Page 474 - IJB-10-5

International Journal of Bioprinting 3D-printed post-otoplasty ear retainer

loading-unloading method. Test samples were initially and sharp edges. The final assembled model, including
stretched from no stress to 20% strain and then unloaded the retainer and positioning cover, was completed to
back to no stress, the cycle of which was repeated five generate a geometric solid STP (Standard for the Exchange
times. This procedure was then applied to 40% strain of Product Dat) STP model of the retainer. The STP
levels, repeated for five cycles, and subsequently extended file was subsequently imported into HyperMesh 14.0
to 60% and 100% strain levels. A 60% strain condition is software (Altair Company, USA) for mesh partitioning,
acceptable for the part and was selected for stabilization. and subsequently exported as a BDF (Boundary Data
For each experiment, the fifth loading up to 60% strain was Format) file for finite element pre-processing using MSC.
isolated from the broader data set, reset to zero stress and Patran2019 software (MSC Software Corporation, USA)
strain, and then utilized as a set of experiments to establish and finite element post-processing using MSC Nastran
a material model. 2019 software (MSC Software Corporation, USA).
The volumetric compression test was performed using The skull, 3D-printed shell, and internal cartilage of the
cylindrical samples with a height of 2 mm and a diameter ear are treated as isotropic, homogeneous, and continuously
of 5 mm; a compression rate of 10 mm/min was used with linear elastic materials. The facial skin, muscle, and soft
indenters to compress the specimens. In their undeformed tissue layers, as well as the soft retainer, are modeled as
state, the nominal stress of the samples was calculated by hyperelastic materials capable of large deformations and
dividing the applied force by the cross-sectional area. non-linear behavior. 18–20 The properties of the retainer are
determined through experimental measurements involving
To evaluate the decrease of stress over time, a stress-
relaxation test was performed. The stress relaxation planar tension, equibiaxial extension, and uniaxial tensile
tests. Table 1 summarizes the properties used in FEA.
properties of a material cause the force acting on it to
gradually weaken during use. The retention rate of stress The boundary conditions for the finite element model
directly reflects the material’s corrective efficiency, making of the ear mold retainer are assumed as follows: a fixed
stress relaxation tests crucial. Stress relaxation tests were constraint is applied to the nodes on the lower surface of
conducted using the ELF 3100 equipment at two strains in the model base, restricting movement in all six degrees of
a 37°C water bath: 5% and 20% strains. Each test involved freedom; a pressure of 14 N was applied vertically to the
a three-part process: initially stretching the specimen by outer surface of the hard shell using multi-point constraint
1.25 mm over 15 s (equivalent to a tensile velocity of 5 mm/ (MPC) technology to simulate the pressure exerted on
min and a total strain of approximately 5%); 5 s system the ear structure when wearing the ear mold retainer.
stabilization and 3 h stretching at 5% strain for observing This setup allows for further observation of the impact
stress relaxation. For each material, three replicates were of wearing the ear mold retainer on the biomechanical
tested in both conditions. Stress values were recorded properties of the human ear. The constrained boundaries
every 0.02 s. To reduce variations within the samples, are illustrated in Figure S1, Supporting Information.
stress relaxation curves were plotted using the percentile
of residual stress. 2.6. Clinical assessments
We conducted a clinical validation to investigate the
2.5. Finite element analysis feasibility of long-term retainer wear in 20 healthy patients.
The structural characteristics were determined by The patients were required to wear the retainers for 8 h
mechanical testing of the BioMed Flex 80A Resin, and per day for a week, and their satisfaction with the retainer
these properties were then applied to the structure. Briefly, was evaluated using the Chinese version of the Quebec
the model was refined to remove any inconsistencies or Auxiliary Technology User Satisfaction Assessment
features that STP (Standard for the Exchange of Product Scale (C-QUEST 2.0). 21,22 C-QUEST 2.0 encompasses
Dat) could lead to computational errors, such as small fillets 12 subscales rated on a scale from 1 to 5, where higher

Table 1. Material properties used in finite element analysis.

Tissue/structure Model Elastic modulus (MPa) Poisson’s ratio
Auricular cartilage Liner elastic 9 0.32
3D-printed positioning cover Liner elastic 3500 0.30
Soft-gel retainer Non-liner hyperelastic Based on the mechanical tests
Hyperelastic (Mooney-Rivlin: C10 =
Facial skin, muscle, and soft tissue 0.7 0.45
0.00165; C01 = 0.00335; D1 = 3.653)

Volume 10 Issue 5 (2024) 466 doi: 10.36922/ijb.3986