Page 488 - IJB-10-5

International Journal of Bioprinting Biocompatible 3D-printed radiotherapy spacer

aid in accurate design of the initial 3D-printed sample. The of 5 mm to ensure that it expanded to more than 10 mm
length expansion ratio was calculated using Equation I: after foaming (Figure 3a). Additionally, to prevent the
spacer from shifting within the body over time, it was
Length expansionratio (%) = designed to wrap around the prostate using CAD (Creo

L after ( mm) − L before ( mm) ×1100 (I) 4.0, PTC Inc., USA).
L before ( mm) 2.6. Sterilization
Surgical medical devices are commonly sterilized using
where L after is the length after foaming and L before is the steam, ethylene oxide (EtO), or gamma radiation.
initial length of the 3D-printed sample before foaming. However, these methods could negatively impact the
Considering the length expansion ratios along the x-, y-, properties of the original materials. For spacers, EtO gas is
and z-axes after foaming (Figure 2b), the length expansion typically used for sterilization, but the use of scCO allows
2
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ratio was set to be the same along the x- and y-axes because sterilization without affecting the material properties.
of the symmetric printing path (Figure 2a). Therefore, To achieve a supercritical state for CO , a pressure above
2
using these two length expansion ratios, the initial sample 7.4 MPa and a temperature above 31°C are required. For
was designed to achieve the final size and shape necessary sterilization, a 6-log reduction in the microbial count can
for insertion between the prostate and rectum. be achieved by exposing bacteria to scCO for 1 min and
2
viruses for 15 min. Conventionally, before performing
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2.5. Spacer pre-foam design surgery to insert a spacer for RT, a total of 10.5 h is required
The length expansion ratio owing to foaming was measured for sterilization—2.5 h for EtO treatment and 8 h for air
along the x-, y-, and z-axes. Using these measurements, bleeding. 29–31 . However, in this study, we reduced the
the final desired shape was reverse-engineered to create a sterilization time from 10.5 h to 15 min using scCO . This
2
3D model for pre-foam 3D printing. Although the design method allows for simultaneous sterilization of the spacer
should consider the shapes of the prostate, rectum, and and gas absorption using a batch process. By doing so, the
surrounding organs of patients undergoing RT, the spacer two separate processes of using EtO for 10.5 h sterilization
was designed to match the shapes of the prostate and rectal and scCO for foaming to create the spacer can be combined
2
wall of a phantom (053S, CIRS, USA), which was used into a single 15-min process. This optimizes both the
for RT planning to verify the function of the spacer. To time and number of steps required to produce the spacer
minimize radiation exposure to the rectum, the distance while simultaneously achieving sterilization and foaming.
between the prostate and the rectum must be maintained Therefore, it is anticipated that the shapes of the patient’s
at a minimum of 7.5 mm. Therefore, the initial pre-foam prostate and rectum obtained from computed tomography
sample (Figure 3b and c) was designed to have a thickness (CT) results can be immediately captured, allowing for the

Figure 2. This figure demonstrates the expansion characteristics of the spacer, which are critical for achieving the desired final dimensions post-foaming
for accurate placement between the prostate and rectum. (a) Design of the 3D-printed sample layer and printing path. (b) Measured length expansion ratio
with respect to the axis. Abbreviations: PCL, polycaprolactone.

Volume 10 Issue 5 (2024) 480 doi: 10.36922/ijb.4252