7 FID- 134 287 COMPOSITE MRATERIALS FOR M RXILLOFACIR PROSTHESES U)' I/i

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COMPOSITE MATERIALS FOR MAXILLOFACIAL PROSTHESES

Annual Progress Report

Robert A. Erb, Ph.D.Stephen W. Osborn, Ph.D.Harold L. Heller

AUGUST 1979

Supported by-7U.S. ARMY MEDICAL RESEARCH AND DEVELOPMENT COMMAND

FORT DETRICK, FREDERICK, MARYLAND 21701

Contract No. DAMD 17-77-C-7059 D T lCELECTE

Division of The Franklin Institute

20th Street and The ParkwayPhiladelphia, Pennsylvania 19103 B

Approved for public release; distribution unlimited.

A.qThe findings in this report are not to be construed as an officialDepartment of the Army position unless so designated by otherauthorized documents.

.j Franklin Research Center 3 I0 2' 058A Dwision of The Franklin Insttute

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SECURITY CLASSIFICATION OF THIS PAGE (heton Does Entered)REPORT DOCUMENTATION PAGE READ INSTRUCTIONSREPORT__ DOCUMENTATIONPAGE_ BEFORE COMPLETING FORM

I. REPORT NUMBER GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER

4. TITLE (and Subtitle) S. TYPE OF REPORT & PERIO'D COVERED

Annual Progress ReportComposite Materials for Maxillofacial 9 Aug. 1978 - 8 Aug. 1979Prostheses 6. PERFORMING ORG. REPORT NUMBER

A-C4842-2 -

7. AUTHOR(s) S. CONTRACT OR GRANT NUMUER(s)

Robert A. Erb, Ph.D.Stephen W. Osborn, Ph.D. DAND 17-77-C-7059Harold L. Heller

9. P .RFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT. PROJECT. TASKAREA & WORK UNIT NUMBERS

Franklin Research Center 62775A

20th St. & Benjamin Franklin ParkwayPhiladelphia, PA 19103 3S162775A825.AB.063

I I CON'ROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE

U.S. Army Medical Research and Development Command. August 1979HQDA (SGRD-RMS) Fort Detrick, Frederick, Maryland 3 NUMBER OF PAGES

21701 22 pages14 MONITORING AGENCY NAME 8 AODRESS(II dilferent from Controlling Office, IS. SECURITY CLASS. (of this report)

Unclassified15a. OECLASSIFICATION DOWNGRADING

SCHEDULE

16. DISTRIBUTION STATEMENT (of hiA. Report)

Approved for public release; distribution unlimited

17 DISTRIBuTION STATEMENT (of the bstract entered in Stark 20, if dIler- from [eport)

;6 SUPPLEMENTARY NOTES

t9. KEY WORDS fContinue on revere side If necessary and Ident y by block number)

MAXILLOFACIAL PROSTHESES; PROSTHETIC MATERIALS; MICROCAPSULES;

SOFT FILLERS; ELASTuMER COMPOSITES

20,_ ABSTRACT 'Continue on reverse side If necesary end Identify by block number)

The purpose of this program is to develop ultrasoft compositematerials to be used as fillers in the fabrication of maxillo-facial prostheses. The projected composite systems are elastomeric-shelled, liquid-fillee m~crocansules. Two experimental approacheswere pursued toward making such microcapsules. One approach involvescoaxial extrusion of a catalyzed elastomer precursor and core liquidinto a recei,,ing bath. The other approach involves interfacial (cont.)

DD ,I D 1473 ED IN OF NOV 5 IS OSOLETL Unclassified

SEC'JRITV CLASSIFICATION OF T 41S PAGE "Wen Del E,#eed;

20. [cont.)

polymerization (e.g., of polyurethane) around droplets of core liquidsuspended in a continuum containing reactive materials (e.g., diisocyanates).

ABSTRACT

The purpose of this program is to develop ultrasoft composite materialsto be used as fillers in the fabrication of maxillofacial prostheses.The projected composite systems are elastomeric-shelled, liquid-filledmicrocapsules. Two experimental approaches were continued toward makingsuch microcapsules. One approach involves coaxial extrusion of acatalyzed elastomer precursor and core liquid. Improvements addedinclude: high-pressure flow-actuation system for shell components,variable-velocity carrier liquid, enclosed chopper, and non-aqueous col-lecting bath. Closed-shell capsules were obtained with the improvedsystem. Further work is needed on increasing the volume fraction ofliquid in the capsules. The other approach involves interfacial polymer-ization. Various reactive combinations to form polyurethanes and othermaterials were studied. A major problem has been to produce a wallwhich is elastomeric by a polymerization reaction fast enough to bepractical. Closed-shell large diameter capsules have been obtained;improvements are needed in making smaller capsules and tougher walls.

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FOREWOID

The concept behind this program is that a multiphase compositesystem should be able to simulate the mechanical properties of humansoft tissue better than a homogeneous system could. The proposedcomposite of particular interest consists of liquid-filled, elastomeric-shelled microcapsules held together to form a deformable mass; this isto simulate the semi-liquid cellular structure of human soft tissue.

The second year's program has continued toward the goal of makingsuitable microcapsules. This task has proved to be considerably moredifficult, and the systems and reactions more complex, than we hadoriginally projected. Nevertheless, sealed, liquid-filled capsuleshave been obtained, and further advances in quality of product andproduction of larger quantities should permit study of the microcapsulesin bonded structures.

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J.LL Franklin Research CenterA Division of The Franklin Institute

CONTENTS

Section Title Page

ABSTRACT . . . . - . .- . . . . . 1

FOREUORD . . . . . . . . . . 2

1 INTRODUCTION. . . . . . . . . . . . 4

2 MICROENCAPSULATION STUDIES . . . . . . . . 5

2.1 Experimental studies with coaxial extrusion. . . 5

2.2 Experimental studies with interfacial polymerization 13

FIGURES

Nwnber T'z.tle Page

1 General View of coaxial extrusion system for formingliquid-filled, elastomeric-shelled microcapsules. . . 6

2 Extrusion head with enclosed chopper. .. . . 7

3 Extrusion head with enclosed chopper, showing orifice . 9

4 Carrier stream coming from extrusion head . . . . 10

5 Microcapsules being formed . . . . . . . . 1

6 Use of mesh bag in collec:ting microcapsule samples . . 12

7 Microcapsules formed by the coaxial extrusion process . 14

8 Capsules formed by the liquid-drop, interfacial polymeriza-

tion process. . . . . 19

TABLE

1 Polyurethane Shell Composition -Bulk Screening Tests . 16

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IFranklin Research CenterA Division of The Franklin Institute

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1. INTRODUCTION

In the previous annual report the history of materials for maxillo-facial prostheses was reviewed. Many materials have been used, but inrecent times poly(vinyl chloride) plastisols, polyurethane compositionsand silicones have been used effectively in simulation of skin and exter-al features.

An area in particular in which further improvement is needed is insimulating the softness or "feel" of underlying soft tissues. This isparticularly important if some movement capability is needed. The soft-est materials presently available are polymeric foams (which have thedisadvantage of taking a permanent set by loss of gas when compressed)and gels (which are often unstable and lose internal liquid by syneresis).

This program is studying a new class of materials for use in fabri-" .cating maxillofacila prostheses: namely, liquid-filled, elastomeric-

shelled microcapsules. Conceptually, such a product is attractive forseveral reasons: (1) the cells in the natural soft tissue are themselvescomposites of liquid (or semi-liquid) material in deformable shells;(2) the liquid-filled microcapsules could be stable entities free fromthe syneresis or gas-leakage of other soft materials; (3) the micro-capsules could be stored as such and used by the prosthetist as anultrasoft filler to modify other materials as needed.

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2. MICROENCAPSULATION STUDIES

2.1 Experimental studies with coaxial extrusion

During this second-year program a new, high-pressure coaxial extru-sion system was designed and fabricated. The previous system, whichused a variable-speed, torque-independent motor acting on a screw drive,could not provide adequate force on the piston actuators for fluid flowof elastomer precursor and catalyst. The new system, which is hydrau-lically actuated, is capable of delivering the elastomer-precursorliquid at pressures up to 4000 psi.

Figure I is a general view of the hydraulically actuated coaxialextrusion system. The hydraulic pusher is in the upper center and thecontrol panel at the upper right. At the right is the variable-speedmotor drive for the core liquid (in a 50 cc polypropylene syringe), withthe speed control box at the far left resting on the variable-speed pumpfor the carrier stream. In the center of Figure 1 are the parallelcylinders containing the elastomer precursor and catalyst. The materialsused were Silastic 382 Medical Grade Elastomer (a polydimethylsiloxane,Dow Corning), 4:1 with 360 Medical Fluid (20 centistoke polydimethylsil-oxane fluid, Dow Corning), which gives a viscosity of about 100 poises.The catalyst used was stannous octoate (Catalyst M, Dow Corning), atabout a 1% level.

Experimental studies were carried out on stream-chopping methods.In the first-year program this was one of the areas of particular diffi-culty because of the viscous nature of the wall elastomer-precursormaterial. Rotating and vibrating cutters became gummed up, and generallydid not produce closed capsules from the coaxial stream. Open rotatingcutters also produced too much turbulence in the receiving bath. In theearly part of the second-year program pulsed liquids were examined as amethod for cutoff. Simpler systems -- such as opposed pulses from dentaljets (e.g., Water Pik) -- did not work well. There were problems inalignment, good pulse shape, and need for varied pulse frequency. Theliquid pulse cutoff method might be able to work, but would require amuch more sophisticated system with a sonic variable-frequency generator,possibly square wave, impressed on a liquid stream passed through pre-cisely positioned multiple ports.

A more effective method was found to be an enclosed rotating bladebuilt into the extrusion head, which was totally redesigned. Figure 2shows the new extrusion head. The elastomer-precursor and catalystenter, unmixed, from their respective piston pumps. These materials flow

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through the in-line static mixer, which is packed with curled stainless-steel ribbons. The mixed materials are then passed through a fine-meshstainless-steel filter, to remove any clumps of material that might clogthe narrow extrusion annulus. From the filter the material passes intothe annular region of the coaxial extrusion head.

The core liquid material (typically a polypropylene glycol) flowsinto the central channel. The coaxial stream is joined by a s, rroundingstream of carrier liquid, and the triple stream passed throtig! :eenclosed rotating chopper. Figure 3 shows the extrusion head semblyfrom another angle to show the orifice for the chopped stream

Figure 4 shows the carrier stream operating alone (witho, hecoaxial stream and chopper for the illustration). The use of iablespeed carrier stream has provided a useful added control on t1. dpsuleformation process. (The concept of a carrier stream at a higher velocitythan the extrudate was originally set forth by G. R. Somerville ofSouthwest Research Institute.)

A further new development has been the use of a nonaqueous (vegetableoil) receiving bath and carrier stream. In the previous year's effortwater and, later, graded-density aqueous salt solutions were used in thereceiving bath. The problem with aqueous systems has been that a slightimperfection or incompleted closure in the unsolidified wall of thecapsule will not heal but will tend to open or remain open. This relatesto the extremely hydrophobic nature of silicone polymers. With the aqueousreceiving bath most of the capsules formed were leakers. Use of thevegetable oil (soy oil) for the bath, with proper adjustment of theoperating parameters, has permitted capsules to close properly, withproduction of closed capsules approaching 100%. The oil is heated toaccelerate the curing of the shell material.

Figure 5 shows microcapsules being formed. The oil in the receivingtank is agitated by means of a magnetic stirrer. This helps to preventthe formed capsules from agglomerating while they are still tacky.Figure 6 shows one of the series of fabricated mesh bags used to collectsequential samples of formed microcapsules during a run. The use of thecollector bags permits samples to be taken from each of several parametersettings during a run. Comparison among the samples helps to define theoptimum parameters fro the process. The overall parameters that can bevaried, with some of these during a run, are: (1) pressure in thepolymer/catalyst system (affecting the flow rate of shell material);(2) motor speed for the core-liquid flow actuator (affecting the flowrate of core liquid); (3) pump speed for the carrier liquid; (4) chopperspeed; (5) bath temperature; (6) bath agitation rate.

Liquid-filled capsules were obtained with closed elastomeric shells.A new problem became evident when shells free of pinholes were obtained.Transport by diffusion of core liquid into the shell caused dimplingafter short-term storage. Systematic studies were carried out of weight

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-L Franklin Research CenterA Division of The Franklin Institute

Figure 3. Extrusion head with enclosed chopper,showing orifice.

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pickup and swelling of Silastic 382 pieces in various core liquids (chosenfrom those listed in Table 1 of the previous annual report). The mostsatisfactory material tested was polypropylene glycol 4000. This wassuccessfully substituted for the polypropylene glycol 1200 used in de-veloping the first sealed capsules.

The capsules formed with Silastic 382 shells and polypropyleneglycol 4000 cores do not show swelling or dimpling, and appear to havelong term stability. Various microscopic techniques for examination ofthe microcapsules were studied. Most show only the outer shape; however,one method allows an undistorted view though the capsule wall. Thisinvolves immersion in and viewing through a layer of xylene. Figure 7is a photomicrograph of closed, liquid-filled microcapsules approximatelyI mm in diameter. The most striking thing seen is the asymmetricalposition of the cavity.

A technique was developed for making compressional stress-strainmeasurements on individual microcapsules. A digital electronic balancewith 0.01 g sensitivity was used to measure stress. Displacement (strain)was measured with a micrometer-screw pusher. To compensate for movementof the balance's transducer a blank calibration curve was first determined.

Proposed further efforts in the coaxial extrusion program include:(1) improvements in the apparatus (replacement of the unreliable pumpfor the carrier stream, improving product collection means, arrangingfor a higher temperature bath, installing a bath cleaning system);(2) investigating a two-component, room-temperature-stable siliconematerial (Silastic 590) as a wall material; (3) studies to obtain improvedgeometrical characteristics with the microcapsules, particularly a highervolume fraction of contained liquid and better symmetry; (4) studies onhow the geometrical characteristics relate to the effective moduli (orsoftness) of the capsules.

Also proposed is the bonding of produced microcapsules in elastomericor gel matrices, and studying the mechanical properties of the combinedsystem. Of particular interest will be the effect of the packing fractionof soft microcapsule filler on the properties of the final composite.Considerations and recommendations will be made as to which types of softtissue might be effectively simulated with the microcapsule compositesfor maxillofacial applications.

2.2 Experimental studies with interfacial polymerization

In the interfacial polymerization part of the program, extensiveexperimental efforts were directed toward various reactive materials, thereaction parameters, and the resultant properties of formed shell material.A major problem has been to produce a wall which is elastomeric and reason-ably strong by means of a polymerization reaction fast enough to be prac-tical, and with a non-discoloring material system. To be avoided arepinholes in the formed walls, cure-through to form solid spheres, and

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-- Franklin Research CenterA Division of The Franklin Insitute

Figure 7. Microcapsules formed by thecoaxial extrusion process.

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UC~Franklin Research CenterA Divsmog of The Frankjin Insftute

agglomeration of formed capsules.

In attempting to define a wall composition with satisfactory mechani-cal properties, more than 60 experimental compositions were formulated,in a bulk screening test, using toluene diisocyanate (TDI) or hexa-methylene diisocvanate (HMDT) reacted with diols. triols, tetrols, amines,and catalyst (stannous octoate). Table 1 lists these combinations and theresults. From these experiments a reaction system consisting of anintermediate MW diol (100 parts) and a low MW tetrol (13.3 parts),reacted either with TDI or HMDI, in the presence of a catalyst waschosen for further study.

An interfacial liquid-drop method was used with diisocyanates inkerosene (constant or gradient concentrations in a cylindrical tube) withdrops of polyglycol mixtures with diamines and an inert material (UCON 50HB-2000, to prevent formation of solid beads). With high diamine levels,a fast cure was obtained, but with excessively brittle walls; with loweramine contents elastomeric properties were retained but with an unaccept-ably slow cure rate.

Studies were directed to a "prepolymer" method. In this approachthe monomers are partially polymerized by a method which produces low MW,liquid polymers with reactive terminals. These are later cured in asecond step which completes the polymerization to (in this case) anelastomeric solid. Isocyanate-terminated (polyols plus excess diiso-cyanate) and acid chloride-terminated (polyols plus excess bis-acidchloride) prepolymers were studied as the core component with inertdiluent liquid. With isocyanate-terminated prepolymer the external phasewas a polyol plus organotin catalyst in kerosene, leading to a poly-urethane wall; cure was too slow, however, and using diamines instead ofpolyol produced fragile shells. The acid chloride-terminated prepolymerswere readily cured using diamines dissolved in kerosene to yield polyesterbeads. Curing rate was good, and properties of the formed wall materialwere very promising. The major problem encountered was a high fractionof leakers; approaches under study toward improving the percentage ofclosed shells include adjustment of the acid chloride/polyol ratio andthe use of various other amine curatives. Figure 8 shows capsules withclosed shells with three different wall materials. As can be seen fromthe scale, the capsules are too large to be practical.

The emulsion process for making finer sized microcapsules, which wasstudied earlier in the program, is projected to be evaluated with the

prepolymer systems which prove effective with the liquid-drop method.The liquid-drop method will also be modified in an attempt to makesmaller capsules. Further development work is needed to define thereaction system more closely so as to maximize the mechanical strength,resiliency, and fluid retention ability of the walls. Characterizationwill be made of suitable microcapsules alone and in matrix systems.

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