First Advisor

Sung Yi

Term of Graduation

Winter 2024

Date of Publication


Document Type


Degree Name

Doctor of Philosophy (Ph.D.) in Mechanical Engineering


Mechanical and Materials Engineering




additive manufacturing, biomaterial, DMSO2, material properties, polymer rheology, scaffold



Physical Description

1 online resource (xvi, 173 pages)


Tissue engineering provides a viable solution to address the shortage of organ donors for allograft transplants. Its primary objective is the replacement or regeneration of damaged tissues which is developed by combining living cells, a scaffold, and a bioreactor. Among these components, the scaffold serves as an artificial extracellular matrix (ECM), which plays a pivotal role in supporting cell adhesion, proliferation, differentiation, and overall tissue development. Additive manufacturing (AM), commonly referred to as three-dimensional (3D) printing, emerges as a promising method for scaffold fabrication due to its capability to create complex and repeatable geometries.

Polycaprolactone (PCL) has been one of the famous biomaterials in the tissue engineering field due to its relatively low melting temperature, excellent thermal stability, and cost-effectiveness. However, certain properties of PCL, such as low cell attraction, low elastic modulus, and long degradation time, have limited its widespread use in the field of tissue engineering. Therefore, a lot of studies on PCL-based biocomposites have been researched to overcome the limitations. Dimethyl sulfone (DMSO2) is a stable and non-hazardous organosulfur compound. DMSO2 has a low viscosity and high surface tension. However, DMSO2 has not been introduced in the tissue engineering field. In this study, PCL and DMSO2 composites were developed to overcome the limitations of PCL and to tailor the properties of biocomposites. A gamma-methacryloxypropyltrimethoxysilane (A-174) was used as a binder to increase the strength of the composite by optimizing the mixing sequence of the PCL matrix, DMSO2 filler, and binder system as well as binder content. Moreover, the 3D printing behavior was examined based on the rheological properties of PCL and DMSO2 composites as well as 3D printing conditions, and a numerical model to predict 3D printed objects was developed.

PCL and DMSO2 were physically mixed with 10, 20, and 30 wt% of DMSO2 to introduce new biocomposites. A dynamic differential scanning calorimetry (DSC), STA 8000 (PerkinElmer, Waltham, Massachusetts, United States), was employed to establish melting and 3D printing temperatures, using a heating and cooling rate of 5 °C/min. Hydrophilicity was evaluated by measuring the water contact angle by the sessile drop method according to the ASTM D7334 standard. Mechanical test specimens were fabricated by mold casting according to the ASTM D790 standard, and then a three-point bending test was performed by model 5ST (Tinius Olsen, Redhill, United Kingdom) to measure the elastic modulus and yield strength. After the mechanical test, the fracture surfaces of the tested specimens were analyzed by scanning electron microscope (SEM), SNE-4500M Plus (SEC Co., Ltd, South Korea). Degradation of the composite in vivo was evaluated by a mass loss test for 9 weeks. The rheological properties of the composites were evaluated by a frequency sweep test using an MCR 702 rotational rheometer (Anton Paar, Graz, Austria) equipped with a parallel plate system featuring a 0.5 mm gap. A pneumatic material extrusion 3D printer using compressed air, Allevi 2 (Allevi, Inc., Philadelphia, PA, United States), was utilized for printing 3D scaffolds. The extrusion behavior was recorded, and the recorded videos were cut into images to calculate the extrusion velocity. The straight line with 2 mm of length was printed with 180 µm of the layer height to measure the printed strut diameter. The extrusion velocity was simulated by using Navier Strokes equations, and the strut diameter was calculated by the surface energy model of the free surface.

The DSC test during the endothermic process showed separated two peaks in the composites while pure PCL and DMSO2 showed only one peak. The melting temperature of PCL in the composites ranged from 55-57 °C, and that of DMSO2 ranged from 106-107 °C. The water contact angle of the composites for the hydrophilicity was decreased by 15.5 % compared to the pure PCL. The elastic modulus of the composites showed 532 MPa, and the degradation time of it was faster than 18 times.

On the other hand, the 0.2 % yield strength was decreased with DMSO2 concentration in the PCL matrix. When the binder and DMSO2 fillers were premixed in the PCL matrix consisting of a DMSO2 filler and an A-174 binder system, the filler surface was coated smoothly and uniformly, and less agglomeration occurred. The yield strength of the composites with the appropriate mixing sequence was 36.71 % higher than that of the specimen without a binder, which was attributed to the improved adhesion between the matrix and fillers. Because of the use of the A-174 silane binder at a concentration of 0.5 phr and the premixing of the binder and filler, the highest performance in terms of strength improvement of a PCL-20 wt% DMSO2 composite was achieved.

The composites demonstrated liquid-like behavior with a higher loss modulus than storage modulus. This behavior exhibited shear-thinning characteristics. The addition of DMSO2 into PCL matrix reduced a zero-shear viscosity of 33, 46, and 74 % compared to PCL. The materials exhibited extrusion velocities spanning from 0.0850 to 6.58 mm/s, with velocity being governed by the reciprocal of viscosity. Extrusion velocities below 0.21 mm/s led to the production of unstable printed lines. This phenomenon allowed the categorization of pore shape into three zones: irregular, normal, and no pore zones.

According to the simulation results from the developed numerical models, the extrusion velocity increased by 23.3 %/°C and 19.1 % per 100 kPa. Similarly, the strut diameter increased by 21.6 %/°C and 16.6 % per 100 kPa. Moreover, the low viscosity and high surface energy of DMSO2 allowed the extrusion velocity and strut diameter to increase. The accuracies of the developed models were confirmed by exceeding 0.95 of the determination coefficient value between measured and predicted data.

This study introduced a new biomaterial and explained its key characteristics. It also uncovered the 3D printing behavior by analyzing the rheological properties and developed numerical models to accurately predict the printed objects.


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