3D Printing Improved Testicular Prostheses: Using Lattice Infill Structure to Modify Mechanical Properties

Abstract

Patients often opt for implantation of testicular prostheses following orchidectomy for cancer or torsion. Recipients of testicular prostheses report issues regarding firmness, shape, size, and position, aspects of which relate to current limitations of silicone materials used and manufacturing methods for soft prostheses. We aim to create a 3D printable testicular prosthesis which mimics the natural shape and stiffness of a human testicle using a lattice infill structure. Porous testicular prostheses were engineered with relative densities from 0.1 to 0.9 using a repeating cubic unit cell lattice inside an anatomically accurate testicle 3D model. These models were printed using a multi-jetting process with an elastomeric material and compared with current market prostheses using shore hardness tests. Additionally, standard sized porous specimens were printed for compression testing to verify and match the stiffness to human testicle elastic modulus (E-modulus) values from literature. The resulting 3D printed testicular prosthesis of relative density between 0.3 and 0.4 successfully achieved a reduction of its bulk compressive E-modulus from 360 KPa to a human testicle at 28 Kpa. Additionally, this is the first study to quantitatively show that current commercial testicular prostheses are too firm compared to native tissue. 3D printing allows us to create metamaterials that match the properties of human tissue to create customisable patient specific prostheses. This method expands the use cases for existing biomaterials by tuning their properties and could be applied to other implants mimicking native tissues.

Keywords: 3D printing; bio-fabrication; bio-materials; implants; meta-materials; soft prostheses; testicular prosthesis.

Figure 1

Method for designing testicular prostheses; (A) material selection, (B) selection of cubic lattice unit cell and identification of relevant design parameters related to relative density length (L) and radius (R), (C) Plot showing how the radius and length of the cubic unit cell beams can be adjusted to achieve a range of relative densities, choosing a radius, or length which is too small may not be manufacturable, while choosing values too large narrows the range of achievable relative densities, (D) populate 3D model of testicle with cubic unit cell, (E) create compression test specimens with uniform cross-sectional area (25 mm²), (F) 3D print the testicles and compression specimens using a Multi-Jet process.

Figure 2

Material jetting technique and post-processing method used to 3D print the testicle prostheses.

Figure 3

(A) 3D printed testicular prostheses and cubic lattice compression samples of relative density 0.2–1.0, (B) Medical prostheses Kiwee, Torosa, and Promedon (L, large; M, medium; S, small) next to a 3D printed testicular prosthesis, (C) Compression testing of cubic lattice samples showing deformation.

Figure 4

(A) Compression test results of cubic lattice specimens showing increase in stiffness with relative density. (B) Comparison of analytical [Equation (1) and (2)] and experimental relative modulus for cubic lattice specimens. (C) Relative densities of 0.3–0.4 match with human testicle stiffness values. (D) The repeatability of the modulus values across a sample size of 3 for each relative density.

Figure 5

Shore Hardness OO scale results for 3D printed testicles and medical prostheses. Notably the Kiwee which is a silicone coated silicone gel filled prostheses shows appropriate hardness results (within ranges of relative density 0.3–0.4); however, Torosa and Promedon prostheses are relatively hard compared to natural tissue. Note that the Hardness results for Torosa small and medium sized prostheses are the same as the large size, as the hardness value is dominated by the material and not minor changes in size.