Minggan Li

A brief review of dispensing-based rapid prototyping techniques in tissue scaffold fabrication: role of modeling on scaffold properties prediction

Artificial scaffolds play vital roles in tissue engineering as they provide a supportive environment for cell attachment, proliferation and differentiation during tissue formation. Fabrication of tissue scaffolds is thus of fundamental importance for tissue engineering. Of the variety of scaffold fabrication techniques available, rapid prototyping (RP) methods have attracted a great deal of attention in recent years. This method can improve conventional scaffold fabrication by controlling scaffold microstructure, incorporating cells into scaffolds and regulating cell distribution. All of these contribute towards the ultimate goal of tissue engineering: functional tissues or organs. Dispensing is typically used in different RP techniques to implement the layer-by-layer fabrication process. This article reviews RP methods in tissue scaffold fabrication, with emphasis on dispensing-based techniques, and analyzes the effects of different process factors on fabrication performance, including flow rate, pore size and porosity, and mechanical cell damage that can occur in the bio-manufacturing process.

Modeling of the Flow Rate in the Dispensing-Based Process for Fabricating Tissue Scaffolds

Made from biomaterials, tissue scaffolds are three-dimensional (3D) constructs with highly interconnected pore networks for facilitating cell growth and flow transport of nutrients and metabolic waste. To fabricate the scaffolds with complex structures, dispensing-based rapid prototyping technique has been employed recently. In such a fabrication process, the flow rate of biomaterial dispensed is of importance since it directly contributes to the pore size and porosity of the scaffold fabricated. However, the modeling of the flow rate has proven to be a challenging task due to its complexity. This paper presents the development of a model for the flow rate in the scaffold fabrication process based on the fundamentals of fluid mechanics. To verify the effectiveness of the developed model, experiments were carried out, in which the chitosan solution (2% w/v) in acetic acid was used for dispensing under different applied pressures (50 kPa, 100 kPa, 150 kPa, 200 kPa, and 250 kPa) and needle heater temperatures (25°C, 35°C, 50°C, and 65°C). The measured flow rates were used to identify the flow behavior of the solution and were compared to the predictions from the developed model to illustrate the model effectiveness. Based on the developed model, simulations were carried out to identify the effects of the needle size and the flow behavior on the flow rate in the scaffold fabrication process. The developed model was also applied to determine the dispensing conditions for fabricating 3D scaffolds from a 50% chitosan-hydroxyapatite colloidal gel. As an example, a scaffold fabricated with a well-controlled internal structure of diameters of 610±43 μm and pore sizes of 850±75 μm in the horizontal plane and of 430 ± 50 pm in the vertical direction is presented in this paper to illustrate the promise of the developed model as applied to the 3D scaffold fabrication.

Modeling of Flow Rate, Pore Size, and Porosity for the Dispensing-Based Tissue Scaffolds Fabrication

Dispensing technique is one of the promising solidfreeform (SFF) methods to fabricate scaffolds with controllable pore sizes and porosities. In this paper, a model to represent the dispensing-based SFF fabrication process is developed. Specifically, the mechanical properties of the scaffold material and its influence on the fabrication process are examined; the flow rate of the scaffold material dispensed and the pore size and porosity of the scaffold fabricated in the process are represented. In order to generate scaffold strands without either tensile or compressive stress, the optimal moving speed of the dispensing head is determined from the flow rate of the scaffold material dispensed. Experiments were also carried out to illustrate the effectiveness of the model developed.

Microarchitecture for a three-dimensional wrinkled surface platform.

Dr. M. Li, N. Hakimi, Dr. R. Perez, Prof. S. Waldman, Prof. D. K. Hwang Department of Chemical Engineering Ryerson University 350 Victoria Street , Toronto , Ontario M5B 2K3 , Canada E-mail: dkhwang@ryerson.ca Dr. R. Perez Department of Nanobiomedical Science Dankook University Cheonan 330–714 , South Korea Prof. S. Waldman Li Ka Shing Knowledge Institute St. Michael’s Hospital 30 Bond Street , Toronto , Ontario M5B 1W8 , Canada Prof. J. A. Kozinski Lassonde School of Engineering York University 4700 Keele Street , Toronto , Ontario M3J 1P3 , Canada

Development of novel hybrid poly(l-lactide)/chitosan scaffolds using the rapid freeze prototyping technique

Engineered scaffolds have been shown to be critical to various tissue engineering applications. This paper presents the development of a novel three-dimensional scaffold made from a mixture of chitosan microspheres (CMs) and poly(L-lactide) by means of the rapid freeze prototyping (RFP) technique. The CMs were used to encapsulate bovine serum albumin (BSA) and improve the scaffold mechanical properties. Experiments to examine the BSA release were carried out; the BSA release could be controlled by adjusting the crosslink degree of the CMs and prolonged after the CMs were embedded into the PLLA scaffolds, while the examination of the mechanical properties of the scaffolds illustrates that they depend on the ratio of CMs to PLLA in the scaffolds as well as the cryogenic temperature used in the RFP fabrication process. The chemical characteristics of the PLLA/chitosan scaffolds were evaluated by Fourier transform infrared (FTIR) spectroscopy. The morphological and pore structure of the scaffolds were also examined by scanning electron microscopy and micro-tomography. The results obtained show that the scaffolds have higher porosity and enhanced pore size distribution compared to those fabricated by the dispensing-based rapid prototyping technique. This study demonstrates that the novel scaffolds have not only enhanced porous structure and mechanical properties but also showed the potential to preserve the bioactivities of the biomolecules and to control the biomolecule distribution and release rate.

Modeling Material-Degradation-Induced Elastic Property of Tissue Engineering Scaffolds

The mechanical properties of tissue engineering scaffolds play a critical role in the success of repairing damaged tissues/organs. Determining the mechanical properties has proven to be a challenging task as these properties are not constant but depend upon time as the scaffold degrades. In this study, the modeling of the time-dependent mechanical properties of a scaffold is performed based on the concept of finite element model updating. This modeling approach contains three steps: (1) development of a finite element model for the effective mechanical properties of the scaffold, (2) parametrizing the finite element model by selecting parameters associated with the scaffold microstructure and/or material properties, which vary with scaffold degradation, and (3) identifying selected parameters as functions of time based on measurements from the tests on the scaffold mechanical properties as they degrade. To validate the developed model, scaffolds were made from the biocompatible polymer polycaprolactone (PCL) mixed with hydroxylapatite (HA) nanoparticles and their mechanical properties were examined in terms of the Young modulus. Based on the bulk degradation exhibited by the PCL/HA scaffold, the molecular weight was selected for model updating. With the identified molecular weight, the finite element model developed was effective for predicting the time-dependent mechanical properties of PCL/HA scaffolds during degradation.

Effects of laminin blended with chitosan on axon guidance on patterned substrates

Axon guidance is a crucial consideration in the design of tissue scaffolds used to promote nerve regeneration. Here we investigate the combined use of laminin (a putative axon adhesion and guidance molecule) and chitosan (a leading candidate base material for the construction of scaffolds) for promoting axon guidance in cultured adult dorsal root ganglion (DRG) neurons. Using a dispensing-based rapid prototyping (DBRP) technique, two-dimensional grid patterns were created by dispensing chitosan or laminin-blended chitosan substrate strands oriented in orthogonal directions. In vitro experiments illustrated DRG neurites on these patterns preferentially grew upon and followed the laminin-blended chitosan pathways. These results suggest that an orientation of neurite growth can be achieved in an artificially patterned substrate by creating selectively biofunctional pathways. The DBRP technique may provide improved strategies for the use of biofunctional pathways in the design of three-dimensional scaffolds for guidance of nerve repair.

Effect of needle geometry on flow rate and cell damage in the dispensing-based biofabrication process.

Biodispensing techniques have been widely applied in biofabrication processes to deliver cell suspensions and biomaterials to create cell‐seeded constructs. Under identical operating conditions, two types of dispensing needles—tapered and cylindrical—can result in different flow rates of material and different cell damage percent induced by the mechanical forces. In this work, mathematical models of both flow rate and cell damage percent in biodispensing systems using tapered and cylindrical needles, respectively, were developed, and experiments were carried out to verify the effectiveness of the developed models. Both simulations and experiments show tapered needles produce much higher flow rates under the same pressure conditions than cylindrical needles. Use of a lower pressure in a tapered needle can therefore achieve the same flow rate as that in a cylindrical needle. At equivalent flow rates, cell damage in a tapered needle is lower than that in a cylindrical one. Both Schwann cells and 3T3 fibroblasts, which have been widely used in tissue engineering, were used to validate the cell damage models. Application of the developed models to specify the influence of process parameters, including needle geometry and air pressure, on the flow rate and cell damage percent represents a significant advance for biofabrication processes. The models can be used to optimize process parameters to preserve cell viability and achieve the desired cell distribution in dispensing‐based biofabrication.

Functional polymer sheet patterning using microfluidics.

Poly(dimethylsiloxane) (PDMS)-based microfluidics provide a novel approach to advanced material synthesis. While PDMS has been successfully used in a wide range of industrial applications, due to the weak mechanical property channels generally possess low aspect ratios (AR) and thus produce microparticles with similarly low ARs. By increasing the channel width to nearly 1 cm, AR to 267, and implementing flow lithography, we were able to establish the slit-channel lithography. Not only does this allow us to synthesize sheet materials bearing multiscale features and tunable chemical anisotropy but it also allows us to fabricate functional layered sheet structures in a one-step, high-throughput fashion. We showcased the techniques potential role in various applications, such as the synthesis of planar material with micro- and nanoscale features, surface morphologies, construction of tubular and 3D layered hydrogel tissue scaffolds, and one-step formation of radio frequency identification (RFID) tags. The method introduced offers a novel route to functional sheet material synthesis and sheet system fabrication.

Characterization of the flow behavior of alginate/hydroxyapatite mixtures for tissue scaffold fabrication

Mixtures of alginate and hydroxyapatite (HA) are promising materials for biomedical applications such as the fabrication of tissue scaffolds. In this paper, the flow behavior of alginate/HA mixtures was investigated and determined to be dependent on the concentration of both alginate and HA, and temperature. The relationships were mathematically established and verified with experimental results. As applied to the tissue scaffold fabrication, the flow rate of the biomaterial solution was predicted from the established flow behavior and verified by experiments. On this basis, the moving speed of the needle was determined and used in the tissue scaffold fabrication. The results obtained show that the knowledge of the flow behavior is essential to the fabrication of tissue scaffolds with an interconnected microstructure.