Additive manufacturing (AM) or 3D printing is a digital manufacturing process and offers virtually limitless opportunities to develop structures/objects by tailoring material composition, processing conditions, and geometry technically at every point in an object. In this review, we present three different early adopted, however, widely used, polymer-based 3D printing processes; fused deposition modelling (FDM), selective laser sintering (SLS), and stereolithography (SLA) to create polymeric parts. The main aim of this review is to offer a comparative overview by correlating polymer material-process-properties for three different 3D printing techniques. Moreover, the advanced material-process requirements towards 4D printing via these print methods taking an example of magneto-active polymers is covered. Overall, this review highlights different aspects of these printing methods and serves as a guide to select a suitable print material and 3D print technique for the targeted polymeric material-based applications and also discusses the implementation practices towards 4D printing of polymer-based systems with a current state-of-the-art approach.

A novel custom-made 3D silicone printer and two-part Ecoflex silicone resins were used to 3D-print standardshaped silicone coupon and irregular-shaped meniscus structures via a heat-cured extrusion-based method. This article is segmented into three parts: (1) study on the effect of 3D printing parameters on dimensional accuracy and mechanical properties of 3D-printed silicone, (2) reliability and failure analysis of 3D-printed silicone according to ASTM D575 standards under monotonic and cyclic compressive loading, and (3) cytotoxicity of 3D-printed silicone by extraction method according to ISO 10993-12 for different extraction time and extract volume/surface area ratios. Based on analysis using regression method and analysis of variance, we found that the dimensional accuracy of lengths and widths is sensitive to both nozzle diameters and bed temperatures (BTs), while the height is only sensitive to BTs. Failure results were analyzed using the two-parameter Weibull probability distribution model and Weibull regression analysis and revealed that the Weibull modulus had a value greater than 1 in all groups, indicating an increasing failure rate with time for Ecoflex 30 and 50 meniscus implants. Results from quantitative cell proliferative assay exhibit statistically insignificant differences for all samples, pointing to the low cytotoxicity and excellent biocompatibility of printed silicone.

The first successful direct 3D printing, or additive manufacturing (AM), of heat-cured silicone meniscal implants, using biocompatible and bio-implantable silicone resins is reported. Silicone implants have conventionally been manufactured by indirect silicone casting and molding methods which are expensive and time-consuming. A novel custom-made heat-curing extrusion-based silicone 3D printer which is capable of directly 3D printing medical silicone implants is introduced. The rheological study of silicone resins and the optimization of critical process parameters are described in detail. The surface and cross-sectional morphologies of the printed silicone meniscus implant were also included. A time-lapsed simulation study of the heated silicone resin within the nozzle using computational fluid dynamics (CFD) was done and the results obtained closely resembled real time 3D printing. Solidworks one-convection model simulation, when compared to the on-off model, more closely correlated with the actual probed temperature. Finally, comparative mechanical study between 3D printed and heat-molded meniscus is conducted. The novel 3D printing process opens up the opportunities for rapid 3D printing of various customizable medical silicone implants and devices for patients and fills the current gap in the additive manufacturing industry.

Osteoarthritis of the knee with meniscal pathologies is a severe meniscal pathology suffered by the aging population worldwide. However, conventional meniscal substitutes are not 3D-printable and lack the customizability of 3D printed implants and are not mechanically robust enough for human implantation. Similarly, 3D printed hydrogel scaffolds suffer from drawbacks of being mechanically weak and as a result patients are unable to execute immediate post-surgical weight-bearing ambulation and rehabilitation. To solve this problem, we have developed a 3D silicone meniscus implant which is (1) cytocompatible, (2) resistant to cyclic loading and mechanically similar to native meniscus, and (3) directly 3D printable. The main focus of this study is to determine whether the purity, composition, structure, dimensions and mechanical properties of silicone implants are affected by the use of a custom-made in-house 3D-printer. We have used the phosphate buffer saline (PBS) absorption test, Fourier transform infrared (FTIR) spectroscopy, surface profilometry, thermo-gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) to effectively assess and compare material properties between molded and 3D printed silicone samples.

The meniscus is the most common damaged structure of the knee, accounting for almost one million cases of knee surgeries performed annually in the United States alone. A complete meniscectomy (complete meniscus removal) was the most common procedure performed in 1889 and was the standard procedure in the next 80 years. However, follow-up radiographic studies from the late 1960s to 1980s reported a high frequency of post-meniscectomy osteoarthritis of the knee.The meniscus functions to transmit load, absorb shock, stabilize the knee joint and nourish the joint. A complete integrity of the meniscus is crucial in maintaining the normal biomechanics of the knee and preventing the onset of premature or traumatic osteoarthritis. 3D Printing of silicone allows arthroscopic replacement of damaged menisci, either totally or partially, enabling the patient to return to work and sports almost instantaneously after surgery.This review summarizes the meniscal structure, biomechanical properties, meniscal lesions, the characteristics and clinical outcomes of various biodegradable synthetic and biological meniscal scaffolds.The menisci are a pair of fibrocartilaginous cushions which sits on the tibial plateau in the knee joint. They act as knee cushions which transmit body weight evenly across the knee joints, thus minimizing contact stresses between femur and tibia and damages to the articular surfaces. Meniscal injuries predisposed the knees to developing premature osteoarthritis (Figure 1). Figure 1: Anatomy of the meniscus.The meniscus is divided into 3 zones, the outermost vascular red-red zone, middle red-white zone and the innermost avascular white-white zone. Cells are spindled-shaped in the outermost redred zone while chondrocyte-like in the innermost white-white region.The meniscus obtains its limited blood supply from the perimeniscal capillary plexus within the synovial and capsular tissues of knee. These plexus, extending for one to three millimeters over the articular surfaces of menisci, are branches of the inferior and superior branches of the lateral and medial geniculate arteries.The vascular supply to meniscus is age dependent. In adult, tears which occur at the most vascularized, peripheral 3 mm of the menisci are most amenable to repair and cellular regeneration, as opposed to the generally avascular tears, greater than 5 mm from the menisci-synovial junction, which are not reparable. For both the medial and lateral menisci, the vascular penetration is about 10-30% ( Figure 2).