GOVERNMENT/INDUSTRY
COLLABORATE TO DEVELOP MERCURY-FREE
DENTAL MATERIAL
http://www.nist.gov/public_affairs/releases/tn5948.htm
FOR IMMEDIATE RELEASE: Roger Rensberger
Oct. 28, 1993 (301) 975-2762
TN-5948
A mercury-free, direct filling alternative for
conventional dental amalgams is being developed at the
National Institute of Standards and Technology. The new
restorative process uses metallic powders in a form that
is easily applied to prepared tooth cavities by dentists
using treatment procedures very similar to those in
current practice.
The dental material project is a collaborative effort
between government and industry. The National Institute of
Dental Research is contributing support for the program
through the American Dental
Association Health Foundation's Center for Excellence.
The ADAHF maintains the Paffenbarger Research Center at
NIST.
A dental materials manufacturer, Dentsply
International, Milford, Del., has a cooperative research
and development agreement with NIST to help develop the
new mercury-free restorative
material.
The new restorative process is based on NIST
electrochemical powder technology. The technique was
invented by David S. Lashmore, leader of the NIST
Electrodeposition Group, and Moshe P. Dariel, guest
scientist from the David Ben-Gurion University, Beer-Sheva,
Israel. "The mercury-free
dental material project offers us an opportunity to
transfer technology developed at NIST to industry and meet
a national need," said Lashmore.
He points out that the mercury
content of amalgams used in the overwhelming majority of
dental restoratives continues to raise concern with regard
to their long-term effect on public health and the
environment. The new
alloy will help reduce the amount of mercury dispersed in
the environment by dental waste.
Joyce Reese, NIDR program director for Biomaterials,
Pulp Biology and Dental Implants, notes that although
there is no scientific evidence linking mercury in amalgam
to systemic diseases, the new NIST mercury-free, dental
restorative material meets an important objective of the
National Institute of Dental Research to find alternative
materials for conventional dental amalgams.
Lashmore said that the principal goal of the NIST
research has been to develop a high-performance, mercury-free
alloy that will consolidate at body temperature
in prepared tooth cavities. The material involves the use
of biocompatible metallic powders such as silver-coated
tin. These pairs of metals undergo fast diffusion or
combine to form an in-situ intermetallic compound at body
temperature.
The treatment of tooth cavities is quite simple. The
dental practitioner will mix the coated powders with an
activating, biocompatible liquid to form a slurry. The
mixture is then pressed into a prepared filling with
conventional dental instruments. After compaction, the
material hardens into a strong,
mercury-free dental alloy.
Lashmore explains that even though silver and tin are
the principal ingredients, investigations are being
conducted with other formulations that may contain copper,
gold, or small amounts of other inert materials such as
silver, alumina and silicon carbide. He emphasizes that all
of the materials under study are free from mercury,
indium or gallium, which are commonly used today in dental
amalgams.
According to Lashmore, the new direct filling
restorative material could be in dental offices within
three years (from Oct. 28, 1993) It
will be tested both at NIST and by independent dental
research laboratories before undergoing review by the Food
and Drug Administration.
As a non-regulatory agency of the Commerce Department's
Technology Administration, NIST promotes U.S. ecomnomic
growth by working with industry to develop and apply
technology, measurements and standards.
NOTE
TO EDITORS:
NIST, which was established in 1901 as the National
Bureau of Standards, has a materials research program that
is recognized worldwide. As a non-regulatory agency, NIST
is conducting research on dental materials to support the
safe, efficient and economical use of materials for the
benefit of consumers and the practicing dental
professional.
The NIST dental materials program is a long-standing
model of cooperation between the private sector and
government. Researchers from the dental profession,
industry and government have worked together at NIST for
more than 65 years to improve dental materials and
devices.
NIDR, one of the National Institutes of Health,
Bethesda, Md., is the primary sponsor of dental research
and related training in the United States.
DENTAL
AND MEDICAL MATERIALS
http://www.metallurgy.nist.gov/techactv1999/AnnualReport1999.html#dental
The Dental and Medical Materials Program provides basic
materials science, engineering, test methods, and
standards to sectors of the health care industry for the
development of new or improved materials and delivery
systems. This program focuses on (1) development of
improved dental restorative materials with greater
durability, wear resistance and clinical acceptability;
(2) development of improved bone fixation materials, and
(3) evaluation of biomaterials.
Dental restorative composites are heterogeneous
materials having three essential phases: (1) a polymeric
matrix which comprises the continuous phase, (2) fillers
of various types, sizes, shapes and morphologies which
constitute the disperse phase and (3) an interfacial phase
that, in varying degree, bonds the continuous and disperse
phases into a unitary material rather than a simple
admixture.
While all three phases are important in determining the
properties of the composites, this program is focused
primarily on the interfacial and polymer matrix phases.
Since the polymerization shrinkage that occurs in the
matrix phase is one of the most commonly cited
deficiencies of dental restorative composites, resources
are allocated to develop high conversion, durable, low
shrinkage polymeric materials for use in dental resin and
composite applications. The polymeric matrix of a dental
composite typically is formed by free radical
polymerization of a resin which is one or more vinyl
monomers, usually of the methacrylate class.
Polymerization is started either by the formation of
initiating radicals from chemical reduction-oxidation
(redox) reactions or by photochemical redox reactions.
Although only a minor component of these composites,
the interfacial phase that develops from the interaction
of the silane coupling agent with the polymer matrix and
the siliceous filler exerts a profound effect on the
properties of the composites. Because these composites are
used in an aggressive, aqueous environment that constantly
challenges the vulnerable silane mediated polymer-filler
bond, understanding of this critical interfacial phase is
being acquired so that strategies can be developed for its
improvement.
The occupational and
environmental hazards associated with the use of
mercury-containing dental alloys are a recurring source of
public concern. Since dental amalgams have
performed exceedingly well over more than one hundred
years, the development of a
direct filling material still based on the common
constituents of dental amalgams, other than mercury, is
desirable. This project is focused on
acid-assisted consolidation of chemically precipitated
silver powders and property measurements of hand
consolidated test compacts prepared with the tools and
procedures normally employed by dentists. The
observed values of flexural strength for the silver
compacts were equal or superior to mercury amalgams.
Corrosion resistance, microleakage and marginal toughness
values of the compacts were found to be superior to those
of amalgams. Wear and biocompatibility studies
on the hand consolidated compacts are in progress.
Besides the dental materials projects, efforts are
directed toward the development of improved bone fixation
materials and the evaluation of biomaterials. A project,
carried out in collaboration with the American Dental
Association and the National Institute of Dental and
Craniofacial Research, is directed at enhancing the
biocompatibility and mechanical properties of composite
bone cements.
The biomaterials evaluation effort centers on the NIST
Orthopedic Wear Consortium which consists of four
companies to develop accelerated wear test procedures for
rapid screening of materials used in hip and knee
replacements. This will accelerate the introduction of new
biomaterials into practice.
Dental and medical research directions in support of
the goals are established in collaboration with the
American Dental Association (ADA), the National Institute
of Dental and Craniofacial Research, the National Heart,
Lung and Blood Institute, the U.S. Food and Drug
Administration, and guest scientists from the U.S. Navy
and the U.S. Public Health Service. NIST has hosted
research associates from ADA since 1928. Currently, the
ADA Health Foundation sponsors 30 research associates at
NIST. The collaborative relationship between that
professional association and the federal government is
unique, and continues to develop and transfer important
new technologies to dentistry and medicine.
Project
Title:
ADVANCED RESTORATIVE DENTAL MATERIALS
Investigators: G. R. Stafford, C. E. Johnson, and D. R.
Kelley
Objectives: The project seeks to provide to the dental
industry with a metallic restorative without the use of
mercury that can be hand consolidated while maintaining
critical mechanical properties and satisfying the
biocompatibility criteria.
Technical Description:
The occupational and
environmental hazards associated with the use of
mercury-containing dental alloys are a recurring source of
public concern. Since dental amalgams have
performed exceedingly well for more than one hundred
years, the development of a direct filling material still
based on some of the common constituents of dental
amalgams, other than mercury,
is the focus of this program. A search for a metallic
substitute to the amalgams has to begin with the problem
of the consolidation of an easily deformable very plastic
material into a strong solid, under the strict
temperature, pressure and time limitations of common
dental practice.
The approach taken by NIST under sponsorship of the
National Institute of Dental Research (NIDR) has been to
provide an appropriate surface treatment to individual
silver powders which are then cold-welded under low
pressures to a cohesive solid. The silver powders are
derived from a chemical precipitation process, resulting
in powders ranging in size from 0.2 F m to 2.0 F m. The
surface treatment involves the use of a dilute acid to
remove the naturally occurring oxide layer on the powders.
Subsequently, a slurry, consisting of the wet mixture of
the surface-treated powder particles, is placed and
consolidated in a prepared dental cavity. The liquid film
surrounding each particle serves both to maintain a clean
surface, and to constrain the micron-size particles, so
that they present no inhalation danger to the patient. The
powders are consolidated into a solid mass using
instruments normally employed in dental practice. The term
"acid-assisted consolidation: was coined for the
consolidation technique.
A series of in vitro biocompatibility tests, for
cytotoxicity, hemolysis, Ames' and Styles' cell
transformation, were used in the evaluation of
hand-consolidated and machine-pressed silver-based
alternative restorative material.
Planned Outcome:
The ability to densify surface-treated silver powder
into a cohesive solid displaying reasonable mechanical
strength, as well as the established and approved use of
silver as a dental restorative material, will lead to a
mercury-free metallic dental restorative in the event that
mercury-containing restoratives are curtailed.
External Collaborations:
The American Dental Association is providing support
for this project by conducting biocompatibility studies on
the silver-based alternative dental restorative.
Collaboration with the American Dental Association
Health Foundation is focused on other factors associated
with the use of the silver-based alternative restorative,
such as the nature and shape of the condensing tools and
the placement procedures to be followed.
Accomplishments:
The development of a
mercury-free metallic alternative to conventional dental
amalgams, which was the subject of a four-year long
program supported by the National Institute of Dental
Research, has concluded. The current technology
is based on the ability of silver surfaces to adhere (cold
weld) to each other after being treated with dilute
fluoboric acid. Silver particles that have been immersed
in such acid can be hand-consolidated into cohesive solids
(78 % theoretical density) using conventional dental tools
at applied pressures of 35 MPa to 50 MPa.
Over the course of the program, several forms of silver
powder were evaluated and it was determined that the best
source of silver was obtained from a two solution chemical
precipitation process (patent pending). Silver powder
properties to best promote hand consolidation were also
determined. One of the most important parameters is the
agglomerate size and individual particle size of the
silver powder. Dramatic increase in both the transverse
rupture strength and density of hand consolidated samples
can be achieved as the maximum agglomerate size of the
silver powder is decreased. The precipitation process was
optimized for agglomerate size less than 40 F m (80 % <
25 F m). Another important parameter is the annealing of
the silver powder prior to consolidation.
Transverse rupture strength and density are improved
dramatically when the precipitated silver powder is
annealed at 750 °C. This has been attributed to a
reduction in yield strength of the powder prior to
consolidation. Minimal sintering and agglomeration occurs
as a result of the 750 °C anneal.
Acid-assisted consolidation
(three patents, one issued and two pending) was the major
finding which allows us to put forth a mercury-free
metallic restorative. Surface spectroscopy and
electrochemical measurement results supported the
assumption that the function of the fluoboric acid is to
remove the silver surface oxide and thereby promote cold
welding.
Using the current technology,
hand consolidated silver equals or exceeds the transverse
rupture strength, shear strength, creep, toughness,
corrosion resistance, microleakage, cyclic contact
fatigue, and wear properties of conventional silver
amalgam. The alternative silver restorative
placement time is twice that for amalgams which warrants
further study with emphasis on reducing the placement
time.
In vitro biocompatibility tests which included
cytotoxicity, hemolysis, and Ames' and Styles' cell
transformation were completed on the component
(precipitated silver powder and fluoboric acid),
hand-consolidated and machine-pressed samples of the
alternative restorative, and corrosion products resulting
from immersion of hand-consolidated material into an
artificial saliva solution. The 7 d, 21 d and 90 d
corrosion products did not reach the threshold level in
any of these tests to be considered non-biocompatible.
Under the conditions employed by the Ames' and Styles'
tests, silver powder, fluoboric acid and hand-consolidated
restorative samples showed no mutagenic potential.
The hemolysis examination indicated that silver powder
and fluoboric acid (0.02 % and greater) were severely
hemolytic. An equal mass of consolidated silver powder was
marginally hemolytic indicating that the extent of
hemolysis is clearly a function of the surface area of
silver exposed to solution. In a clinical situation,
silver powder is quickly consolidated and the bioavailable
surface area is quickly reduced. Most of the consolidated
Ag would be surrounded by hard tissue. Initial
cytotoxicity tests showed the powder and consolidated
material to be severely cytotoxic while the dilute
fluoboric acid is not cytotoxic. Subsequent testing of
consolidated silver disk extracts were found to be
severely toxic in tissue culture medium. The cytotoxicity
was exacerbated by 30 % with cell to metal contact.
However, all extracts in artificial saliva were not
cytotoxic. Thus it appeared that the majority of the
cytotoxicity was an artifact of the corrosive properties
of the tissue culture medium, not likely to be present in
the oral cavity. Furthermore, the cytotoxicity rapidly
decreases upon dilution. Dilution of released silver
within the oral cavity is very likely to occur with
salivary flow. Thus there may be little or no cytotoxic
effect under clinical conditions.
Impacts:
The program has demonstrated that a metallic
mercury-free dental restorative material, based
essentially on metallic silver, can be obtained using
chemically precipitated silver powder and acid-assisted
consolidation. Technologies developed during the program
have been transferred to industry by way of exclusive
licensing of patents. Patents involving electrochemical
coating of powders and acid-assisted consolidation of
metallic powders have been licensed to Materials
Innovation, Inc., for use other than dental applications.
These technologies are presently in use in the manufacture
of thermal management devices. The
American Dental Association Health Foundation has been
given exclusive license to use the acid-assisted
consolidation patent for dental applications.
Outputs:
Patents Granted:
Electrochemical Fluidized Bed Coating of Powders, U.S.
Patent No. 5,603,815, February 18, 1997. C. E. Johnson, D.
R. Kelley, et. al.
Acid Assisted Cold Welding and Intermetallic Formation
and Dental Applications Thereof, U.S. Patent No.
5,711,866, January 27, 1998. C. E. Johnson, D. R. Kelley,
et. al.
Patents Pending:
Acid Assisted Cold Welding and Intermetallic Formation
and Dental Applications Thereof, NIST Docket No. 93-031
CIP2, U.S. Patent Application No. 08/317,729. C. E.
Johnson, D. R. Kelley, et. al.
Acid Assisted Cold Welding and Intermetallic Formation
and Dental Applications Thereof, NIST Docket No. 95-038D,
U.S. Patent Application No. 08/437,650. C. E. Johnson, D.
R. Kelley, et. al.
Method for the Chemical Precipitation of Metallic
Silver Powder Via a Two Solution Technique, NIST Docket
No. 98-027US, U.S. Patent Application No. 09/256,073. C.
E. Johnson and G. R. Stafford
RESEARCH STAFF
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NIST
in Your Mouth
http://www.nist.gov/public_affairs/nhouse/nhmouth.htm
By helping to invent and improve materials, tools and
methods, NIST has been advancing the practice of dentistry
for nearly 80 years. The practice of dentistry and the
National Institute of Standards and Technology (NIST) go
back a long way--back to 1919 in
fact, when the U.S. Army asked NIST, then the National
Bureau of Standards, to look into the physical factors
behind good and bad metal-based amalgams for filling
teeth. Ten years later, NIST's laboratory
predecessor began what continues to be a collaboration
with the American Dental Association whose goal has been
the development, refinement and general improvement of
medical practice through the invention of new dental
materials, tools and methods.
In addition to the introduction of new polymeric and
mineral-based materials for aesthetic tooth restoration
and the development of metallic alloys for amalgams, other
results from the collaborative research between NIST and
the ADA include the panoramic X-ray and the water-driven
precursor of today's air-driven handpiece with which
dentists wield drills, cleaning heads and other tools.
Besides increasing the quality of patient care, both of
these instruments have saved the nation several billion
dollars by reducing the time required by dentists to treat
patients and by increasing the comfort and effectiveness
of dental treatment. It was estimated in 1987 that the
increased durability of composite restorations, and
thereby the reduction of replacement costs of previously
used materials, saved Americans more than the combined
appropriated budgets of NIST, the ADA and the National
Institute of Dental Research.
There are many ongoing recent research projects at NIST
that aim to improve dentistry. One project seeks better
understanding of the mechanism by which dental
biomaterials adhere to tissues. Biodegradable materials
are being developed for hard tissue repair and are being
evaluated clinically. Improved resins that have higher
resistance to degradation by oral fluids and that reduce
polymerization shrinkage are being developed. An in-mouth
radiation shield to protect cancer patients from secondary
radiation emitted from metallic restorations during
radiation therapy is in clinical trials with industrial
sponsorship. The interfaces between fillers and resins of
resin-based composite restoratives are being investigated
with the goal of improving interfacial strength and
durability of composite restorations. Work
is in progress with the NIST Metallurgy Division to
develop a metallic, mercury-free restorative that can be
used like dental amalgam.
Link:
Dental Fact Sheet: questions and answers about dental
research at NIST.
Send feedback to Sharon Shaffer
ADVANCED
MATERIALS PROGRAMS: Dental & Medical Materials
The Dental and Medical Materials
Program provides basic materials science, engineering,
test methods, and standards to sectors of the health care
industry for the development of new or improved materials
and delivery systems. The focus of this program is the
development of improved dental restorative materials with
greater durability, wear resistance and clinical
acceptability.
Dental restorative composites are
heterogeneous materials having three essential phases: (1)
a polymeric matrix which comprises the continuous phase,
(2) fillers of various types, sizes, shapes and
morphologies which constitute the disperse phase and (3)
an interfacial phase that, in varying degree, bonds the
continuous and disperse phases into a unitary material
rather than a simple admixture. While all three phases are
important in determining the properties of the composites,
this program is focused primarily on the interfacial and
polymer matrix phases. Since the polymerization shrinkage
that occurs in the matrix phase is one of the most
commonly cited deficiencies of dental restorative
composites, resources are allocated to develop high
conversion, durable, low shrinkage polymeric materials for
use in dental resin and composite applications. The
polymeric matrix of a dental composite typically is formed
by free radical polymerization of a resin which is one or
more vinyl monomers, usually of the methacrylate class.
Polymerization is started either by the formation of
initiating radicals from chemical reduction-oxidation (redox)
reactions or by photochemical redox reactions.
Although only a minor component of
these composites, the interfacial phase that develops from
the interaction of the silane coupling agent with the
polymer matrix and the siliceous filler exerts a profound
effect on the properties of the composites. Because these
composites are used in an aggressive, aqueous environment
that constantly challenges the vulnerable silane mediated
polymer-filler bond, understanding of this critical
interfacial phase is being acquired so that strategies can
be developed for its improvement.
The occupational and environmental
hazards associated with the use of mercury-containing
dental alloys are a recurring source of public concern.
Since dental amalgams have performed exceedingly well over
more than one hundred years, the development of a direct
filling material still based on the common constituents of
dental amalgams, other than
mercury, is desirable. This project is focused
on acid-assisted consolidation of chemically precipitated
silver powders and property measurements of hand
consolidated test compacts prepared with the tools and
procedures normally employed by dentists. The observed
values of flexural strength for the silver compacts were equal
or superior to mercury amalgams. Corrosion
resistance, microleakage and marginal toughness values of
the compacts were found to be superior to those of
amalgams. Wear and biocompatibility studies on the hand
consolidated compacts are in progress.
Dental research directions in support
of the goals are established in collaboration with the
American Dental Association (ADA), the National Institute
of Dental Research (NDIR), and guest scientists from the
U.S. Navy and the U.S. Public Health Service. NIST has
hosted research associates from ADA since 1928. Currently,
the ADA Health Foundation sponsors 32 research associates
at NIST. The collaborative relationship between that
professional association and the federal government is
unique, and continues to develop and transfer important
new technologies to dentistry and medicine.
Dental
Information
