Jiankang He

Synthesis and characterization of photocrosslinkable gelatin and silk fibroin interpenetrating polymer network hydrogels.

To effectively repair or replace damaged tissues, it is necessary to design scaffolds with tunable structural and biomechanical properties that closely mimic the host tissue. In this paper, we describe a newly synthesized photocrosslinkable interpenetrating polymer network (IPN) hydrogel based on gelatin methacrylate (GelMA) and silk fibroin (SF) formed by sequential polymerization, which possesses tunable structural and biological properties. Experimental results revealed that IPNs, where both the GelMA and SF were independently crosslinked in interpenetrating networks, demonstrated a lower swelling ratio, higher compressive modulus and lower degradation rate as compared to the GelMA and semi-IPN hydrogels, where only GelMA was crosslinked. These differences were likely caused by a higher degree of overall crosslinking due to the presence of crystallized SF in the IPN hydrogels. NIH-3T3 fibroblasts readily attached to, spread and proliferated on the surface of IPN hydrogels, as demonstrated by F-actin staining and analysis of mitochondrial activity (MTT). In addition, photolithography combined with lyophilization techniques was used to fabricate three-dimensional micropatterned and porous microscaffolds from GelMA-SF IPN hydrogels, furthering their versatility for use in various microscale tissue engineering applications. Overall, this study introduces a class of photocrosslinkable, mechanically robust and tunable IPN hydrogels that could be useful for various tissue engineering and regenerative medicine applications.

Convection-driven generation of long-range material gradients.

Natural materials exhibit anisotropy with variations in soluble factors, cell distribution, and matrix properties. The ability to recreate the heterogeneity of the natural materials is a major challenge for investigating cell-material interactions and for developing biomimetic materials. Here we present a generic fluidic approach using convection and alternating flow to rapidly generate multi-centimeter gradients of biomolecules, polymers, beads and cells and cross-gradients of two species in a microchannel. Accompanying theoretical estimates and simulations of gradient growth provide design criteria over a range of material properties. A poly(ethylene-glycol) hydrogel gradient, a porous collagen gradient and a composite material with a hyaluronic acid/gelatin cross-gradient were generated with continuous variations in material properties and in their ability to regulate cellular response. This simple yet generic fluidic platform should prove useful for creating anisotropic biomimetic materials and high-throughput platforms for investigating cell-microenvironment interactions.

Cartilage Repair and Subchondral Bone Migration Using 3D Printing Osteochondral Composites: A One-Year-Period Study in Rabbit Trochlea

Increasing evidences show that subchondral bone may play a significant role in the repair or progression of cartilage damage in situ. However, the exact change of subchondral bone during osteochondral repair is still poorly understood. In this paper, biphasic osteochondral composite scaffolds were fabricated by 3D printing technology using PEG hydrogel and β-TCP ceramic and then implanted in rabbit trochlea within a critical size defect model. Animals were euthanized at 1, 2, 4, 8, 16, 24, and 52 weeks after implantation. Histological results showed that hyaline-like cartilage formed along with white smooth surface and invisible margin at 24 weeks postoperatively, typical tidemark formation at 52 weeks. The repaired subchondral bone formed from 16 to 52 weeks in a “flow like” manner from surrounding bone to the defect center gradually. Statistical analysis illustrated that both subchondral bone volume and migration area percentage were highly correlated with the gross appearance Wayne score of repaired cartilage. Therefore, subchondral bone migration is related to cartilage repair for critical size osteochondral defects. Furthermore, the subchondral bone remodeling proceeds in a “flow like” manner and repaired cartilage with tidemark implies that the biphasic PEG/β-TCP composites fabricated by 3D printing provides a feasible strategy for osteochondral tissue engineering application.

Fabrication of Nature‐Inspired Microfluidic Network for Perfusable Tissue Constructs

A microreplication method is presented to transfer nature optimized vascular network of leaf venation into various synthetic matrixes. The biomaterial hydrogel with these microfluidic networks is proven to facilitate the growth of endothelial cells and simultaneously function as convection pathways to transport nutrients and oxygen in a pump-free bioreactor setup, which is crucial for the long-term viability of encapsulated cells.

Surface-tension-driven gradient generation in a fluid stripe for bench-top and microwell applications

A simple and inexpensive method is presented employing passive mechanisms to generate centimeters-long gradients of molecules and particles in under a second with only a coated glass slide and a micropipette. A drop of solution is pipetted onto a fluid stripe held in place on a glass slide by a hydrophobic boundary. The resulting difference in curvature pressure drives the flow and creates a concentration gradient by convection. Experiments and theoretical models characterize the flows and gradient profiles and their dependence on the fluid volumes, properties, and stripe geometry. A bench-top rapid prototyping method is outlined to allow the user to design and fabricate the coated slides using only tape and hydrophobic spray. The rapid prototyping method is compatible with microwell arrays, allowing soluble gradients to be applied to cells in shear-protected microwells. The methods simplicity makes it accessible to virtually any researcher or student and its use of passive mechanisms makes it ideal for field use and compatible with point-of-care and global health initiatives.

Designer Hydrophilic Regions Regulate Droplet Shape for Controlled Surface Patterning and 3D Microgel Synthesis

A simple technique is presented for controlling the shapes of micro- and nanodrops by patterning surfaces with special hydrophilic regions surrounded by hydrophobic boundaries. Finite element method simulations link the shape of the hydrophilic regions to that of the droplets. Shaped droplets are used to controllably pattern planar surfaces and microwell arrays with microparticles and cells at the micro- and macroscales. Droplets containing suspended sedimenting particles, initially at uniform concentration, deposit more particles under deeper regions than under shallow regions. The resulting surface concentration is thus proportional to the local fluid depth and agrees well with the measured and simulated droplet profiles. A second application is also highlighted in which shaped droplets of prepolymer solution are crosslinked to synthesize microgels with tailored 3D geometry.

Ice-template-induced silk fibroin-chitosan scaffolds with predefined microfluidic channels and fully porous structures.

Scaffold-based tissue engineering has made great progress in fabricating relatively simple tissues. One of the major challenges in creating thick complex organs is to achieve sufficient nutrient supply as well as uniform cell distribution in a three-dimensional (3D) scaffold. Here we employed microstructured ice templates to fabricate silk fibroin-chitosan (SF-CS) scaffolds with predefined microfluidic channels, open-pore surface and oriented porous structures. The effects of these structural organizations in ice-template-induced (ITI) scaffolds on nutrient delivery, cell seeding as well as cell growth were well investigated in comparison with that of polydimethylsiloxane-template-induced scaffolds. The ITI scaffolds exhibited better structural properties in promoting mass transport, facilitating uniform cell distribution and growth. The ITI scaffolds uniformly seeded with living cells could be further rolled up to form a thick tissue-engineered construct with predefined microfluidic channels. We envision that our ITI scaffolds can be potentially used to engineer thick prevascularized organs when the oriented porous structures are uniformly seeded with primary cells and the predefined microfluidic channels are incorporated with endothelial cells.

Microfluidic synthesis of composite cross-gradient materials for investigating cell-biomaterial interactions.

Combinatorial material synthesis is a powerful approach for creating composite material libraries for the high‐throughput screening of cell–material interactions. Although current combinatorial screening platforms have been tremendously successful in identifying target (termed “hit”) materials from composite material libraries, new material synthesis approaches are needed to further optimize the concentrations and blending ratios of the component materials. Here we employed a microfluidic platform to rapidly synthesize composite materials containing cross‐gradients of gelatin and chitosan for investigating cell–biomaterial interactions. The microfluidic synthesis of the cross‐gradient was optimized experimentally and theoretically to produce quantitatively controllable variations in the concentrations and blending ratios of the two components. The anisotropic chemical compositions of the gelatin/chitosan cross‐gradients were characterized by Fourier transform infrared spectrometry and X‐ray photoelectron spectrometry. The three‐dimensional (3D) porous gelatin/chitosan cross‐gradient materials were shown to regulate the cellular morphology and proliferation of smooth muscle cells (SMCs) in a gradient‐dependent manner. We envision that our microfluidic cross‐gradient platform may accelerate the material development processes involved in a wide range of biomedical applications. Biotechnol. Bioeng. 2011; 108:175–185.

Human lower extremity joint moment prediction: A wavelet neural network approach

Joint moment is one of the most important factors in human gait analysis. It can be calculated using multi body dynamics but might not be straight forward. This study had two main purposes; firstly, to develop a generic multi-dimensional wavelet neural network (WNN) as a real-time surrogate model to calculate lower extremity joint moments and compare with those determined by multi body dynamics approach, secondly, to compare the calculation accuracy of WNN with feed forward artificial neural network (FFANN) as a traditional intelligent predictive structure in biomechanics. To aim these purposes, data of four patients walked with three different conditions were obtained from the literature. A total of 10 inputs including eight electromyography (EMG) signals and two ground reaction force (GRF) components were determined as the most informative inputs for the WNN based on the mutual information technique. Prediction ability of the network was tested at two different levels of inter-subject generalization. The WNN predictions were validated against outputs from multi body dynamics method in terms of normalized root mean square error (NRMSE (%)) and cross correlation coefficient (@r). Results showed that WNN can predict joint moments to a high level of accuracy (NRMSE 0.94) compared to FFANN (NRMSE 0.89). A generic WNN could also calculate joint moments much faster and easier than multi body dynamics approach based on GRFs and EMG signals which released the necessity of motion capture. It is therefore indicated that the WNN can be a surrogate model for real-time gait biomechanics evaluation.

Prediction of in vivo joint mechanics of an artificial knee implant using rigid multi-body dynamics with elastic contacts

Lower extremity musculoskeletal computational models play an important role in predicting joint forces and muscle activation simultaneously and are valuable for investigating functional outcomes of the implants. However, current computational musculoskeletal models of total knee replacement rarely consider the bearing surface geometry of the implant. Therefore, these models lack detailed information about the contact loading and joint motion which are important factors for evaluating clinical performances. This study extended a rigid multi-body dynamics simulation of a lower extremity musculoskeletal model to incorporate an artificial knee joint, based upon a novel force-dependent kinematics method, and to characterize the in vivo joint contact mechanics during gait. The developed musculoskeletal total knee replacement model integrated the rigid skeleton multi-body dynamics and the flexible contact mechanics of the tibiofemoral and patellofemoral joints. The predicted contact forces and muscle activations are compared against those in vivo measurements obtained from a single patient with good agreements for the medial contact force (root-mean-square error = 215 N, ρ = 0.96) and lateral contact force (root-mean-square error = 179 N, ρ = 0.75). Moreover, the developed model also predicted the motion of the tibiofemoral joint in all degrees of freedom. This new model provides an important step toward the development of a realistic dynamic musculoskeletal total knee replacement model to predict in vivo knee joint motion and loading simultaneously. This could offer a better opportunity to establish a robust virtual modeling platform for future pre-clinical assessment of knee prosthesis designs, surgical procedures and post-operation rehabilitation.