Real‐time Monitoring of Force Response Measured in Mechanically Stimulated Tissue‐Engineered Cartilage

Abstract

Mechanical stimulation improves tissue-engineered cartilage development both in terms of biochemical composition and structural properties. However, the link between the compositional changes attributed to mechanical stimulation and the changing structural properties of the engineered cartilage is poorly understood. We hypothesize that transient events associated with construct stiffening can be documented and used to understand milestones in construct development. To do this, we designed and built a mechanical stimulation bioreactor that can continuously record the force response of the engineered construct in real time. This study documents the transient changes of the stiffness of tissue-engineered cartilage constructs over the first 14 days of their development under cyclic loading. Compressive strain stimulation (15%, 1 Hz) was applied to poly(ethylene glycol) (PEG) hydrogels seeded with primary articular chondrocytes. The average compressive modulus of strain-stimulated constructs was 12.7 +/- 1.45 kPa after 2 weeks, significantly greater (P < 0.01) than the average compressive moduli of both unstimulated constructs (10.7 +/- 0.94 kPa) and nonviable stimulated constructs (11.2 +/- 0.91 kPa). The system was able to document that nearly all of the stiffness increase occurred over the last 2 days of the experiment, where live-cell constructs demonstrated a rapid 20% increase in force response. The system's ability to track significant increases in stiffness over time was also confirmed by Instron testing. These results present a novel view of the early mechanical development of tissue-engineering cartilage constructs and suggest that the real-time monitoring of force response may be used to noninvasively track the development of engineered tissue.

Cartilage Engineering: Current Status and Future Trends

“…Results from bioreactor studies show increases in production of ECM molecules, cell proliferation, and mechanical properties [87,88]. For example, PGA scaffolds in a perfusion system showed increases in both DNA and GAG content compared to controls [89].…”

The Design and Development of a High-Throughput Magneto-Mechanostimulation Device for Cartilage Tissue Engineering

“…Numerous short-and long-term studies have shown that dynamic compression protocols, at moderate levels (2%-15% strain; 0.5-1 MPa) spanning a wide range of physiological frequencies (0.01-1.0 Hz) can modulate chondrocyte viability, gene expression, and biosynthesis of various extracellular matrix molecules. [44][45][46] Therefore, the force range of our device is 0.5 to 30 N and the frequency range is 0.01 to 1 Hz; both the force range and loading profiles are easily adjustable and definable by the user.…”

A Novel Bioreactor for the Dynamic Stimulation and Mechanical Evaluation of Multiple Tissue-Engineered Constructs

“…For standard specimen diameters of 4-8 mm, these loads correspond to stresses of 2-500 kPa, and cover the spectrum of stimulation magnitudes and frequencies normally applied in cartilage TE studies. 17,22 This level of loading accuracy for a batch-testing bioreactor is uncommon, and differentiates the MATE from contemporary bioreactors that load multiple TE constructs. In an independent review of 205 peer-reviewed articles that applied compressive loads to TE constructs, 84% displaced multiple specimens with a single mechanoelectric actuator, and 95% did not report accounting for alterations in specimen thickness during culture.…”

“…Static loads were applied at low magnitudes (0.1-1.0 N) and high magnitudes (1.0-10.0 N) to represent the forces commonly used to compress soft hydrogels and mature cartilage, respectively. 17,22 For dynamic testing, a linear spring (Lee Spring, Brooklyn, NY; 10 N/mm) was loaded with variable sinusoidal amplitudes (0.1-10.0 N) and frequencies (1, 10 Hz). The spring was preloaded to 0.2 N before applying oscillations.…”

“…Mapping the material properties of TE constructs could guide the development of effective culture protocols. 17 This is particularly important in the design of biodegradable scaffolds, in which the rate of scaffold degradation should coincide with the rate of matrix deposition. 18 A bioreactor is therefore needed that can provide researchers a platform to efficiently and accurately apply mechanical stimulations and assess material properties.…”