The design and development of a high-throughput magneto-mechanostimulation device for cartilage tissue engineering

Abstract

To recapitulate the in vivo environment and create neo-organoids that replace lost or damaged tissue requires the engineering of devices, which provide appropriate biophysical cues. To date, bioreactors for cartilage tissue engineering have focused primarily on biomechanical stimulation. There is a significant need for improved devices for articular cartilage tissue engineering capable of simultaneously applying multiple biophysical (electrokinetic and mechanical) stimuli. We have developed a novel high-throughput magneto-mechanostimulation bioreactor, capable of applying static and time-varying magnetic fields, as well as multiple and independently adjustable mechanical loading regimens. The device consists of an array of 18 individual stations, each of which uses contactless magnetic actuation and has an integrated Hall Effect sensing system, enabling the real-time measurements of applied field, force, and construct thickness, and hence, the indirect measurement of construct mechanical properties. Validation tests showed precise measurements of thickness, within 14 μm of gold standard calliper measurements; further, applied force was measured to be within 0.04 N of desired force over a half hour dynamic loading, which was repeatable over a 3-week test period. Finally, construct material properties measured using the bioreactor were not significantly different (p=0.97) from those measured using a standard materials testing machine. We present a new method for articular cartilage-specific bioreactor design, integrating combinatorial magneto-mechanostimulation, which is very attractive from functional and cost viewpoints.

FIG. 1.

The developed high-throughput magneto-mechanostimulation bioreactor can apply two modes of biophysical stimulation: (A) magneto-stimulation (static and time-varying magnetic fields); and (B) mechano-stimulation (static and dynamic loading). The device consists of actuating (contactless magnetic coupling) and sensing (Hall Effect sensor) components. Dynamic mechanical loading is enabled through the cyclic coupling of a permanent magnet pair via the extension and retraction of each linear actuator (as depicted by the green arrow in the schematics). Similarly, application of time-varying magnetic fields is achieved through cyclic extension and retraction of the linear actuator. The device is capable of stimulating 18 articular cartilage constructs independently and simultaneously (C).

FIG. 2.

Schematic depicting the geometry of the system; from the known system dimensions (shown here), it is possible to determine the distance between the magnets (D_mag) at every position of the stepper motor (i.e., as the shaft extends and retracts). Similarly, the thickness of the construct can be determined using a simple algebraic formula, as described in the text.

FIG. 3.

Typical Hall Effect sensor calibration curve (voltage vs. distance between magnets [D_mag]). Sensor output voltage is proportional to the magnetic field strength, which in turn depends on the distance between magnets, D_mag. Based on known geometric factors, both construct thickness and applied force can be monitored in real time as described in the text.

FIG. 4.

The user interface for the high-throughput mageto-mechanostimulation device was developed using LabView. The control panel (A) enables users to input 18 independent magneto-mechanical loading regimens and activate force-feedback (FFB) as required. The visualization panel (B) enables the real-time data visualization of the applied force and construct thickness; this image is of one channel, and the user can select which channel they want to view at any instant. Color images available online at www.liebertpub.com/tec

FIG. 5.

For magneto-stimulation experiments, the relationship between magnetic field strength (mT) and distance (mm) of neodymium iron boron (NdFeB) magnets was established (A). Similarly, for mechanostimulation experiments, the relationship between force (N) and distance (mm) between a pair of NdFeB magnets was established (B).

FIG. 6.

Stability and repeatability was established over a 3-week dynamic loading period, during which time, the stepper motors were cycled (1 Hz, 30 min on, 1 h off) to apply a target force of 1.2 N. Force-time traces show three representative 30-min cycling periods (only peak force recorded) in which, there was a maximum deviation from target force of±0.04 N.

FIG. 7.

Biocompatibility of the polyetherimide bioreactor chambers was assessed using standard biophysical (A; stiffness) and biochemical (B; sulfated glycosaminoglycan [sGAG] production) tests. Bovine chondrocyte (BC)-seeded 2.0wt% agarose gels were cultured for 2 weeks in six-well plates and bioreactor chambers. No significant difference (p=0.68) was observed in construct stiffness (p=0.68) or sGAG production (p=0.76).

FIG. 8.

Comparison of stiffness values obtained from the stress–strain curves for two representative materials; hard (Rubber) and soft (White Foam). Data were gathered by both the bioreactor and a standard materials testing (Instron) device.