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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

31 1.05-1 1-3 1 1056 1 June 30, 1970 1 0 272

Progress Report

on DEVELOPMENT OF A RADIOPAQUE DENTURE BASE MATERIAL

by H. H. Chandler,* R. L. Bowen+

and G. C. Paffen barger +

* Research Associate, American Dental Association Research Program

in the Dental Research Section, National Bureau of Standards,

Washington, D. C. on a two-year leave of absence from The Ohio

State University College of Dentistry, Columbus, Ohio 43210.

h Research Associates, American Dental Association Research program

in the Dental Research Section, National Bureau of Standards,

Washington, D. C. 20234.

This investigation was supported in part by Research Grant DE02742-0 1 to the American Dental Association from the National Institute of Dental Research and is part of the dental research program conducted

by the National Bureau of Standards in cooperation with the Council

on Dental Research of the American Dental Association, the Dental Research Division of the U. S. Army Medical Research and Development

Command; the Dental Sciences Division of the School of Aerospace

Medicine, USAF, the National Institute of Dental Research, and the

Veterans Administration.

IMPORTANT NOTICE NATIONAL BUREAU OF STA'

for use within the Government. B and review. For this reason, the whole or in part, is not authorizi Bureau of Standards, Washington

the Report has been specifically p

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

; accounting documents intended

ubjected to additional evaluation

isting of this Report, either in Office of the Director, National

the Government agency for which

lies for its own use. <NBS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
I
DEVELOPMENT OF A RADIOPAQUE DENTURE BASE MATERIAL

ABSTRACT

Radiopaque glass-resin composites were made for evaluation

as denture base materials.

The addition of silane-treated, radiopaque, powdered

glass to clear poly (methyl methacrylate) resulted in composites that had greater optical translucency than commercially available pink denture base resins. These formulations could be pigmented and opacified to produce materials

that simulated oral soft tissues in color and translucency.

Other composites were made using various monomer, polymer

and glass combinations. Those made from mixes of BIS-GMA,

methyl methacrylate, poly (methyl methacrylate), and radiopaque

glass had excellent translucency but handling characteristics

were less than ideal.

Removal of very small glass particles (less than a few

pm) appeared to improve optical translucency.
2

A radiographic survey of 12 specimens that contained

from 29 to 57% of the radiopaque glass, indicated that all

had sufficient radiopacity to aid in localization of

swallowed or aspirated dentures made from such materials.

Finishing procedures on composite specimens were more

difficult and time-consuming and did not produce surfaces

as smooth as those obtained on poly (methyl methacrylate).

The finished surfaces felt smooth to the tongue but might

stain more easily in the mouth.
3

Introduction

In a previous paper, the need for radiopaque denture

base materials was demonstrated by a review of 123 cases of

swallowed or aspirated dentures or denture fragments. 1 The

difficulties and time involved in locating and orientating

the missing materials would, in many cases, have been decreased if they had been sufficiently radiopaque to be discernible in radiographs.

Previous methods of making denture base resins

detectable on radiographs have been either the addition

of certain salts of heavy metals to the polymer powder

of the placement of radiopaque inserts into the mixed

material during the molding procedure. 2 ” 6 Neither has

had wide acceptance. The addition of salts of heavy

metals tends to decrease the optical translucencv of

the cured material, as well as cause a deterioration of

certain physical properties. 3 ' 5 Addition of radiopaque

inserts causes a concentration of stresses at the interface between the insert and the resin which sometimes results in fracture of the base material.
4

In this study, several experimental radiopaque composite

denture base materials were made from mixtures of a silane-treated

radiopaque powdered glass and various monomer or monomer -polymer

combinations. The mixes were evaluated for general handling,

working, and molding characteristics; the polymerized specimens

were evaluated for optical translucency , radiopacity, surface finis

and overall suitability as a denture base.

One formulation, based on the addition of silanetreated radiopaque glass to poly (methyl methacrylate) (PMMA)

,

was selected for measuring some pertinent physical properties

and for constructing a number of technic dentures. The results

will be reported in subsequent papers . 6 ’ 7 methods* Materials and *

An x-ray opaque glass was previously developed for

use as a filler in dental restorative materials . 8 This

glass has been used as one of the fillers in an experi-

* Certain commercial materials and equipment are identified

in this paper to specify adequately the experimental

procedure. In no instance does such identification imply

recommendation or endorsement by the National Bureau of S

Standards or that the material or equipment identified is

necessarily the best available for the purpose.
mental anterior filling material, and the radiopacity of

the finished restorations was useful in dental radiographic

diagnosis. 9 The batch formulation of the glass was Si02 , 44;

BaF2 , 28; B2 03 / 16; and A1 3 03 , 12, in mole percent. 8 The density

2 5 of the glass was about 3.5 Gm/cc and its refractive index n D

was 1.535. The powder particle sizes ranged from about 1 to

50 pm.

For reasons of expediency this glass powder was the

primary one investigated in this study as an x-ray opacifying

agent for denture base materials. Other glass and resin

compositions, especially formulated for use in denture base

materials, would probably yield better overall results.

The glass powder was slurried with 0.5% (with respect

to the powder) 3-methacry loxypropy Itr imethoxy silane in

a solution of 95% acetone. After intermittent stirring, the

mixture was dried in a vacuum oven at 120° C and about 33.7

kN/m2 (250 mm Hg ) and passed through a U. S. Standard Sieve

No. 100 (maximum opening 149 pm). Silane treatment can significantly increase the bonding between a filler and a resin

matrix 10 and can make the powder hydrophobic. This glass,

unless otherwise specified, was used in making the specimens

in this study.
6

Initial testing indicated that glass particles of

very small size (less than a few pm in diameter) caused

a large reduction in optical translucency of the cured

specimens. Therefore, another powder was prepared like

the foregoing, except that before silane treatment, the

glass was mixed about 8 times with 95% ethanol and all

but the last mixture was allowed to stand undisturbed for

2 to 16 hours, after which the cloudy supernatant liquid

was decanted and discarded. The silane was added to the

last glass-ethanol mixture. Microscopic examination of the

dried powder revealed that the range in particle size

remained about the same, but presumably there was a decrease in the number of the smallest (colloidal) particles.

A few specimens were made with two other glasses. One

had a batch formulation of Al2 Q3 , 12; BaF2 , 8; B 2 0 3 , 16; and

Si0 2 , 44, in mole percent. 8 It was composed of silane-treated

particles of 44 to 480 pm and its refractive index n^p was 1.564.

The other had a batch formulation of Si02 , 78; B 2 0 3 , 6; BaF 2 ,

11; and A1 2 03 , 5, in mole percent. The powder particle sizes

2 5 ranged from about 1 to 50 pm and the refractive index n D

was 1.50 with slight variations in some particles.
7

One or another of the foregoing four glasses was added

in varying amounts to combinations of monomers (or monomers

and polymers) containing suitable initiators to cause polymerization. The monomers and polymers used were:

1.

) Ternary eutectic dimethacrylate. This monomer is

composed of the eutectic composition of three

crystalline dimethacrylates. They are bis (2-methacry 1-

oxyethyl ) isophthalate, bis (2-methacry loxyethy 1 ) terephthalate, and bis (2-methacry loxyethy 1 )phthalate

1

This monomer has been used previously in an experimental

anterior composite filling material and in a posterior

temporary composite filling material. 9 ’ 12 It is a

non-volatile liquid having a viscosity of about 2.4

2 5 poises at 27° C. The refractive index n^ of its

polymer is 1.548.

2.

) Stypol 46-4005 (Freeman Chem. Carp.). This monomer

is of the type described as BIS-GMA10 and is the

type currently being used as one of the monomers

in several commercial composite filling materials

for anterior teeth. It is very viscous, non-volatile,

2 5 and light yellow in color. The refractive index n Q

of its polymer is 1.568.
8

3.

) Methyl methacrylate (MMA) (Rohm and Haas) CO.

4.

) PMMA (Fisher Scientific Co.)

5.

) PMMA and poly (ethyl methacrylate) (PEMA) copolymer.

Type DJ (Rohm and Haas Co.).

6.

) In many instances, certain commercially available

denture base powders and liquids were used. These

will be identified as they appear.

Some specimens were opacified with titanium oxide (Fisher

Scientific Co.) and pigmented with cadmium red (United Color

and Pigment Co.).

Some of the composite specimens made in this study were

subjected to a finishing procedure similar to that in common

use for acrylic resin denture base. The procedure consisted

of initial finishing with a wet rag wheel charged with wet

pumice followed by a polishing agent (Shure Shine, Aurora

Dental Specialties Co.) on a. dry rag wheel. They were then

compared with PMMA specimens finished in an identical manner.

Fifteen specimens were made to determine the relative percentage of glass needed to give adequate radiopacity for diagnostic purposes. Twelve of these specimens were formed by
9

polymerizing mixture, of PMMA, PMMA-PEMA copolymer, MMA, and

from 29 to 57.5% by weight glass. One specimen was clear

denture base resin (Acri-Luc, Acr i-Lux Dental Manufacturing

Co., Inc.), one specimen was pink denture base resin (JustiTone T-3, H. D. Justi Co., Division of Williams Gold Refining

Co., Inc.) and one specimen was a pink denture base resin

containing barium sulfate [Stellon (pink radiopaque). Amalgamated Dental Co., Ltd.]. The specimens (as most of those

in this investigation) were made by compression molding the

doughy mixtures in a gypsum mold 28 mm square and about 3 mm

deep. The square specimens were ground to a thickness of

2.0 ± 0.1 mm, were cut in two, and had the corners rounded

so that the test pieces would more nearly simulate portions

of broken dentures than square pieces. They were then placed

in various positions on a tissue equivalent phantom (Rando

Phantom, Alderson Research Laboratories, Inc.).

Radiographs were made at 80 KV for 12.5 milliampere

seconds (50 MA for 1/4 second) at a source-film distance of

72 inches. Medium speed films ( 14 x 17 in.) were developed

for 5 minutes at 68°F. Filtration equaled a total of 2.5 mm

of aluminum.
- 10

The refractive indexes of 33 commercially available

denture base resins were determined using an oil immersion

method

.

Results and discussion

In order for denture base material to have wide

acceptance, it must be non-toxic, have translucency and

color that simulates gingival tissue, have adequate physical

properties, and should be workable by methods that are in

common use.

To obtain maximum translucency in a resin-glass composite,

it is necessary that the refractive indexes of the matrix and

filler be as nearly alike as possible. The refractive index

n^ of the primary glass selected as the filler was 1.535.

Therefore, it was necessary for the refractive index of the

resin matrix to be near enough to this value so that when

opacifiers such as coloring pigments were added, the composites would have translucency simulating oral soft tissues.

The refractive indexes of the 33 commercially available

denture base resins were, with one exception, in a range of
11

1.491 to 1.496. This indicates that they were all composed

2 5 essentially of PMMA (n ^ ~1.492). The one exception (Unibase,

2 5 Austenal Dental Division, Howmet Corp.) had an index n D of

1.5635 and is a styrene-acrylonitrile copolymer. Although

the refractive index of Unibase was nearly ideal for the

purpose of this investigation the material was not studied

further because it required techniques and equipment not

commonly utilized. Many styrene containing copolymers also

2 5 have refractive indexes n ^ above 1.5. Although these types

of materials represent interesting areas for possible future

development, they were not studied in this investigation

because of their rather unique handling and molding characterist ics

Composite specimens were made using two pink denture base

resins (Justi-Tone T-3, H. D. Justi Co and Acralite 88,

Acralite Co., Inc.) as the matrix and 40% by weight of the

2 5 glass as the filler. Both resins had a refractive index n Q

of about 1.494. The mixes reached a doughy consistency and

could be packed using techniques like those used with conventional powder-liquid denture base systems. The addition of

glass appeared to slightly reduce the flow of the doughy

i
12

material. The optical translucency of the finished specimens

was somewhat lower than identical specimens with no glass and

the color was changed to an unlifelike gray-pink. Both the

color and optical translucency would be unacceptable in a

denture base resin. A similar specimen made with a clear

denture base resin (Acri-Lux, Acri-Lux Dental Mfg. Co.) and

containing 40% by weight glass was yellowish-gray in color

and had greater translucency than the pink colored specimens

with no glass. These results indicate that the opacifiers

present in commercially available pink denture base resins

result in greater opacity than that obtained by the addition

of 40% glass to a clear resin alone. This gives some latitude for the use of additional opacifiers in attempts to

arrive at the proper color for a denture base material containing the glass filler.

Specimens were made of mixes containing the ternary

eutectic dimethacrylate monomer (n ^

= 1.548 for polymer)

with 25%, 50%, and 79% by weight glass. The mixes did not

reach a doughy state and did not flow under pressure in

the manner of a mix of PMMA-MMA. The resulting specimens

were reasonably translucent, but contained many voids and

air bubbles. Because the character of mixes containing this
13

monomer does not seem well adapted to pressure molding

techniques and because the specimens contained air bubbles

and imperfections that were not easily avoided or removed,

they were not investigated further.

Another series of specimens was made utilizing BIS-GMA

and MMA without glass. These materials, being liquids, were

heat-cured in test tubes, and the resulting specimens were

transparent (Fig. 1). By adjusting the ratio of BIS-GMA to

MMA, transparent specimens could be obtained with refractive

indexes almost identical to that of the glass. For example,

the polymer of a mix containing 60% BIS-GMA and 40% MMA by weight

2 5 had a refractive index n^ of 1.538. Addition of glass to

mixes such as this yielded specimens of excellent translucency

PMMA powder was added to mixes containing the glass,

BIS-GMA and MMA in order to achieve a doughy consistency.

Some of the resulting specimens still exhibited excellent

translucency. An example of one such mix contained 34.5%

glass, 34.5% polymer (Acri-Lux, clear), 14.8% BIS-GMA and

16.2% MMA by weight. The specimen from this mix approached

transparency (Fig. 2). However, these mixes did not have

doughy characteristics but became somewhat rubber-like in
14

texture. The rubber-like texture increased as the amount of

BIS-GMA was increased. Mixtures containing 15% or more by weight

of BIS-GMA were very difficult to pack and adhered severely to

the polyethylene films used for trial packing. Sticking also

occurred when cellophane and Teflon^ films were used. Mixes

made with less than 15% BIS-GMA could be pressure molded. The

same poor handling characteristics remained but to a lesser

extent and there was a loss of translucency as less BIS-GMA

was used. Because of their rather poor packing and handling

characteristics, mixes of this type were not investigated

further. As with the styrene-containing polymers, these

materials offer an interesting avenue for further development

of a composite denture base having a high degree of light

translucency

.

Another method of obtaining greater light translucency

is to use glass containing particles of larger size. This

acts to reduce the area of glass-resin interface and thereby

reduces the amount of light scatter. To illustrate this

factor, a specimen was made using glass particles with a

size range of 44 to 480 pm mixed with Acri-Lux clear polymer
15

O C and Acri-Lux monomer. The glass had a refractive index n^

of 1.564. The specimen was extremely translucent (Fig. 3)

but would not be acceptable as a denture base resin because it

could not be finished to a smooth surface using methods in

common use. One might expect such a material to have poorer

strength properties than a similar specimen made with smaller

particles

1

Attempts to gain greater translucency by using the glass

powder from which the smallest particles had been removed were

only partially successful in that the specimens had only slightly

better translucency than those made with the primary glass powder.

Apparently, the method for removing the particles was not

entirely successful. It is these small particles (less than

a few pm) that are most effective in scattering light.

Specimens were made using the experimental glass containing 11 mole percent of the BaF2 . The refractive index

n^1 of this glass was about 1.50. The specimens were made

from mixes containing 41% glass, 40% PMMA-PEMA copolymer,

and 19% MMA by weight. The optical translucency of these

specimens was not improved over similar specimens containing

I I I 1 I V T I
16

the glass with 28 mole percent of BaF 2 hut there was much

less gray discoloration. Whether the absence of the gray

color was due to fewer impurities is not known. In any case,

these specimens lacked the same degree of radiopacity due

to the reduced amount of BaF 2 in the glass filler. The

possibility of obtaining a whiter material by using

different glasses exists. Such materials would be easier

to tint to the color of the gingival tissues.

The foregoing investigation of light translucency and

handling characteristics reveals that several combinations

of polymers and radiopaque glasses have characteristics that

would make them possibilities for development into acceptable composite denture bases. However, specimens composed

essentially of PMMA and glass seemed to offer the best

combination of properties for immediate development. In order

to determine if acceptable color and translucency could be

achieved, several specimens were made from mixes containing

PMMA, MMA and the glass to which was added cadmium red pigment

with and without titanium oxide opacifier.
17

The composite specimens colored with cadmium red with

no titanium oxide opacifier were reddish brown in color

and totally unacceptable. Composite specimens colored with

the cadmium red pigment and with an equal amount of titanium

oxide had acceptable translucency and color. A moderate blue

or gray tinge remained. A typical specimen vileding these

results was made from a mix containing 38.5% PMMA, 38.5%

unsedimentated glass, and 23.0% MMA to which was added 0.029%

by weight each of the cadmium red and titanium oxide. It

should not be concluded that this combination of pigment

and opacifier, either in amount or type, represents the best

available for the purpose intended. These results only point

out that a composite material of this type can be developed

with ranges in color and optical translucency that simulate

oral tissues.

Finishing procedures were more time-consuming and difficult

for the glass-containing specimens and removal of gross scratches

required more effort. The finished surfaces did not reflect

light as well as specimens containing no glass. Evidently,

the pumice particles are not hard enough to give the exposed

T I I 1
- 18

glass particles a planed surface. Hence, the glass interfered with the incident light and caused it to be scattered.

The finished surfaces of the glass filled specimens felt

smooth to the tongue and were barely distinguishable in

this respect from the surfaces of unfilled specimens. They

appeared to be sufficiently smooth and reflected enough

light to be esthetically acceptable. Such surfaces might

however, be susceptible to staining in the oral environment.

One of the radiographs made on the tissue equivalent

phantom is seen in Figure 4. This and other radiographs

were examined by two radiologists to determine the weight

percent of glass required to yield adequate radiopacity

for diagnostic purposes. It may be seen in Figure 4 that

all of the 11 glass-containing specimens are visible to some

degree even when superimposed over the mediastinal area.

This area was selected because foreign bodies lodged there

are relatively difficult to locate. It was agreed that even

the specimen containing 29% glass would be of aid in locating

a swallowed or aspirated denture but that the more glass

present, the better it. would be for diagnostic purposes.
19

The specimen made of pink PMMA in Figure 4 is invisible and

demonstrates the problem of locating foreign bodies of this

composition. Other radiographs showed that the specimen containing BaS04 was only slightly radiopaque and would be of

little diagnostic value in locating a foreign body.

Summary and conclusions

A series of radiopaque resin-glass composite specimens

was made for the purpose of evaluating them for use as a

denture base material. The evaluation included molding and

handling characteristics, light translucency , radiopacity,

and overall suitability for use as denture base materials.

Several combinations of monomer, polymer, and radiopaque

glasses were used. The addition of a silane treated radio25

paque glass having a refractive index n Q of 1.535 to clear

PMMA denture base resin resulted in specimens of greater

optical translucency than commercially available pink denture

base resins. Such formulations could be pigmented and

opacified to produce specimens that simulated oral soft tissues

in color and translucency. These formulations appeared to

i i i
20

have the best characteristics for immediate development.

Other monomer, polymer* and glass combinations were investi

gated. Specimens made from mixes containing BIS-GMA, PMMA,

MMA and the glass gave the greatest translucency but handling

and molding characteristics were less than ideal. This combination and others using styrene copolymers offer areas

for further investigation.

Optical translucency was also improved by using glass

particles of larger size. One such specimen (using glass

particle sizes of 44 to 480 pm) was extremenly translucent.

Some evidence was obtained that removal of very small glass

particles (less than a few microns) would also be an effective

method for improving optical translucency.

A radiographic survey indicated that specimens containing 29 to 57% of the glass by weight all had sufficient

radiopacity to be of aid in localization of swallowed or

aspirated denture fragments made from such materials.

Specimens containing the higher percentages of glass would

be more easily located.
21

Finishing procedures on the composite specimens did

not yield as smooth a surface and required more time than

identical procedures on PMMA specimens. The finished surfaces

of the composite specimens felt smooth to the tongue but

might stain easily in the oral environment.
22

References

1. Chandler, H. H.; Bowen, R. L., and Paf f enbarger , G. C.

The need for radiopaque denture base materials: A

review of the literature. (in manuscript)

2. Alvarez, Luiz C. Radiopacity of acrylic resins. Oral

Surgery 22:318 Sept 1966.

3. Atkinson, H. F. Partial denture problem. The lack of

radio-pacity in methyl methacrylate. Aust Dent J 58:349

Dec 1954.

4. Bursey, Don C., and Webb, J. J. Incorporation of

radiopaque materials into denture plastics. U. S. Armed

Forces Med J 11:561 May 1960.

5. Saunsbury, P. Radio-paque denture resins. Dental

Practitioner 14:243 Feb 1964.

6. Chandler, H. H. ; Bowen, R. L „ , and Paf f enbarger , G. C.

Physical properties of a radiopaque denture base material,

(in manuscript).
23

7. Chandler, H. H. ; Bowen, R. L., and Paf f enbarger , G. C.

Radiopaque denture base materials: Technic dentures.

(in manuscript)

8. Bowen, R. L., and Cleek, G. W. X-ray-opaque reinforcing

fillers for composite materials. J Dent Res 48:79

Jan-Feb 1969.

9. Chandler, H. H.; Bowen, R. L.; Paf f enbarger , G. C., and

Mullineaux, A. L. Clinical evaluation of a radiopaque

composite material. Has been accepted for publication in

The Journal of the American Dental Association.

10. Bowen, R. L. Properties of a silica-reinforced polymer

for dental restorations. JADA 66:57 Jan 1963.

11. Bowen, R. L. Crystalline dimethacrylate monomers.

Has been submitted to the J Dent Res for publication.

12. Burns, Claire; Barton, J. A., Jr., and Chandler, H. H.

Development of a composite resin material for temporary

restorative materials. I ADR Program and Abstracts of

Papers March 1969, Abstract No. 145.

i i i
24

13. Bowen, R. L., and Rodriguez, M. S. Tensile strength

and modulus of elasticity of tooth structure and

several restorative materials. JADA 64:378 Mar 1962.
->? -

Legends

FIGURE Is A 5 mm thick specimen polymerized from a mix

containing 60% BIS-GMA and 40% MMA by weight.

The almost total transparency can be seen from

the clarity of the typewritten letter beneath

2 5

the specimen. The refractive index n ^

of the

copolymer is 1.538.

FIGURE 2: The specimen on the left (B) is a commercially

available PMMA pink denture base resin. The

specimen on the right (C) was from a mix containing 34.5% of the glass, 34.5% clear polymer

(n^ 5 = 1.492), 14.8% BIS-GMA, and 16.2% MMA by weight.

Both specimens are about 2.75 mm thick and are against

a narrow black tape on frosted glass illuminated from

behind. Note the greater translucency of the glasscontaining specimen.

FIGURE 3: The specimen on the left (D) is a commercially

available pink denture base resin with no glass

i i i i i
- ^aP'-

(refractive index = 1.492). The specimen

in the middle (E) contains 40% glass (particle

size 44 to 480 pm; refractive index n^ = 1.564)

and 60% clear denture base resin (n^ = 1.492).

The specimen on the right (F) is composed of

40% glass (particle size 1 to 50 pm; refractive index n^ 1.535) and 60% PMMA. The

illumination is the same as in Figure 2. Note

the extreme translucency achieved in specimen E

with the large particles even though the glass

used has a refractive index higher than that of

the glass used in specimen F.

FIGURE 4: Chest radiograph made on a tissue equivalent

phantom on which 11 glass-containing specimens

and one commercially available denture base

resin were superimposed over the mediastinal

area. The radiograph was exposed at 80KV for

12.5 MAS at a target film distance of 72 inches.

Filtration equalled 2.5 mm of aluminum (AP

view)

.

USCOMM-NBS-DC
Specimen No. 1 is composed of a commerciaily available

pink denture base resin. Specimen No. 2 contains 29%

glassNo. 3 / 33% glass

No. 4, 36% glass? No. 5

glass; No. 6 , 42% glassNo. 7

44% glass. No. 8

glass

No. 9, 49% glass 7 No. 10 , 51% glass 7 NO

53% glass, and No. 12, 54% glass. All of the glasscontaining specimens are visible to some degree

whereas the commercially available resin is invisible .

The authors wish to thank Ray Alcox, DDS and his associates

Division of Medical Radiation Exposure, Bureau of Radiological

Health of the United States Public Health Service, for their

assistance in the radiographic portions of this investigation.
*

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