Tissue engineering: use of scaffolds for ligament and tendon healing and regeneration

The field of Tissue Engineering has experienced exponential growth since its inception a little more than 20 years ago. In 1987, a small number of bioengineers were invited by the US National Science Foundation (yours truly was fortunate enough to be included) to discuss an emerging concept of combining biology and engineering to appropriately address the structure and function relationship of biological tissues. At that meeting, the term ‘‘tissue engineering’’ was coined by Professor Y. C. Fung of the University of California, San Diego in La Jolla, CA, USA. The first ‘‘Tissue Engineering’’ workshop was then organized and was held in the following year in Lake Tahoe, CA, USA. A number of biologists and bioengineers sat together for 4 days and the discussion centered on vascular endothelial cell technology, skin and connective tissue, implants, musculoskeletal system and orthopaedic surgery, artificial organs, the nervous system, the hematopoietic system, and mathematical modeling [10]. Since then, the field of Tissue Engineering has attracted many players. During the last 3 or 4 years, not a month went by without a major tissue engineering conference around the world. At present, this field has evolved and can be represented by a triad of cells, bioactive molecules (growth factors and cytokines), and scaffolds. The purpose of the latter two is to encourage the cells to proliferate rapidly and to synthesize proteins vigorously. The work on scaffolds, especially bioscaffolds, actually came later, but its importance as a structural support has now been well recognized, and more and more innovations are being made each day. Thus, reading the manuscript by Iwasa, Engebretsen, Shima and Ochi on ‘‘Clinical application of scaffolds for cartilage tissue engineering’’ and its positive view of scaffolds being as effective as conventional ACI treatment in the current issue of KSSTA helps us to realize the potential of these new and exciting approaches, and that is why there are loads of enthusiasm on scaffolds over the last decade. I would like to further encourage the readers to examine a recent review paper on bilayer scaffolds designed for osteochondral tissue engineering [8]. There, the current status of synthetic polymers [such as poly (lactic acid) (PLA) and poly (glycolic acid) (PGA)], and bioceramics [hydroxyl carbonate apatite (HCA)], were beautifully reviewed. In addition, three strategies for osteochondral healing and regeneration were presented, and they are (1) chondrocytes or neo-cartilage tissue (scaffold free) seeded directly onto a bone scaffold base; (2) assembled bilayered scaffolds consisting of distinct cartilage and bone scaffolds joined (or assembled) together either before or during surgical implantations, and (3) integrated bilayered scaffolds of two uniquely different segments amalgamated together via the integration of a mutual material common to both layers. In the tendon and ligament area, much interest has been given to the use of bioactive molecules including hyaluronic acid (HA), EGF, TGF-beta; and more recently, the ubiquitous platelet rich plasma (PRP) matrices for applications in orthopaedic sports medicine. PRPs have been used to enhance the healing of Achilles tendon tears [9] as well as muscle injuries [2]. The potential of these bioactive molecules has renewed the interest of using them for anterior cruciate ligament (ACL) healing and regeneration. Early works using HA, bFGF, collagen gel with platelet rich plasma (C-PRP) as well as stem cells to heal central ACL S. L-Y. Woo (&) Musculoskeletal Research Center, Department of Bioengineering, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, PA, USA e-mail: ddecenzo@pitt.edu