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View Full Version : Strain gauge coatings, etc.



Thomas G. Loebig
08-19-1994, 05:25 AM
About a month ago I posted the following:=20

>Hi Netters,
>
>To anyone out there with some bio-strain gauging experience:
>We would like to know,
>1) what's the best and most economical coating to use on wet bone in=
a
saline
>sprayed (not submerged) environment? We are considering M-Coat C
>(solvent-thinned RTV silicone), M-Coat D (acrylic lacquer) or Dow-Co=
rning
>Silicone Rubber 3140. We have been using M-Coat A, and after experi=
encing
>some progressively increasing signal drift, we want to use something=
a
little
>more substantial than polyurethane.
>2) Are freeze/thaw cycles harmful to cyanoacrylate adhesives such as=
M-Bond
>200?
>3) Exactly how long is cyanoacrylate effective when wetted with sali=
ne?
>4) Is there any literature on thermal characteristics of bone to hel=
p in
>evaluting STC performance of gauges, thermal output, heat dissipatio=
n, power
>requirements, etc.?
>
>Any info will be greatly appreciated, and of course, a summary of re=
plies
>will be posted. Thanx in advance.
> =20

To date, I have received 3 responses, apparently indicating low inter=
est in
strain gauging, so I will forego a complete summary. I have compiled=
more
complete information and sent the info only to those who showed inter=
est, in
order to save bandwidth. To supplement the information obtained from
responses to my posting, we have done some additional research and va=
lidation
work ourselves. The answers to my questions are included as well as =
the most
helpful literature. If anyone has or wants any other info, or would =
like to
comment on the subject, please contact me.

Q:
>1) what's the best and most economical coating to use on wet bone in=
a
saline
>sprayed (not submerged) environment?...

A:
We had some difficulty with installations that were protected with M-=
Coat A
(polyurethane) and bonded in the conventional manner used by Finlay e=
t al
[3]. In our experience, M-Coat A (gauges bonded with methyl cyanoacry=
late)
provided about 24 hours of reliable output. We ran some experiments =
using
M-Coat D topped with M-Coat C and the installation technique of Butte=
rmann
[1]. Gauge zero stability is affected by the curing processes of the
adhesive and both coatings for nearly 24 hours and thus should not be=
trusted
until all materials have cured. Strain gauge output was consistent f=
or 2
days after which severe drift occurred. As recommended by Measuremen=
ts Group
for a saline spray environment, 3140 RTV should be used for "high hum=
idity,
water splash" environment. Using the cleaning and coating methods of
Buttermann et al. combined with M-Coat C has provided us with useful =
output
lasting for 2 days and will be sufficient for our testing needs. Com=
bining
these techniques with a better adhesive will improve the quality of s=
train
readings.

Q:
>2) Are freeze/thaw cycles harmful to cyanoacrylate adhesives such as=
M-Bond
>200?
A: Studies in our lab show that strain gauge debonding is exacerbate=
d by
freeze/thaw cycles. Expansion of water when frozen is likely to furt=
her
damage the already weakening bond between cyanoacrylate and bone. A =
word
about Measurements Group(MG) and cyanoacrylates(CA): MG doesn't alwa=
ys
supply the exact same CA when M-Bond 200 is ordered. Some of our tro=
uble was
caused by using methyl CA that was inferior for bone, but, to MG's cr=
edit,
superior for metals.=20


Q:
>3) Exactly how long is cyanoacrylate effective when wetted with sali=
ne?
A:
Technical Data Sheets from Loctite show that, after 20 hours salt spr=
ay
exposure, bond strength decreases by 15% and 20% for ethyl and methyl
cyanoacrylates, respectively. After 40 hours, the decrease is 20% fo=
r ethyl
and 28% for methyl. Isobutyl or n-butyl cyanoacrylates (Ethicon IBC-=
2 or 3M
Vetbond, respectively) have been recommended for better moisture and =
salt
resistance. Mfg's of cyanoacrylates are 3M Animal Care Division, Loc=
tite,
Henkel, and Pacer Technologies.

Q:
>4) Is there any literature on thermal characteristics of bone to hel=
p in
>evaluting STC performance of gauges, thermal output, heat dissipatio=
n, power
>requirements, etc.?
A:
STC numbers for dry bone have been estimated by Cochran [4] to be 9 p=
pm/=A1F
for dry cortical in the long axis and 15 ppm/=A1F for the transverse =
axis.=20
recommended STC numbers are thus 9,13, and 15. No other published da=
ta has
been found. We have experimentally determined the heat-sink capabili=
ty (a
measure of thermal conductivity) of both wet and dry bone to be appro=
ximately
0.6 W/in^2 (0.96 kW/m^2) for the in vitro situation. Power densities=
above
this level will cause rapid gauge self-heating. In vivo testing may
accomodate higher power densities due to heat transfer through tissue=
s
contacting the top of the gauge surface. Additional considerations
concerning stacked rosettes are discused in MG's catalogs and Tech No=
tes.

References Cited:
[1] Buttermann, G.R., Kahmann, R.D., Lewis, J.L., Bradford, D.S. (199=
1). "An
Experimental Method for Meauring Force on the Spinal Facet Joint: Des=
cription
and Application of the Method." Journal of Biomechanical Engineering=
, 113
(4): 375-386.

[2] Cochran, G.V.B. (1972). =D2Implantation of Strain Gages on Bone =
In Vivo
(Technical Note).=D3 Journal of Biomechanics, 5: 119-123.

[3] Finlay, J.B., Bourne, R.B., and McLean, J. (1982). =D2A Techniqu=
e for the
In Vitro Measurement of Principal Strains in the Human Tibia.=D3 Jou=
rnal of
Biomechanics, 15: 10: 723-739.

Other References:

[4] Caler, W.E., Carter, D.R., and Harris, W.H. (1981). =D2Technique=
s for
Implementing an In Vivo Bone Strain Gage System (Technical Note).=
=D3 Jounal of
Biomechanics, 14: 7: 503-507.

Also consult Measurements Group Tech Notes and Technical Data Sheets
available from individual product manufacturers. You can call toll-f=
ree
directory assistance 800-555-1212 for toll-free numbers for all above
manufacturers.

Thanks to Dr. J.B Finlay, John Bertram, Young Hui Chang, and Barry Sp=
igel for
their help.

Tom


Thomas G. Loebig, MSME Research Associate
Allegheny-Singer Research Institute tom@biomechanics.asri.edu
320 E. North Avenue,10th Floor ST voice: (412)359-6773
Pittsburgh, PA 15212-4772 fax: (412)359-3494