The apparatus has provisions for a “skim’substrate” or “substrate/substrate” interface where the adke-

231

TAM,

FAULKNER,

AND

WOLFAARDT

5. Load cell detail and strain gauge locations. All measurement5in millimeters. Fig.

Fig.

Fig.

6. Calibration curve for torsional load cell.

sivesto be tested are applied at the interface. The skin/silicone interface is accomplishedby constructing a mold in which the silicone is processedbefore it is bonded by adhesivesto skin surfaces.To evaluate shear strength of the adhesives,the mechanismis designedto measurethe shear strength while the prosthesisis twisted off of the skin. To measurethe tensile strength the test surfaceis pulled away from the skin surface. The apparatusalsopermits simultaneousshear and tensile tests. Sincethe device is intended to perform tests on humans, the designof a stand is crucial becauseit must allow flexibility in positioning the subject for testing on the face or limbs.

TEST

CLUSTER

The test cluster is the most complex part of the device becauseits function is to apply and measurethe loads on the test specimen(Fig. 3). A movable motor platform [2] is driven vertically by a variable speedtranslation motor [l] by meansof a cable and pulley assembly.A secondvariable speedmotor and gearassembly(for rotation) [3] ismounted on the platform and coupled to the specimen.These two motors provide the torsional and tensile loading. The shaft

232

7. Calibration curve for axial load cell.

8. Rotational speed-voltageinput characteristic for torsional motor. Fig.

from the rotational motor is coupled to the applicator (receptacle die), which is attached rigidly to the silicone test specimen.This coupling shaft acts asthe load cell to measure both tensile/compressiveloads and torsional loads. The vertical motion of the motor platform [2j is designed to allow smoothtravel alongthe 2 direction while resisting rotation in the plane perpendicular to it. A linear bearing [4] and two ball bearings were used to mount the motor platform on a 12.7 mm diameter shaft backed by a “shaft support T-rail” [5] (Fig. 4). The function of the linear bearing was to allow smooth displacementshaft (2 direction), and the two ball bearingswere usedto resistrotation on the XY plane. The load cell is the element responsiblefor measuring the torque neededto twist the silicone specimenfrom the test surface and the tensile force neededto pull the specimen from the test surface (Fig. 5). The torque is measured with four strain gaugesmounted in a 45-degreehelix and

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Fig. 9. Translational speed-voltage for translational motor.

TESTER

input

characteristic Fig.

Fig.

Fig. 10. Details of dies for silicone specimen All measurements in millimeters.

preparation.

tensile load is measured by four additional gauges mounted axially on the 0.77 mm thick tube. Each of these sets of four gauges (two of each are shown in Fig. 5) is connected to a four-arm Wheatstone bridge that automatically provides temperature compensation. The calibration curves for the torsional and axial load cells are shown in Figs. 6 and 7. Both cells were calibrated three times with dead weight. The curves indicate that the ceils exhibit a consistent linear behavior. A best-fit regression analysis was also completed. The resolution of the torsional cell was 1.25 Nmm, and for the tensile mode the resolution was better than kO.5 N. Also shown on the calibration curves (Figs. 6 and 7) are the theoretical outputs of the load cells. These lines were used in the original design and indicate that in both tests the design performed well.

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11. Processed

12. Schematic

silicone

diagram

specimen.

of die set closure.

The torsional and tensile loads were applied with the torsional and translational motors respectively. Varying the voltage applied to them varied the speed of the DC motors to allow testing to be done at several different speeds, Because no feedback control was used to regulate the motors, their speed was a function of the load applied. The rotation speed input voltage characteristic for the torsional motor is shown in Fig. 8 for different applied torques. This figure indicates that the rotational speed will vary as torque is applied. For example, with 8 volts input, the rotation speed can vary from approximately 1 rpm with 500 Nmm applied to 2.8 rpm with no torque applied. The curves also indicate the range of possible rotation speeds and torques that can be obtained. Fig. 9 is a similar characteristic for the translational motor. The speeds are for motion in the vertical direction and the curves show the variation with load. Again the curves delineate the range of speeds that can be obtained by varying the applied voltage. The test specimen is mounted on the end of the load cell with a holder that is specifically designed for the material

233

TAM,

Fig. 15. Schematic ated instrumentation. Fig.

Fig.

13. Die set retainer

14. Isometric

schematic

AND

WOLFAARDT

of test apparatus

and associ-

fins.

of test stand.

being tested. With silicone specimens, this holder is the receptacle portion of the die set which is used to form the silicone specimen. A die set, consisting of an attachment and a receptacle, is constructed so as to process silicone specimens in much the same fashion that prostheses are manufactured (Fig. 10). The specimen provides a 5 mm rim for the application of adhesives (Fig. 11). The die set is conical in shape so that, during the closure of the mold, extra material can flow along the taper of the attachment die and eventually overflow through the bleed holes (Fig. 12). The die is machined of stainless steel for durability and to prevent degradation of the die surface. Three stainless steel screws are used at the head of the attachment section for separating the mold sections after the silicone has cured.

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diagram

FAULKNER,

Fig. 16. Typical strip chart output sional and tensile test.

from combined

tor-

The holder must remain attached to the silicone specimen to provide resistance to torsion and tensile loads. A hollow and conical V-shaped retentive structure was designed to be attached to the inner wall of the receptacle section. The V-shaped retainer consists of fins positioned at an angle of approximately 0 to 60 degrees (Fig. 13). The fins are the essential feature responsible for resisting torsional and tensile forces so that forces with both Fx and Fy components can be resisted.

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CONCLUSION The stand is designed to accommodate the head and limbs for testing between the upper and lower plates (Fig. 14). The upper plate is capable of translating in all three directions, allowing adjustment for various subjects. It can accommodate motions of 168 mm, 300 mm, and 356 mm in the X, Y, and 2 directions. The stand is made of Plexiglass (Rohm and Haas, Philadelphia, Pa.) acrylic material, which is lightweight and transparent to allow easy monitoring of the tested portion of the skin. The test cluster is mounted on plate 2 (Fig. 14) and in operation the holder and prosthetic specimen are slightly beneath the plate, with the circular hole acting as a centering device as well as a stop in case large loads are accidentally applied laterally to the load cell. Plate 1 can move in the X and 2 directions by using rack and pinion gears whereas plate 2 can move only in the Y direction but is attached to plate 1 so that in essence it can be adjusted in all three directions. The instrumentation necessary to operate and monitor the apparatus is shown in Fig. 15. The two variable voltage power supplies provide power to the translational and rotational motors. The speed control is accomplished by varying the voltage on these power supplies. The output from the torsional and axial load cells is sensed by the two channel strain gauge conditioning systems monitored by the multimeters and recorded on the strip chart recorder. This recording allows the torsional and axial loads to be continuously monitored during testing as well as being recorded versus time. The entire output instrumentation is calibrated so that the graphic output is read directly in the appropriate units on the two-channel strip chart. Fig. 16 illustrates the output obtained from a combined torsional and tensile test. The upper curve is the tensile load, which is at approximately 5 N before the test begins. This value corresponds to the weight of the specimen and die, which is suspended from the load cell. Once the translational motor is started, the load rises until the maximum adhesion is achieved. The lower curve measures the torsional load, which is effectively zero until the test is begun. (The offset of the two curves is to allow the pens to overlap.) The full-scale values for this test were 10 N in tension and 250 Nmm for torsion.

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The apparatus described has been developed to more closely simulate the situation in which most facial adhesive systems would function. By including both tensile and torsional loading capability, the apparatus has allowed Aexibility in the type of information that can be obtained. If necessary, the output of the load cells can be digitized to allow further manipulation of the data in a desktop computer. No standards exist for the testing of maxillofacial prosthetic adhesives. The application of industrial standards to this group of materials appears inappropriate. The design of the adhesive testing apparatus presented provides a means of testing maxillofacial prosthetic adhesives in a meaningful manner. REFERENCES 1. Russouw C. The bond strength of facial prosthetic adhesive systems [Dissertation]. Johannesburg, South Africa: University of the Witwatersrand, 1987. 2. Page K. Assessment of the mechanical properties of some facial prosthetic adhesives: a preliminary report. Proc Int Gong Maxillofacial Prosthetics and Technology. Southampton: Millbrook Press, 1983:41049. 3. Rahn AO, Boucher LJ. Maxillofacial prosthetics-principles and concepts. 1st ed. Philadelphia: WB Saunders, 1970:113-87. 4. Roberts AC. Facial reconstruction by prosthetic means. Br J Oral Surg 1966;4:157-82. 5. Jani RM, Schaff NG. An evaluation of facial prostheses. J PR~STHET DENT

1978;39:546-50.

6. Pare1 SM. Diminishing

dependence on adhesives for retention of facial prostheses. J PROSTHET DENT 1980;43:553-60. I. Hulland CV, Hulland SM, Turner TD. Adhesion to skir-principles and applications. Proc Int Cong Maxillofacial Prosthetics and Technology. Southampton: Millbrook Press, 1983:402-g. 8. Andrew EH, Khan TA, Majid HA. Adhesion to skin. Part I-peel tests with hard and soft machines. J Mater Sci 1985,20:3621-30. 9. American Society for Testing and Materials. Standards D816-55, D897-68, D1002-64. Part 16. Philadelphia: 1968;250-306. Reprint

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to:

DR. JOHN F. WOLFAARDT 4036 DENTISTRY/PHARMACY FACULTYOFDENTISTRY UNIVERSITYOFALBERTA EDMONTON, CANADA

ALTA.

T6G

CENTRE

2N8

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Apparatus for the mechanical testing of maxillofacial prosthetic adhesives.

The use of adhesives to retain facial prosthesis remains controversial. This controversy arises in part from a lack of information on the biomechanica...
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