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November 22, 2012

Hybrid printing of mechanically and biologically improved constructs for cartilage tissue engineering applications

A new tissue printer can print cartilage.


Bioprinting is an emerging technique used to fabricate viable, 3D tissue constructs through the precise deposition of cells and hydrogels in a layer-by-layer fashion. Despite the ability to mimic the native properties of tissue, printed 3D constructs that are composed of naturally-derived biomaterials still lack structural integrity and adequate mechanical properties for use in vivo, thus limiting their development for use in load-bearing tissue engineering applications, such as cartilage. Fabrication of viable constructs using a novel multi-head deposition system provides the ability to combine synthetic polymers, which have higher mechanical strength than natural materials, with the favorable environment for cell growth provided by traditional naturally-derived hydrogels. However, the complexity and high cost associated with constructing the required robotic system hamper the widespread application of this approach. Moreover, the scaffolds fabricated by these robotic systems often lack flexibility, which further restrict their applications. To address these limitations, advanced fabrication techniques are necessary to generate complex constructs with controlled architectures and adequate mechanical properties. In this study, we describe the construction of a hybrid inkjet printing/electrospinning system that can be used to fabricate viable tissues for cartilage tissue engineering applications. Electrospinning of polycaprolactone fibers was alternated with inkjet printing of rabbit elastic chondrocytes suspended in a fibrin–collagen hydrogel in order to fabricate a five-layer tissue construct of 1 mm thickness. The chondrocytes survived within the printed hybrid construct with more than 80% viability one week after printing. In addition, the cells proliferated and maintained their basic biological properties within the printed layered constructs. Furthermore, the fabricated constructs formed cartilage-like tissues both in vitro and in vivo as evidenced by the deposition of type II collagen and glycosaminoglycans. Moreover, the printed hybrid scaffolds demonstrated enhanced mechanical properties compared to printed alginate or fibrin–collagen gels alone. This study demonstrates the feasibility of constructing a hybrid inkjet printing system using off-the-shelf components to produce cartilage constructs with improved biological and mechanical properties.






Future direction

Cartilage constructs similar to those created in this experiment could be applied clinically by using MRI images of a defect in the knee as a 'blueprint' to bioprint a matching construct. Careful selection of scaffold material for each patient's construct would allow the implant to withstand mechanical forces while encouraging new cartilage to organize and fill the defect. This technique can also be extended to other tissue types. Although electrospun scaffolds can direct chondrocytes growth [30], they can also direct the formation of myotubes to enhance strategies for muscle tissue engineering [28]. The use of hybrid system allows creation of highly aligned electrospun fibers and targeted application of myotubes. This technique can then be used to layer patterns of endothelial cells and nerve cells on the aligned myotubes to create functional muscle. By overcoming one of the major obstacles associated with conventional bioprinting, we have expanded the utility of biofabrication technology.

Conclusions

This study demonstrates the feasibility of generating layered cartilage constructs using a combination of electrospinning and inkjet printing. The hybrid scaffold demonstrated enhanced mechanical properties compared to conventional hydrogel constructs generated using inkjet printing alone. Printed cells maintained their viability and produced cartilage-specific ECM both in vitro and in vivo. Further refinement of the technique could produce functional cartilage constructs by using oriented fibers to direct chondrocyte growth. Combining this tight control of scaffold properties with the precise cell delivery of the printing process, as well as with advances in hydrogel technology, will enable production of highly functional tissue constructs. This work indicates that the hybrid electrospinning/inkjet printing technique is a promising new technology that could simplify production of complex tissues.




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