Functionally graded (FG) scaffolds are becoming mainstream in tissue engineering, since most human tissues and organs are too complex to be restored with a single synthetic material or monolithic implant once damaged. More than a decade ago, pioneering studies dealing with FG-scaffolds reported the applicability of rapid prototyping and additive manufacturing technologies, such as fused-deposition modeling (FDM), 3D fiber deposition, inkjet printing or selective laser sintering, for achieving the complex geometries required for these graded constructs [1–3]. Since then, most additive manufacturing technologies capable of processing biomedical polymers, ceramics, alloys and biomaterials have been applied to prototyping FG-scaffolds.
Currently, two main strategies for achieving the desired gradients coexist, one based on mono-material manufacturing of scaffolding geometries designed with gradients, the other based on multi-material combinations for producing scaffolds. Both strategies can be combined for additional versatility. As regards mono-material solutions, graded pore sizes or lattices with varying truss thicknesses can be straightforwardly achieved with CAD modeling resources and employed for mechanobiological studies [4]. The use of triply periodic minimal surfaces with thickness variations has been also reported [5–6]. More recently, different additive manufacturing technologies for geometrically graded alloys and scaffolds have been reviewed [7]. Concerning multi-material options, polymer-polymer composite scaffolds have been also reviewed [8] and the use of coatings and fillers is also a remarkable option towards FG-scaffolds [9]. Stacked, bilayered, trilayered or multi-layered combinations of materials are also an option, like trilayered combinations of CaP, cryogel and hydrogel for osteochondral repair [10]. Recent advances combining both main strategies open the path towards digital biomaterials, with improved control of composition and properties at every point of the scaffolding construct [11].
Articular injures and osteochondral defects can clearly benefit from FG-scaffolds, due to the need for dealing with the repair and regeneration of various involved tissues with very relevant differences of mechanical properties and biochemical composition. The potentials of 3D printing, bioprinting and electrospinning for achieving adequate scaffolds with gradients of properties for osteochondral repair has been previously analyzed [12]. Biomimetic bi-phasic and multi-phasic scaffolds have been also reported, as potential solutions for repairing cartilage lesions of the knee, in which soft cartilage over hard subchondral bone with a calcified cartilage interface in-between should be emulated [13]. Bilayered gene-activated osteochondral scaffolds, with a plasmid TGF-β1-activated chitosan-gelatin scaffold (chondrogenic layer) and a plasmid BMP-2-activated hydroxyapatite/chitosan-gelatin scaffold (osteogenic layer), put forward the beneficial combination of biomimetic multi-phasic scaffolds, spatially controlled gene delivery and mesenchymal stem cells [14].
Some years ago, our team described the functionalization of composite scaffolds using human mesenchymal stem cell (h-MSC) conditioned medium (CM) for improving scaffold’s response and a combination of titanium for the hard osteogenic phase and polydimethylsiloxane (PDMS) for the soft chondrogenic phase [15]. The titanium lattice was geometrically graded and obtained additively and used as insert within a mold, in which PDMS with porogens (sugar grains) was casted. After leaching, a porous PDMS network or sponge was obtained. However, porogens may lead to irregular uncontrolled structures with suboptimal mechanical properties and with eventual remaining debris from the pore generators [16], which cannot always escape from the scaffold if the network of porogens is not fully connected. Depending on the actual nature of particles acting as porogens, this may have unwanted effects for cells in culture and for scaffolds’ viability.
In consequence, to achieve PDMS components with complex design-controlled shapes, the use of design-controlled sacrificial molds may be preferred [17]. In connection to this, sacrificial templates designed using minimal surfaces have been described [18], as well as different sacrificial processes and templates from the micro-electro-mechanical systems (MEMS) and microfluidics fields, some of them involving the use of water-soluble poly(vinyl alcohol) (PVA) [19–20]. Other techniques, previously reviewed for the promotion of vascularization, including bioprinting with fugitive inks or the use of temporal inserts, may be also applicable for complex-shaped PDMS elements with porous or hollow regions [21].
In this study an innovative approach for engineering design-controlled and functionally graded scaffolds, combining hard-phase 3D printed lattices and soft-phase PDMS sponges or networks, is presented. According to our experience, sacrificial templates employed as inserts, within a mold prepared for casting, cannot be always completely washed-away after PDMS casting and polymerization. Normally a thin PDMS layer that engulfs the whole insert is formed, hence preventing washing with water or the appropriate solvent. Hence, in this approach, modified PVA lattices are integrated with the actual mold’s walls for facile dissolution in water. The actual freeform design of these molds and their printing with PVA enable the incorporation of hard inserts and the casting of PDMS. The obtained constructs (PVA molds-lattices, hard inserts and casted PDMS) after water immersion and PVA washing lead to interwoven instead of stacked multi-material scaffolds with hard and soft phases. Geometries are controlled from the design stage, which enables the use of both planar and non-planar inserts, depending on the final purpose of the scaffolds, respectively, in vitro test probes or personalized repairs. The developed procedure is illustrated through two case studies: one dealing with the creation of PDMS and PDMS-PLA constructs, as chondral and osteochondral plugs; another focused on the prototyping of personalized PDMS-PLA/resin constructs, as scaffolds for the tissue engineering or repair of the meniscus. Promising potentials for the tissue engineering of complex-shaped and large-size osteochondral and meniscal defects are discussed, and a systematic description of future research directions for enhanced results is included. Before presenting the main research results and analyzing them, next section deals with the materials and methods employed for this work.