Research Program
Our research interests are in the areas of biomaterials, implant devices, drug delivery, and musculoskeletal tissue engineering. In particular, we are interested in developing bio-inspired biomaterials and platform technologies to engineer tissues and organs. We aim to improve understanding of tissue-like chemistry and structure approaches of implant device design and fabrication, how these lead to tissue-like properties and functions, and the extent to which they can enhance clinical outcomes. Our research methodology includes concept design and development, characterization and evaluation, in vitro and in vivo validation of novel biomaterials and implant devices. Our current program comprises the following themes: Enabling technology for musculoskeletal tissue engineering, surface nanotechnology for osseointegrated implant devices, and naturally derived novel biomaterials for cancer treatment.
- Enabling technology for musculoskeletal tissue engineering
- Surface nanotechnology for osseointegrated implant devices
- Naturally based novel biomaterials for cancer treatment
1. Enabling technology for musculoskeletal tissue engineering
Tissue engineering consists of three components: scaffolds, cells and signals. In our lab, we are interested in developing clinically-applicable platform technologies to manipulate scaffolds, cells and signals, mimic musculoskeletal tissue, and promote tissue regeneration. More specifically, our research aims (1) to integrate microfabrication (bottom-up) with scaffolding (top-down) approaches to re-vascularize engineered cortical and cancellous bones at a large scale, and (2) to achieve temporally and spatially controlled signals that regulate tissue regeneration, leading to a functional tissue regeneration with biomimetic complexity and enhanced functionality. Currently, we are interested in applying the tissue engineering principles to repair and treat musculoskeletal diseases such as bony birth defects, large load-bearing bony defects, and dental and orthopaedic infections.
One of our major endeavors is to develop bio-inspired scaffolds to recapitulate in vivo bony microenvironment. In our lab, we developed functionally graded scaffolds by enabling gradual and spatial variation in scaffold chemistry, structure, property, and signals, which could seamlessly integrate different interfacial properties and achieve multiple functions. Recently, we invented a method to regulate porosity and pore arrangement across a calcium phosphate-based scaffold. Figure 1 shows biodegradable porous bone mineral scaffolds with different pore sizes and spatial arrangements. The porous structure of a calcium phosphate scaffold is critical to facilitate bone regeneration as a template.
Figure 1 Representative digital images of biodegradable porous calcium phosphate scaffolds with different pore sizes and spatial arrangements. A is a scaffold with uniform big pores; B is a scaffold with uniform small pores; C is a scaffold with central small pores and peripheral big pores; D is a scaffold with central big pores and peripheral small pores.
In addition to porous structure, the chemistry and signals on the bone mineral scaffold are also very important for bone cells and mesenchymal stem cells. Our recent study was to re-create bony microenvironment with cell-derived extracellular matrix (ECM) and biodegradable β-tricalcium phosphate (β-TCP). More specifically, we investigated whether the ECM produced by bone marrow-derived mesenchymal stem cells (hBMSC) on a β-TCP scaffold can bind bone morphogenetic protein-2 (BMP-2) and control its release in a sustained manner. We further examined the effect of ECM and the BMP-2 released from ECM on cell behavior. The loading and release kinetics of the BMP-2 on the β-TCP/ECM were significantly slower than those on the β-TCP. Furthermore, the BMP-2-loaded β-TCP/ECM stimulated reorganization of the actin cytoskeleton (shown in Figure 2), and increased cell expression of alkaline phosphatase and calcium deposition compared to those without BMP-2 loading and the β-TCP with BMP-2 loading.
Figure 2 Representative immunofluorescent images for F-actin distribution in hBMSCs taken by a confocal laser scanning microscope after 3 days incubation. (A) Cells in the β-TCP scaffolds group; (B) Cells in the BMP-2-loaded β-TCP scaffolds; (C) Cells in the β-TCP/ECM-7 scaffolds; (D) Cells in the BMP-2-loaded β-TCP/ECM-7 scaffolds; (E) Cells in the β-TCP/ECM-14 scaffolds; and (F) Cells in the BMP-2-loaded β-TCP/ECM-7 scaffolds. (from Biomaterials 2011; 32:6119)
In another study, we evaluated bone formation efficacy aided by biodegradable calcium phosphate scaffolds in the absence of growth factors and stem cells using a rat cranial critical size bone defect (Figure 3). Newly-formed bone can be clearly observed by micro CT images and histological stains. We are engineering scaffolds, growth factor delivery, and stem cells for accelerating bone formation.
Figure 3 CaP scaffolds aided bone healing in the absence of BMP-2 and cells using a rat cranial critical size bone defect (8 mm in diameter) after one-month implantation. Images of μCT (top) show the newly-formed bone in a scaffold. Histological data (bottom) show that the newly-formed bone by H&E stain filled the gap in the bone defect.
PB = pre-existing bone
2. Surface nanotechnology for osseointegrated implant devices
Surface engineering in our lab focuses on implant surface chemistry, texture and mechanical properties to improve performance of osseointegrated implant devices. In clinics, osseointegration, which is defined as the direct bone-implant contact, is critical for initial fixation and long-term success of endosseous dental and orthopedic implants. The initial host response after implantation is similar to a common bone wound healing modified by the presence of the implant. The new bone formation in the gap between the implant surface and host bone consists of three categories: osteogenesis at the implant surface (contact osteogenesis), within the surgical microgap at sites of neovasculization, and the surgical host bone margin (distance osteogenesis). As such, surface features that may influence any or all of these rates of bone formation will have the potential to enhance osseointegration. In our lab, the implant surface chemistry, texture and mechanical properties have been modified via plasma spraying, sputtering, ion implantation and chemical treatment. The chemistry includes titanium, hydroxyapatite, zirconia, titania, and biomolecule and growth factor. The enhancement of osseointegration has been evidenced by in vitro cell culture and in vivo animal study. Some examples in the implant surface modification, characterization, and biological evaluation are presented below.
Figure 4Plasma sprayed hydroxyapatite and titanium coated cobalt hip implant devices. (a) Digital photos. The left shows a cobalt metallic hip stem implant. The center shows a white hydroxyapatite coating on the proximal stem. The right shows a grey titanium coating on the proximal stem. (b) Scanning electron micrographs of plasma sprayed hydroxyapatite coating. (c) Scanning electron micrographs of plasma sprayed titanium coating. The plasma sprayed coatings roughened the surface textures that facilitate and promote osseointegration.
Figure 5 Scanning electron micrographs of nanoscale implant surfaces. Using different techniques, we can fabricate different nanoscale morphologies. For example, we can use vapor based deposition to make dense surfaces with different nanosize grains. We can use solution based deposition to produce porous surfaces with different nanosize pores. Also, we can use solid based deposition to prepare nano rods with different nanosize diameters. The amazing fact that different nano patterns have different effects on cell responses may enable us to guide cells towards our desired response.
Figure 6 Biological evaluation of plasmas sprayed titanium implants in vitro and in vivo. The left panel shows in vitro osteoblast cells’ adhering, migrating and ingrowth on the plasma sprayed titanium porous coating under a scanning electron microscope. The right panel shows osseointegration of new bone–titanium interface under a fluorescent microscope. The plasma sprayed functionally porous graded titanium coated samples were placed into a dog femur. At eight weeks after implantation, the new bone was not only directly in contact with the implant, but also grew into the interconnected pores and formed mechanical interlocking, thereby enhancing the fixation of implants with bone. Yellow indicates new bone; black indicates titanium.
3. Naturally based novel biomaterials for cancer treatment
We are interested in developing naturally based novel biomaterials for improving the efficacy of cancer treatment and reducing side effects. Glioblastoma is the most common malignant tumor of the nervous system in adult humans. The survival rates of patients have not changed in the past 30 years because high grade gliomas mostly recur locally. Therefore, local therapies combined with surgical intervention are an ideal approach to improve the efficacy of treatments. These methods administer drugs directly into the brain, bypass the blood brain barrier (BBB), and deliver the drugs in a concentration-dependent manner. Some clinical studies have demonstrated that local chemotherapy delivered by polymer carrier significantly prolongs the survival time of patients. Currently we are developing a novel naturally based ellagic acid-chitosan composite that is safe and highly efficacious for an improved local chemotherapy.
Figure 7 Fluorescence images of GFP (green fluorescent protein) tagged rat C6 glioma in nude mouse right flanks on the 5th and 21st days after tumor inoculation. The chitosan-ellagic acid composite significantly inhibited the tumor growth in a mouse flank model. (A) tumor-bearing control group; (B) chitosan carrier control group; (C) chitosan-ellagic acid experimental group. The arrows indicate the tumors.
