Biomedical Engineering Reference
In-Depth Information
Fig. 5.28 a Determination of the actual thickness of the deformed skin/fat and muscle layers,
b separated in vivo creep test data of gluteal skin/fat and passive muscle tissue and compound test
data along the indenter axis in the time range of 0-270 s and c detail showing the time range of 0-5 s
5.2.4.1 Creep Experiments
Experimental Set Up: The developed magnetic resonance imaging (MRI) com-
patible loading device as described in
Sect. 5.2.2.1
(cf. Fig.
5.13
a) was modified so
that creep testing could be conducted at the buttocks in the MRI environment (cf.
Fig.
5.27
a). Loading was applied to the skin via a cylindrical shaped indenter head
(Ø35 mm) with a head corner radius of 2 mm. A constant total weight of 3.9 kg
was mounted to a guiding shaft carrying the indenter head. The shaft was sup-
ported by a bushing integrated into the test rig. Bushing and shaft were arranged
vertically and lubricated to minimize frictional effects. To record total skin surface
displacement over time, the shaft was equipped with an incremental position
sensor (WS10; ASM; Germany). Tablets (cf. Fig.
5.27
b, position [1]) with a high
fraction of sodium hydrate (L
AXOBERAL
,B
OEHRINGER
I
NGELHEIM
P
HARMA
, Ger-
many) were incorporated into the indenter head. This conferred a clearly detect-
able signal to localise the position of the indentation axis in the MRI-environment
for reproducibility in the FE-modelling process. The ground frame of the test rig
was fixed to the MRI table.
Location of the Test Point: The location of the test point was chosen according
to the procedure described in
Sect. 5.2.2.1
. In this process, the test person's hip
region was fixed using a plaster mold. The mold was structured so that the pelvis
was tilted at an angle of 45 degrees to ensure the indenter meeting the pelvic bone
(cf. Fig.
5.27
b, position [2]) surface nearly orthogonally. Creep indentation at the
buttocks was performed at constant loading using the cine-MR-imaging method to
record one MR-image every 0.675 s.
MRI Scan Data: MR-recording was triggered with the initial release of the
indenter shaft. The data was acquired using a 1.5T M
AGNETOM
E
SPREE
(S
IEMENS
M
EDICAL
S
OLUTIONS
, Erlangen/Germany) scanner. A combination of a flex multi-
channel body coil and a spine matrix coil was employed. For the scan a T1-weighted
