Composition options for tissue-engineered bone

Characterizing the efficacy of calcium channel agonist-release strategies for bone tissue engineering applications

We have previously reported on the use of Bay K8644-release strategies in combination with perfusion-compression bioreactor systems for up regulating bone formation in three-dimensional PLLA scaffolds. Here we report on the analysis of Bay activity following its release from our PLLA scaffolds over the culture period imposed in our tissue engineering protocol using UV spectroscopy in combination with whole cell patch clamping techniques. Bay was released continually from scaffolds within the physiological range required for agonist activity (1-10 microM). Patch clamping allowed for the effects of Bay released from scaffolds to be monitored directly with respect to osteoblast electrophysiology. A characteristic shift in the current-voltage (I-V) relationship of L-type VOCC currents was observed in rat osteoblast sarcoma (ROS) cells patched in a solution with Bay released from scaffolds following 14 and 28 days incubation, with statistically significant differences observed in peak currents compared to non-Bay controls. An increase in the magnitude of the peak inward currents was also noted. The electrophysiological response of osteoblasts in the presence of Bay released from scaffolds demonstrates that the released Bay is stable and maintains its bioactivity following culture of up to 28 days.

A Novel Approach to Regenerating Periodontal Tissue by Grafting Autologous Cultured Periosteum

In the field of oral and maxillofacial surgery, tissue-engineering techniques have been found useful in regenerating lost tissues. Periodontal disease causes severe destruction of periodontal tissue, including the alveolar bone. In this study we attempted to regenerate canine periodontal tissue defects by grafting autologous cultured membrane derived from the periosteum. Under appropriate culture conditions, periosteal cells produce enough extracellular matrix to form sheets. Periosteum specimens were peeled from the mandibular body of adult hybrid dogs and were cultured until cells formed membrane. ALP activity was measured to determine an optimal time for grafting. The cultured periosteum (CP) was grafted and sutured on a mechanically made Class III furcation defect in the 4th mandibular premolars. After 3 months, the samples were harvested and observed radiologically and histologically. In cases of CP, the bone defects were regenerated and filled with newly formed hard tissue, whereas in the controls the defects remained. These results show that our novel treatment is effective in regenerating alveolar bone for the treatment of periodontal disease.

Chemical modification of chitosan by phosphorylation: an XPS, FT-IR and SEM study

In the present work, the surface of chitosan membranes was modified using a phosphorylation method carried out at room temperature. Phosphorylation may be of particular interest in materials for orthopaedic applications, due to the cation-exchange properties of phosphate functionalities. Phosphate groups chelate calcium ions, thus inducing the deposition of an apatite-like layer known to improve the osteoconduction of polymer-based implants. Additionally, the negatively charged phosphate functionalities, together with the positively charged amine groups from chitosan, are expected to provide chitosan with an amphoteric character, which may be useful as a combinatorial therapeutic strategy, by simultaneously allowing the immobilization of signalling molecules like growth factors. Phosphorylation was carried out at room temperature using the H3PO4/Et3PO4/P2O5/butanol method. Surface characterization was performed by XPS, ATR-FT-IR, and SEM. Cross-sections were analyzed by SEM fitted with EDS. The phosphate content increased with the reaction time, as shown by XPS and ATR-FT-IR, a P/N atomic ratio of 0.73 being obtained after 48 h of treatment. High-resolution XPS spectra regarding C1s, O1s, N1s and P2p are discussed. The introduction of a neutralization step led to a reduction of P content, which pointed out to the presence of phosphates ionically bound to protonated amines, in addition to phosphate esters. EDS analysis of cross-sections revealed a gradual P reduction up to 50% towards the inner part of the membrane.

A digital micro-mirror device-based system for the microfabrication of complex, spatially patterned tissue engineering scaffolds

Our ability to create precise, pre-designed, spatially patterned biochemical and physical microenvironments inside polymer scaffolds could provide a powerful tool in studying progenitor cell behavior and differentiation under biomimetic, three-dimensional (3D) culture conditions. We have developed a simple and fast, layer-by-layer microstereolithography system consisting of an ultra-violet light source, a digital micro-mirror masking device, and a conventional computer projector, that allows fabrication of complex internal features along with precise spatial distribution of biological factors inside a single scaffold. Photo-crosslinkable poly(ethylene glycol) diacrylates were used as the scaffold material, and murine bone marrow-derived cells were successfully encapsulated or seeded on fibronectin-functionalized scaffolds. Fluorescently-labeled polystyrene microparticles were used to show the capability of this system to create scaffolds with complex internal architectures and spatial patterns. We demonstrate that precisely controlled pore size and shapes can be easily fabricated using a simple, computer-aided process. Our results further indicate that multi-layered scaffolds with spatially distributed factors in the same layer or across different layers can be efficiently manufactured using this technique. These microfabricated scaffolds are conducive for osteogenic differentiation of marrow-derived stem cells, as indicated by efficient matrix mineralization.

Low level laser irradiation stimulates osteogenic phenotype of mesenchymal stem cells seeded on a three-dimensional biomatrix

Mesenchymal stem cells (MSCs) seeded on three-dimensional (3D) coralline (Porites lutea) biomatrices were irradiated with low-level laser irradiation (LLLI). The consequent phenotype modulation and development of MSCs towards ossified tissue was studied in this combined 3D biomatrix/LLLI system and in a control group, which was similarly grown, but was not treated by LLLI. The irradiated and non irradiated MSC were tested at 1-7, 10, 14, 21, 28 days of culturing via analysis of cellular distribution on matrices (trypan blue), calcium incorporation to newly formed tissue (alizarin red), bone nodule formation (von Kossa), fat aggregates formation (oil red O), alkaline phosphatase (ALP) activity, scanning electron microscopy (SEM) and electron dispersive spectrometry (EDS). The results obtained from the irradiated samples showed enhanced tissue formation, appearance of phosphorous peaks and calcium and phosphate incorporation to newly formed tissue. Moreover, in irradiated samples ALP activity was significantly enhanced in early stages and notably reduced in late stages of culturing. These findings of cell and tissue parameters up to 28 days of culture revealed higher ossification levels in irradiated samples compared with the control group. We suggest that both the surface properties of the 3D crystalline biomatrices and the LLLI have biostimulatory effects on the conversion of MSCs into bone-forming cells and on the induction of ex-vivo ossification.

Endothelial progenitor cell sprouting in spheroid cultures is resistant to inhibition by osteoblasts: A model for bone replacement grafts

Survival of tissue transplants generated in vitro is strongly limited by the slow process of graft vascularization in vivo. A method to enhance graft vascularization is to establish a primitive vascular plexus within the graft prior to transplantation. Endothelial cells (EC) cultured as multicellular spheroids within a collagen matrix form sprouts resembling angiogenesis in vitro. However, osteoblasts integrated into the graft suppress EC sprouting. This inhibition depends on direct cell-cell-interactions and is characteristic of mature ECs isolated from preexisting vessels. In contrast, sprouting of human blood endothelial progenitor cells is not inhibited by osteoblasts, making these cells suitable for tissue engineering of pre-vascularized bone grafts.

Regeneration of articular cartilage--evaluation of osteochondral defect repair in the rabbit using multiphasic implants

OBJECTIVE

To investigate whether two different multiphasic implants could initiate and sustain repair of osteochondral defects in rabbits. The implants address the malleable properties of cartilage while also addressing the rigid characteristics of subchondral bone.

DESIGN

The bone region of both devices consisted of D, D-L, L-polylactic acid invested with hyaluronan (HY). The cartilage region of the first device was a polyelectrolytic complex (PEC) hydrogel of HY and chitosan. In the second device the cartilage region consisted of type I collagen scaffold. Eighteen rabbits were implanted bilaterally with a device, or underwent defect creation with no implant. At 24 weeks, regenerated tissues were evaluated grossly, histologically and via immunostaining for type II collagen.

RESULTS

PEC devices induced a significantly better repair than untreated shams. Collagen devices resulted in a quality of repair close to that of the PEC group, although its mean repair score (19.0+/-4.2) did not differ significantly from that of the PEC group (20.4+/-3.7) or the shams (16.5+/-6.3). The percentage of hyaline-appearing cartilage in the repair was highest with collagen implants, while the degree of bonding of repair to the host, structural integrity of the neocartilage, and reconstitution of the subchondral bone was greatest with PEC devices. Cartilage in both device-treated sites stained positive for type II collagen and GAG.

CONCLUSIONS

Both implants are capable of maintaining hyaline-appearing tissue at 24 weeks. The physicochemical region between the cartilage and bone compartments makes these devices well suited for delivery of different growth factors or drugs in each compartment, or different doses of the same factor. It also renders these devices excellent vehicles for chondrocyte or stem cell transplantation.

Fibrin Gel-Immobilized Primary Osteoblasts in Calcium Phosphate Bone Cement: In vivo Evaluation with Regard to Application as Injectable Biological Bone Substitute

Osteogenic injectable bone substitutes may be useful for many applications. We developed a novel injectable bone substitute based on osteoblast-fibrin glue suspension and calcium phosphate bone cement (BC). Human osteoblasts were isolated from trabecular bone samples and cultured under standard conditions. Osteoblasts were suspended in fibrinogen solution (FS). BC was cured with thrombin solution. 8 x 4 mm injectable bone discs were prepared using silicon molds and a custom-made applicator device. Discs containing BC, BC/FS, or BC/FS/osteoblasts were implanted subcutaneously into athymic nude mice. After 3, 9 and 24 weeks, specimens were explanted and subjected to morphologic and biomechanical evaluation. In vitro fibrin gel-embedded osteoblasts displayed a differentiated phenotype as evidenced by alkaline phosphatase, collagen type 1 and von Kossa stains. A proportion of osteoblasts appeared morphologically intact over a 3-day in vitro period following application into the BC. BC/FS and BC/FS/osteoblast discs were sparsely infiltrated with vascularized connective tissue. There was no bone formation in implants from all groups. However, positive von Kossa staining only in BC/FS/osteoblast groups suggests engraftment of at least some of the transplanted cells. Biomechanical evaluation demonstrated initial stability of the composites. Young's modulus and maximal load did not differ significantly in the BC/FS and BC/FS/osteoblast groups. The practicability of osteoblast-containing injectable bone could be demonstrated. The dense microstructure and the suboptimal initial vascularization of the composites may explain the lack of bone formation. Modifications with regard to enhanced osteoblast survival are mandatory for a possible application as injectable osteogenic bone replacement system.

Novel PCL-based honeycomb scaffolds as drug delivery systems for rhBMP-2

This study investigated a novel drug delivery system (DDS), consisting of polycaprolactone (PCL) or polycaprolactone 20% tricalcium phosphate (PCL-TCP) biodegradable scaffolds, fibrin Tisseel sealant and recombinant bone morphogenetic protein-2 (rhBMP-2) for bone regeneration. PCL and PCL-TCP-fibrin composites displayed a loading efficiency of 70% and 43%, respectively. Fluorescence and scanning electron microscopy revealed sparse clumps of rhBMP-2 particles, non-uniformly distributed on the rods' surface of PCL-fibrin composites. In contrast, individual rhBMP-2 particles were evident and uniformly distributed on the rods' surface of the PCL-TCP-fibrin composites. PCL-fibrin composites loaded with 10 and 20 microg/ml rhBMP-2 demonstrated a triphasic release profile as quantified by an enzyme-linked immunosorbent assay (ELISA). This consisted of burst releases at 2 h, and days 7 and 16. A biphasic release profile was observed for PCL-TCP-fibrin composites loaded with 10 microg/ml rhBMP-2, consisting of burst releases at 2 h and day 14. PCL-TCP-fibrin composites loaded with 20 microg/ml rhBMP-2 showed a tri-phasic release profile, consisting of burst releases at 2 h, and days 10 and 21. We conclude that the addition of TCP caused a delay in rhBMP-2 release. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and alkaline phosphatase assay verified the stability and bioactivity of eluted rhBMP-2 at all time points.

Preservation and Regeneration of Alveolar Bone by Tissue-Engineered Implants

Bone maintenance after dental extraction has a significant impact on the success of future treatment. The purpose of this study was to regenerate bone by implanting an engineered porous scaffold seeded with bone marrow mesenchymal stem cells (BMSCs) in a socket created by extraction of the lower left central incisor in rabbits, utilizing the principles of tissue engineering. It involved preparation and characterization of three-dimensional porous hollow root form scaffolds consisting of a poly-L-lactic acid:polyglycolic acid composite (PLG, 50:50), using a solvent casting/compression molding/particulate leaching technique. Porosity of the scaffolds was 83.71% with good interconnectivity and uniform distribution of the various pore sizes. The degraded scaffolds maintained their porosity and form for the first 2 weeks and their mass loss continued up to 6 weeks. The scaffolds developed viscoelastic behavior under dynamic compression; yet they lost their mechanical characteristics as they degraded. The scaffolds were seeded with BMSCs and examined by scanning electron microscopy. Cell proliferation and scaffold degradation were shown up to 2 weeks in vitro. The cultivated scaffolds were implanted in empty extraction sockets immediately after tooth removal. Four weeks later, bone regeneration was evaluated histologically in the healed sockets in three experimental groups: sockets left empty, sockets that received PLG without cells, and sockets that received PLG with cells. Radiographic evaluation, performed 4 weeks later for the three experimental groups, demonstrated preservation of alveolar bone walls in the extraction sockets that received PLG with cells as compared with the other two groups. The bone density profile for the healed sockets confirmed both histological and radiographic findings. The results of this study show promise in the area of dentoalveolar surgery, yet longitudinal studies under variable clinical situations would encourage the current application.

Tissue-engineered collagen meniscus implants: 5- to 6-year feasibility study results

PURPOSE

In this feasibility study, a 5- to 6-year clinical follow-up evaluation was conducted on 8 patients who had undergone reconstruction of 1 injured medial meniscus with a tissue-engineered collagen meniscus implant. The hypothesis was that these patients would show significant clinical improvement over their preoperative status and would have maintained their status determined at the 2-year follow-up evaluation.

TYPE OF STUDY

Prospective longitudinal feasibility study follow-up evaluation.

METHODS

Eight patients underwent arthroscopic placement of a collagen meniscus implant by a single surgeon to reconstruct and restore the irreparably damaged medial meniscus of 1 knee. All patients returned for clinical, radiographic, magnetic resonance imaging, and arthroscopic examinations an average of 5.8 years (range, 5.5-6.3 y) after collagen meniscus implant placement.

RESULTS

Lysholm scores improved significantly (P = .045) from 75 preoperatively to 88 at most recent follow-up evaluation. Average Tegner activity scores improved significantly (P = .001) from 3 to 6. Patient self-assessment improved significantly (P = .046) from 2.4 to 1.9 (1 = normal, 4 = severely abnormal). Pain scores improved from 23 to 11 (0 = no pain, 100 = worst pain). Imaging studies confirmed that the chondral surfaces of the medial compartment had not degenerated further since the placement of the implant 5.8 years earlier. Relook arthroscopy with direct measurement of the newly generated tissue revealed 69% defect filling. Histologic assessment of tissue biopsy specimens from 3 patients showed the presence of fibrocartilage with a uniform extracellular matrix.

CONCLUSIONS

The meniscus-like tissue that developed after collagen meniscus implant placement has maintained its structure and functioned without negative effects for more than 5 years. The hypothesis was affirmed that these patients were improved significantly compared with their preoperative status and unchanged compared with 2-year evaluations.

LEVEL OF EVIDENCE

Level IV.

Synthetic extracellular matrices for tissue engineering and regeneration

The need for replacement tissues or organs requires a tissue supply that cannot be satisfied by the donor supply. The tissue engineering and regeneration field is focused on the development of biological tissue and organ substitutes and may provide functional tissues to restore, maintain, or improve tissue formation. This field is already providing new therapeutic options to bypass the limitations of organ?tissue transplantation and will likely increase in medical importance in the future. This interdisciplinary field accommodates principles of life sciences and engineering and encompasses three major strategies. The first, guided tissue regeneration, relies on synthetic matrices that are conductive to host cells populating a tissue defect site and reforming the lost tissue. The second approach, inductive strategy, involves the delivery of growth factors, typically using drug delivery strategies, which are targeted to specific cell populations in the tissues surrounding the tissue defect. In the third approach, specific cell populations, typically multiplied in culture, are directly delivered to the site at which one desires to create a new tissue or organ. In all of these approaches, the knowledge acquired from developmental studies often serves as a template for the tissue engineering approach for a specific tissue or organ. This article overviews the development of synthetic extracellular matrices (ECMs) for use in tissue engineering that aim to mimic functions of the native ECM of developing and regenerating tissues. In addition to the potential therapeutic uses of these materials, they also provide model systems for basic studies that may shed light on developmental processes.

Development of multifunctional polymer-mineral composite materials for bone tissue engineering

The main goal of this article is the development of a novel approach to construct multifunctional composite scaffolds for bone tissue engineering. For this purpose, different kinds of mineral macroporous supports, water-soluble aldehyde-containing copolymers of N-vinylpyrrolidone, as well as different nonspecific and biospecific ligands governing cell adhesion and growth have been used. The composite materials were tested initially for cytotoxicity in cell culture experiments using a model cell line.

The microenvironment around total hip replacement prostheses

The metal stem of the totally replaced hip carries load and resists fatigue, but it is electrochemically corroded. Metallic atoms act as haptens, induce type 1 T-helper cells/Th1-type immune responses and enhance periprosthetic osteolysis. Stiff metal implants, which do not have the same elasticity as the surrounding bone, cause stress shielding. Cyclic loading and lack of ligamentous support lead to mechanical and ischemia reperfusion injury and particle formation from bone, polymethylmethacrylate, and porous implant surfaces, which accelerate third-body polyethylene wear. Surgical injury and micromotion induce the formation of a fibrous capsule interface. Type-B lining cells produce lubricin and surface-active phospholipids to promote solid-to-solid lubrication but may loosen the implant from bone. The pumping action of the cyclically loaded joint and synovial fluid pressure waves dissect the implant-host interface and transports polyethylene particles and pro-inflammatory mediators to the interface. Hyaluronan induces formation of a synovial lining like layer. Because of its localization close to bone, foreign body inflammation at the interface stimulates osteoclastogenesis and peri-implant bone loss. Metal-on-metal and ceramic-on-ceramic pairs might minimize third body wear, but can lead to high-impact load of the acetabulum. Diamond coating of a metal-on-polyethylene couple might solve both of these problems. The basic biomaterial solutions allow good mechanical performance and relatively long life in-service, but surface modifications (porous coating, hydroxyapatite, diamond, bioglass, and others) may facilitate performance of the implant and improve the biomaterial and body interfaces.

Evolving concepts in bone tissue engineering

The field of tissue engineering integrates the latest advances in molecular biology, biochemistry, engineering, material science, and medical transplantation. Researchers in the developing field of regenerative medicine have identified bone tissue engineering as an attractive translational target. Clinical problems requiring bone regeneration are diverse, and no single regeneration approach will likely resolve all defects. Recent advances in the field of tissue engineering have included the use of sophisticated biocompatible scaffolds, new postnatal multipotent cell populations, and the appropriate cellular stimulation. In particular, synthetic polymer scaffolds allow for fast and reproducible construction, while still retaining biocompatible characteristics. These criteria relate to the immediate goal of determining the ideal implant. The search is becoming a reality with widespread availability of biocompatible scaffolds; however, the desired parameters have not been clearly defined. Currently, most research focuses on the use of bone morphogenetic proteins (BMPs), specifically BMP-2 and BMP-7. These proteins induce osteogenic differentiation in vitro, as well as bone defect healing in vivo. Protein-scaffold interactions that enhance BMP binding are of the utmost importance, since prolonged BMP release creates the most osteogenic microenvironment. Transition into clinical studies has had only mild success and relies on large doses of BMPs for bone formation. Advances within the field of bone tissue engineering will likely overcome these challenges and lead to more clinically relevant therapies.

Biomaterial challenges and approaches to stem cell use in bone reconstructive surgery

As life expectancy increases, so does the need to treat large bone defects. New biomaterials combined with osteogenic cells are now being developed as an alternative to autogenous bone grafts. The goal is to make the stem cells adhere to the scaffold, and then grow to differentiate into functional osteogenic cells and organize into healthy bone as the scaffold degrades. Decisive improvements have been made in the fields of stem cell biology, 3-D scaffold fabrication and tissue engineering, but the ideal bone substitute that fulfils all functional and safety requirements has yet to be developed.

Bi-directional cell contact-dependent regulation of gene expression between endothelial cells and osteoblasts in a three-dimensional spheroidal coculture model

Multiple cell-cell interactions control bone morphogenesis and vascularization. We have employed a spheroidal coculture system of endothelial cells (EC) and osteoblasts (OB) to study cell contact-dependent gene regulation between these two cell types that may play a role in regulating OB differentiation and EC angiogenic properties. Coculture spheroids differentiate spontaneously to organize into a core of OB and a surface layer of endothelial cells. Individual spheroid culture of EC or OB leads to significant alterations in gene expression compared to standard monolayer culture (upregulation of Tie-2 in EC; upregulation of angiopoietin-2 in osteoblasts). More importantly, spheroidal coculture of endothelial cells and osteoblasts leads to significant changes of gene expression in both cell populations (upregulation of VEGFR-2 in EC; downregulation of VEGF, and upregulation of alkaline phosphatase in osteoblasts). These changes are dependent on cell-cell contact and are not seen in stimulation experiments with conditioned supernatants. Collectively, the data demonstrate complex bi-directional gene regulation mechanisms between EC and OB that are likely to play a critical role during OB differentiation and in controlling the properties of angiogenic EC.

Skeletal tissue engineering-from in vitro studies to large animal models

Bone is a tissue with a strong regenerative potential. New strategies for tissue engineering of bone should therefore only focus on defects with a certain size that will not heal spontaneously. In the development of tissue-engineered constructs many variables may play a role, e.g. the source of the cells used, the design and mechanical properties of the scaffold and the concentration and mode of application of growth factor(s). Models for studying new strategies for tissue engineering of bone should be based on the target tissue to be restored. However, in light of the many potential variables, which may also interact if used in combination(s), there is also a large need for relatively simple models in which variables can be tested in a limited number of animals. Moreover, in compromised bone there may be a problem with the load-bearing capacity of the remaining healthy bone. In this light, an important prerequisite for tissue-engineering constructs is that they can be tested in loaded conditions. Particularly, this latter prerequisite is very difficult to achieve. Therefore, in vitro tests for mechanical stability are very useful for evaluating the mechanical consequences of a particular reconstruction procedure prior to the animal experiment. Before a tissue-engineered construct can be introduced into a clinical trial, a final test should be available in a large animal model that is as close and relevant to a particular problematic clinical situation as possible.In the past, a series of models were developed in our laboratory that are very useful for testing tissue-engineered constructs. In this paper, we focus on the use of relatively new simple in vitro and in vivo models for hip revision surgery, segmental bone defect restoration and tumour surgery.

Bone tissue engineering: recent advances and promising therapeutic agents

Bone regeneration can be accomplished with growth factors, cells and delivery systems. This review is a summary of these components that may be used for tissue regeneration. Support for the potential therapeutic applications of transcription factors in bone tissue engineering will also be discussed.