Two-dimensional C-arm robotic navigation system (i-Navi) in spine surgery: a pilot study.

PURPOSE: Pedicle screws placement is very common procedure in spinal surgery. Robotic assisted surgery has been widely used in this operation. We assessed the accuracy of thoracolumbar spine trans-pedicle screws (TPS) implantation utilizing a noval robotic navigation system (i-Navi robotic navigation system) by planning with two-dimensional (2-D) C-arm. METHODS: This study was approved by the Institutional Review Board of the Cathay General Hospital on June 21, 2018 (IRB number: CGH-P 106,092), and written informed consents were obtained from all the patients. There are 18 patients were enrolled in the study. All the patients received the posterior fusion with TPS insertion under the assistant of our robotic navigation system. RESULTS: There are 18 patients were included into our study, there are 2 patients were quitted from the study due to the equipment setup was not complete. Other 16 patients completed the entire procedure successfully. There is total 88 pedicle screws were inserted through i-Navi robotic navigation system. There are 79 of 88 screws were graded A, and 9 screws were graded B; no screws were graded C or D. No vascular or nerve injuries were noted after the operations. CONCLUSION: We present our i-Navi robotic navigation system, by planning with 2-D C-arm imaging and pre-operative CT scans. According to the results of study, we think it can provide a reliable and easy tool to perform the TPS in thoracic lumbar spine surgery.

Implementation and performance evaluation of a drilling assistive device for distal locking of intramedullary nails.

Intramedullary nailing is a common treatment for long bone fractures. The nail might deform during implantation because of the shape of medullary cavity. Thus, surgeons take many X-ray images to position distal locking holes and check the drilling process. In this study, we developed a positioning algorithm with a passive or active (robot arm) assistive device for promptly positioning of distal locking holes and stably drilling guidance and support. Using the passive device, the surgeon could manually align the positioning probe with locking hole within 60 seconds based on 20 test cases. In 36 test cases, the active device aligned the positioning probe with locking hole automatically with average errors of 2.2 mm in position and 3.19° in direction. The passive device provides a reliable and low-cost solution for distal locking of intramedullary nails, while the active device is easy and friendly to use.

C-Arm Image-Based Surgical Path Planning Method for Distal Locking of Intramedullary Nails.

Due to the curvature of the bone marrow cavity, the intramedullary nail used in long bone fracture fixation can be deformed, causing displacement of the locking holes. In this study, an algorithm using only one C-arm image to determine the center positions and axial directions of locking holes was developed for drilling guidance. Based on conventional method that the axial direction of locking hole would be identified when locking hole contour is presented as a circle, the proposed method can locate the circle contour centroid by using one C-arm image including two elliptical contours. Then the two distal locking holes' axial direction and centers would be determined. Three experiments were conducted to verify the performance of the proposed algorithm, which are (1) computer simulation, (2) use of real intramedullary nails, and (3) actual drilling test with the bone model. The experimental results showed that the average error of the axial direction and center position were 0.62 ± 0.6°, 0.73 ± 0.53 mm (simulation) and 3.16 ± 1.36°, 1.10 ± 0.50 mm (actual nail), respectively. The last ten drilling test sets were completed successfully (with an average duration of 48 seconds). Based on the experimental results, the proposed algorithm was feasible for clinic applications.

An Ultrasound Imaging-Guided Robotic HIFU Ablation Experimental System and Accuracy Evaluations.

In recent years, noninvasive thermal treatment by using high-intensity focused ultrasound (HIFU) has high potential in tumor treatment. The goal of this research is to develop an ultrasound imaging-guided robotic HIFU ablation system for tumor treatment. The system integrates the technologies of ultrasound image-assisted guidance, robotic positioning control, and HIFU treatment planning. With the assistance of ultrasound image guidance technology, the tumor size and location can be determined from ultrasound images as well as the robotic arm can be controlled to position the HIFU transducer to focus on the target tumor. After the development of the system, several experiments were conducted to measure the positioning accuracy of this system. The results show that the average positioning error is 1.01 mm with a standard deviation 0.34, and HIFU ablation accuracy is 1.32 mm with a standard deviation 0.58, which means this system is confirmed with its possibility and accuracy.

Using Free Navigation Reference Points and Prefabricated Bone Plates for Zygoma Fracture Model Surgeries.

Surgical navigation systems have been an important tool in maxillofacial surgery, helping surgeons create a presurgical plan, locate lesions, and provide guidance. For secondary facial bone reductions, a good presurgical plan and proper execution are the key to success. Previous studies used predetermined markers and screw holes as navigation references; however, unexpected situations may occur, making the predetermined surgical plan unreliable. Instead of determining positions preoperatively, this study proposes a method that surgeons can use intraoperatively to choose surface markers in a more flexible manner. Eight zygomatic fractures were created in four skull models, and preoperative computed tomography (CT) image data were imported into a self-developed navigation program for presurgical planning. This program also calculates the ideal positions of navigation references points for screw holes. During reduction surgery, markers on fractured bone are selected, registered, and calculated as free navigation reference points (FNRPs). The surface markers and FNRPs are used to monitor the position of the dislocated bone. Titanium bone plates were prefabricated on stereolithography models for osteosynthesis. Two reductions with only FNRPs, as well as six reductions with FNRPs and prefabricated bone plates, were successfully performed. Postoperative CT data were obtained, and surgical errors in the six-reduction group were evaluated. The average deviation from the screw hole drilling positions was 0.92 ± 0.38 mm. The average deviation included displacement and rotation of the zygomas. The mean displacement was 0.83 ± 0.38 mm, and the average rotations around the x, y, and z axes were 0.66 ± 0.59°, 0.77 ± 0.54°, and 0.79 ± 0.42°, respectively. The results show that combining presurgical planning and the developed navigation program to generate FNRPs for assisting in secondary zygoma reduction is an accurate and practical method. Further study is necessary to prove its clinical value.

Water-based polyurethane 3D printed scaffolds with controlled release function for customized cartilage tissue engineering.

Conventional 3D printing may not readily incorporate bioactive ingredients for controlled release because the process often involves the use of heat, organic solvent, or crosslinkers that reduce the bioactivity of the ingredients. Water-based 3D printing materials with controlled bioactivity for customized cartilage tissue engineering is developed in this study. The printing ink contains the water dispersion of synthetic biodegradable polyurethane (PU) elastic nanoparticles, hyaluronan, and bioactive ingredients TGFß3 or a small molecule drug Y27632 to replace TGFß3. Compliant scaffolds are printed from the ink at low temperature. These scaffolds promote the self-aggregation of mesenchymal stem cells (MSCs) and, with timely release of the bioactive ingredients, induce the chondrogenic differentiation of MSCs and produce matrix for cartilage repair. Moreover, the growth factor-free controlled release design may prevent cartilage hypertrophy. Rabbit knee implantation supports the potential of the novel 3D printing scaffolds in cartilage regeneration. We consider that the 3D printing composite scaffolds with controlled release bioactivity may have potential in customized tissue engineering.

Preparation and characterization of a biodegradable polyurethane hydrogel and the hybrid gel with soy protein for 3D cell-laden bioprinting.

3D printing shows great potential for fabricating customized scaffolds for tissue regeneration. Using hydrogel as a bioink for cell printing provides a biological platform for basic research and potential medical treatments. In this study, a waterborne poly(ε-caprolactone) (PCL)-based biodegradable polyurethane (PU) with a soft segment replaced with 20 mol% of poly(l-lactide) (PLLA) diol or poly (d,l-lactide) (PDLLA) diol was prepared. These two PUs formed compact packing structures at temperatures ≥37 °C. They responded differently to temperature changes and the presence of electrolytes because of the difference in the free volume. With their thermal-responsive properties, both PU dispersions could form a gel in 3 min with the gel modulus reaching about 6-8 kPa after 30 min. To enhance the structural integrity during layer-by-layer deposition, the hybrid hydrogel of PU and soy protein isolate (PU/SPI hybrid) was further developed. The PU/SPI hybrid dispersion could undergo rapid gelation at 37 °C with the modulus reaching 130 Pa in 1 min. Moreover, the PU/SPI hybrid gel was readily blended with cells and printed at 37 °C without preheating. Neural stem cells (NSCs) were embedded in the hydrogels and analyzed for cell viability, metabolism, proliferation, and gene expression of neural-related markers. Cells cultured in the PU/SPI hybrid construct had better survival and proliferation than those in the PU gel. The PU/SPI hybrid ink may provide unique rheological properties for direct cell/tissue printing at 37 °C and a biomimetic microenvironment for cell survival, growth, and differentiation.

Placement-induced effects on high tibial osteotomized construct - biomechanical tests and finite-element analyses.

BACKGROUND: High tibial osteotomy (HTO) with a medially opening wedge has been used to treat osteoarthritic knees. However, the osteotomized tibia becomes a highly unstable structure and necessitates the use of plate and screws to stabilize the medial opening and enhance bone healing. A T-shaped plate (e.g. TomoFix) with locking screws has been extensively used as a stabilizer of the HTO wedge. From the biomechanical viewpoint, however, the different plate sites and support bases of the HTO plate should affect the load-transferring path and wedge-stabilizing ability of the HTO construct. This study uses biomechanical tests and finite-element analyses to evaluate the placement- and base-induced effects of the HTO plates on construct performance. METHODS: Test-grade synthetic tibiae are chosen as the standard specimens of the static tests. A medial wedge is created for each specimen and stabilized by three plate variations: hybrid use of T- and I-shaped plates (TIP), anteriorly placed TomoFix (APT), and medially placed TomoFix (MPT). There are five tests for each variation. The failure loads of the three constructs are measured and used as the load references of the fatigue finite-element analysis. The residual life after two hundred thousand cycles is predicted for all variations. RESULTS: The testing results show no occurrence of implant back-out and breakage under all variations. However, the wedge fracture consistently occurs at the opening tip for the APT and MPT and the medially resected plateau for the TIP, respectively. The testing results reveal that both failure load and wedge stiffness of the TIP are the highest, followed by the MPT, while those of the APT are the least (P < 0.05). The fatigue analyses predict comparable values of residual life for the TIP and MPT and the highest value of damage accumulation for the APT. Both experimental and numerical tests show the biomechanical disadvantage of the APT than their counterparts. However, the TIP construct without locking screws shows the highest stress at the plate-screw interfaces. CONCLUSIONS: This study demonstrates the significant effect of placement site and support base on the construct behaviors. The TIP provides a wider base for supporting the HTO wedge even without the use of locking screws, thus significantly enhancing construct stiffness and suppressing wedge fracture. Compared to the APT, the MPT shows performance more comparable to that of the TIP. If a single plate and a smaller incision are considered, the MPT is recommended as the better alternative for stabilizing the medial HTO wedge.

Registration of 2D C-Arm and 3D CT Images for a C-Arm Image-Assisted Navigation System for Spinal Surgery.

C-Arm image-assisted surgical navigation system has been broadly applied to spinal surgery. However, accurate path planning on the C-Arm AP-view image is difficult. This research studies 2D-3D image registration methods to obtain the optimum transformation matrix between C-Arm and CT image frames. Through the transformation matrix, the surgical path planned on preoperative CT images can be transformed and displayed on the C-Arm images for surgical guidance. The positions of surgical instruments will also be displayed on both CT and C-Arm in the real time. Five similarity measure methods of 2D-3D image registration including Normalized Cross-Correlation, Gradient Correlation, Pattern Intensity, Gradient Difference Correlation, and Mutual Information combined with three optimization methods including Powell's method, Downhill simplex algorithm, and genetic algorithm are applied to evaluate their performance in converge range, efficiency, and accuracy. Experimental results show that the combination of Normalized Cross-Correlation measure method with Downhill simplex algorithm obtains maximum correlation and similarity in C-Arm and Digital Reconstructed Radiograph (DRR) images. Spine saw bones are used in the experiment to evaluate 2D-3D image registration accuracy. The average error in displacement is 0.22 mm. The success rate is approximately 90% and average registration time takes 16 seconds.

Biomechanical effects of plate area and locking screw on medial open tibial osteotomy.

Medial open high tibial osteotomy (HTO) has been used to treat osteoarthritis of the medial compartment of the knee. However, weaker plate strength, unstable plate/screw junction and improper surgery technique are highly related to the HTO outcomes. Two π-shape plates were designed and eight variations (two supporting area × four locking stiffness) were compared by finite-element method. The computed tomography-based tibia was reconstructed and both wedge micromotion and implant stresses were chosen as the comparison indices. The construct was subjected to surgical and physiological loads. The medial-posterior region is the most loaded region and the load through the posterior leg is about four times that through the anterior leg. This indicates that the two-leg design can form a force-couple mechanism to effectively reduce the implant stresses. The use of locking screws significantly decrease the screw and hole stresses. However, the extending plate reduces the stresses of screws and holes above the wedge but makes the distal screws and holes much stressed. Wedge micromotion is affected by extending plate rather than locking screw. Three factors contribute to effective stabilisation of unstable HTO wedge: (1) intimate tibia-plate contact at medial-posterior regions, (2) sufficient rigidity at plate-screw junctions and (3) effective moment-balancing design at distal tibia-plate interfaces.

Effects of stemmed and nonstemmed hip replacement on stress distribution of proximal femur and implant.

BACKGROUND: Despite improvements in shape, material, and coating for hip stem, both stress shielding and aseptic loosening have been the major drawbacks of stemmed hip arthroplasty. Some nonstemmed systems were developed to avoid rasping off the intramedullary canal and evacuating the bone marrow due to stem insertion. METHODS: In this study, the finite-element models of one intact, one stemmed, and two nonstemmed femora with minimal removal of the healthy neck were investigated to evaluate their biomechanical effects. The resurfacing (ball-shaped) and fitting (neck-shaped) systems were respectively selected as the representative of the ready- and custom-made nonstemmed implants. The stress distribution and interface micromotion were selected as the comparison indices. RESULTS: The results showed that stress distributions of the two nonstemmed femora are consistently more similar to the intact femur than the stemmed one. Around the proximal femur, the stem definitely induces the stress-shielding phenomenon of its counterparts. The fitting system with the anatomy-shaped cup can make intimate contact with the neck cortex and reduce the bone-cup micromotion and the implant stress. Comparatively, the reamed femoral head provides weaker support to the resurfacing cup causing higher interfacial micromotion. CONCLUSIONS: The reserved femoral neck could act as the load-transferring medium from the acetabular cup, femoral neck, to the diaphysial bone, thus depressing the stress-shielding effect below the neck region. If the hip-cup construct can be definitely stabilized, the nonstemmed design could be an alternative of hip arthroplasty for the younger or the specific patients with the disease limited only to the femoral head.

Synthesis and 3D printing of biodegradable polyurethane elastomer by a water-based process for cartilage tissue engineering applications.

Biodegradable materials that can undergo degradation in vivo are commonly employed to manufacture tissue engineering scaffolds, by techniques including the customized 3D printing. Traditional 3D printing methods involve the use of heat, toxic organic solvents, or toxic photoinitiators for fabrication of synthetic scaffolds. So far, there is no investigation on water-based 3D printing for synthetic materials. In this study, the water dispersion of elastic and biodegradable polyurethane (PU) nanoparticles is synthesized, which is further employed to fabricate scaffolds by 3D printing using polyethylene oxide (PEO) as a viscosity enhancer. The surface morphology, degradation rate, and mechanical properties of the water-based 3D-printed PU scaffolds are evaluated and compared with those of polylactic-co-glycolic acid (PLGA) scaffolds made from the solution in organic solvent. These scaffolds are seeded with chondrocytes for evaluation of their potential as cartilage scaffolds. Chondrocytes in 3D-printed PU scaffolds have excellent seeding efficiency, proliferation, and matrix production. Since PU is a category of versatile materials, the aqueous 3D printing process developed in this study is a platform technology that can be used to fabricate devices for biomedical applications.

Biomechanical effects of bone-implant fitness and screw breakage on the stability and stress performance of the nonstemmed hip system.

BACKGROUND: Some nonstemmed hip systems have been developed to avoid stress shielding and aseptic loosening, which are major drawbacks of stemmed hip arthroplasty. Without the stem, the cup over the femoral head can be stabilized by anatomic fitness of the cup interior and mechanical fixation of the auxiliary screws. METHODS: Using finite-element method, neck-shaped systems with two bone-cup fitness situations and four types of screw breakages are systematically investigated to evaluate their biomechanical effects on construct performances. The construct stresses and interfacial micromotion were chosen for comparison between two bone-cup fitness situations and four types of screw breakages. FINDINGS: The screw breakage deteriorates the stresses of the mating screw and the neck cup and loosens the bone-cup interfaces. The breakages of central and locking screws decrease the bone stress by about 43.2% and 12.7%, respectively. This indicates that the central screw is a more effective load-bearer for the superimposed cup than the locking screw. As compared with the fitting cup, the stress of cup and the bone stresses of the unfitting cup obviously increase. This demonstrates that the load-transferring path at the cup bottom is important in directly relieving the prosthetic stresses. INTERPRETATION: Any screw design inducing stress concentration should be validated to avoid screw breakage. Comparatively, surgical unfitness has a more significant effect on the construct performance than does the screw breakage. Even for custom-made cups, cautious preparation of the neck resection is still necessary to ensure intimate bone-cup contact.

The substrate-dependent regeneration capacity of mesenchymal stem cell spheroids derived on various biomaterial surfaces.

Mesenchymal stem cells (MSCs) are widely used for their self-renewal and multipotent abilities, which can be further enhanced by growing MSCs as three-dimensional (3D) cellular spheroids on certain substrates. Although various surfaces have been used to generate 3D MSC spheroids, the answer to whether all these spheroids have similar in vitro and in vivo properties remains unclear. In this study, adipose-derived adult stem cells (ADSCs) were cultured on a non-adherent Petri dish, polyvinyl alcohol, chitosan (CS), or chitosan-hyaluronan (CS-HA) to form 3D spheroids. The expression of the cell adhesion molecule, N-cadherin, was analyzed by qRT-PCR and Western blotting. The functional migration ability was tested using the transwell assay. The capacity for chondral regeneration of various ADSC spheroids was further evaluated in a rabbit model. We demonstrated that ADSC spheroids derived on the CS or CS-HA surface had the greater expression of N-cadherin and better migration ability. The latter was consistent with the higher expression levels of chemokine/receptor SDF-1/CXCR4 for the spheroids derived on CS or CS-HA. Animal studies also revealed significantly better cartilage repair in defects loaded with CS- or CS-HA-derived spheroids. In particular, CS-HA-derived spheroids gave rise to the best regeneration when combined with a 3D printed scaffold. This study suggested that MSC spheroids derived on different surfaces may have distinct in vitro and in vivo properties, which appeared to be associated with the surface-bound calcium as well as the calcium-dependent N-cadherin and CXCR4 signaling. The substrate-dependent properties may eventually lead to different regeneration capacities of various MSC spheroids in vivo.

Stress and stability comparison between different systems for high tibial osteotomies.

BACKGROUND: High tibial osteotomy (HTO) with a medial opening wedge has been used to treat medial compartment osteoarthritis. However, this makes the proximal tibia a highly unstable structure and causes plate and screws to be the potentials sources for mechanical failure. Consequently, proper design and use of the fixation device are essential to the HTO especially for overweight or full weight-bearing patients. METHODS: Based on the CT-based images, a tibial finite-element model with medial opening was simulated and instrumented with one-leg and two-leg plate systems. The construct was subjected to physiological and surgical loads. Construct stresses and wedge micromotions were chosen as the comparison indices. RESULTS: The use of locking screws can stabilize the construct and decrease the implant and bone stresses. Comparatively, the two-leg design provides a wider load-sharing base to form a force-couple mechanism that effectively reduces construct stresses and wedge micromotions. However, the incision size, muscular stripping, and structural rigidity are the major concerns of using the two-leg systems. The one-leg plates behave as the fulcrum of the leverage system and make the wedge tip the zone of tension and thus have been reported to negatively affect the callus formation. CONCLUSIONS: The choice of the HTO plates involved the trade-off between surgical convenience, construct stability, and stress-shielding effect. If the stability of the medial opening is the major concern, the two-leg system is suggested for the patients with heavy load demands and greater proximal tibial size. The one-leg system with locking screws can be used for the majority of the patients without heavy bodyweight and poor bone quality.

Air plasma treated chitosan fibers-stacked scaffolds.

Chitosan is a nontoxic, biodegradable and biocompatible polymer. Rapid prototyped chitosan scaffolds were manufactured by liquid-frozen deposition of chitosan fibers in this study. To investigate if the air plasma (AP) treatment could be used to improve the surface properties of these scaffolds for cell attachment, chitosan films were first prepared and treated with AP under different conditions. Under the optimized condition, the water contact angle of chitosan films was significantly reduced from 90 ± 1° to 19 ± 1° after AP treatment. On the other hand, the surface charge and nanometric roughness of chitosan films increased after AP treatment. X-ray photoelectron spectroscopy measurement on AP-treated three-dimensional chitosan scaffolds showed that nitrogen and oxygen increased at each location inside the scaffolds as compared to the untreated ones, which indicated that AP could permeate through the fibrous stacks of the scaffolds and effectively modify the interior (visible) surface of the scaffolds. Moreover, AP treatment enabled the migration of MC3T3-E1 cells into the scaffolds, facilitated their proliferation and promoted the bone mineral deposition. These results suggested that fibers-stacked chitosan scaffolds may be produced by liquid-frozen deposition and treated with AP for bone tissue engineering applications.

Chondrogenesis from human placenta-derived mesenchymal stem cells in three-dimensional scaffolds for cartilage tissue engineering.

Human placenta-derived mesenchymal stem cells (hPMSCs) represent a promising source of stem cells. The application of hPMSCs in cartilage tissue engineering, however, was less reported. In this study, hPMSCs were grown in a three-dimensional (3D) environment for cartilage tissue formation in vitro. To select proper scaffolds for 3D culture of mesenchymal stem cells (MSCs), rat adipose-derived MSCs were initially employed to optimize the composition and condition of the 3D environment. The suitability of a poly(D,L-lactide-co-glycolide) (PLGA) precision scaffold previously developed for seeding and culture of primary chondrocytes was tested for MSCs. It was established that MSCs had to be embedded in alginate gel before seeded in the PLGA precision scaffold for cartilage-like tissue formation. The inclusion of nano-sized calcium-deficient hydroxyapatite (nCDHA) and/or a recombinant protein containing arginine-glycine-aspartate (RGD) into the alginate gel enhanced the chondrogenesis for both rat adipose-derived MSCs and hPMSCs. The amount of extracellular matrix such as glycosaminoglycan and type II collagen accumulated during a period of 21 days was found to be the greatest for hPMSCs embedded in the alginate/nCDHA/RGD gel and injected and cultivated in the precision scaffold. Also, histological analyses revealed the lacunae formation and extracellular matrix production from the seeded hPMSCs. Comparing human bone marrow-derived MSCs (hBMSCs) and hPMSCs grown in the previous composite scaffolds, the secretion of glycosaminoglycan was twice as higher for hPMSCs as that for hBMSCs. It was concluded that the alginate/nCDHA/RGD mixed gel in the aforementioned system could provide a 3D environment for the chondrogenesis of hPMSCs, and the PLGA precision scaffold could provide the dimensional stability of the whole construct. This study also suggested that hPMSCs, when grown in a suitable scaffold, may be a good source of stem cells for building up the tissue-engineered cartilage.

Effect of recombinant galectin-1 on the growth of immortal rat chondrocyte on chitosan-coated PLGA scaffold.

The effect of galectin-1 (GAL1) on the growth of immortal rat chondrocyte (IRC) on chitosan-modified PLGA scaffold is investigated. The experimental results showed that water absorption ratio of chitosan-modified PLGA scaffold was 70% higher than that of PLGA alone after immersion in ddH(2)O for 2 weeks, indicating that chitosan-modification significantly enhances the hydrophilicity of PLGA. The experimental results also showed that GALl efficiently and spontaneously coats the chitosan-PLGA scaffold surface to promote adhesion and growth of immortal rat chondrocyte (IRC). To investigate the effect of endogenous GAL1, the full-length GAL1 cDNAs were cloned and constructed into pcDNA3.1 vectors to generate a plasmid expressed in IRC (IRC-GAL1). The results showed that IRC-GAL1 growth was significantly higher than that of IRC on chitosan-PLGA scaffold. The GAL1-potentiated IRC growth on chitosan-PLGA scaffold was dose-dependently inhibited by TDG (specific inhibitor of GAL1 binding). These results strongly suggest that GAL1 is critical for enhancing IRC cell adhesion and growth on chitosan-PLGA scaffold. Moreover, GAL1-coating or expression tends to promote IRC cell-cell aggregation on chitosan-PLGA scaffold and significantly enhances IRC migration. These results suggest that GAL1 probably could induce tissue differentiation and facilitates cartilage reconstruction. In conclusion, the experimental results suggest that both GAL1 and chitosan are important for enhancing IRC cell adhesion and growth on PLGA scaffold, and GAL1 is a potential biomaterial for tissue engineering.