Design and Validation of a Compressive Tissue Stimulator with High-Throughput Capacity and Real-Time Modulus Measurement Capability

Abstract

Mechanical stimulation has been shown to impact the properties of engineered hyaline cartilage constructs and is relevant for engineering of cartilage and osteochondral tissues. Most mechanical stimulators developed to date emphasize precision over adaptability to standard tissue culture equipment and protocols. The realization of mechanical characteristics in engineered constructs approaching native cartilage requires the optimization of complex variables (type of stimulus, regimen, and bimolecular signals). We have proposed and validated a stimulator design that focuses on high construct capacity, compatibility with tissue culture plastic ware, and regimen adaptability to maximize throughput. This design utilizes thin force sensors in lieu of a load cell and a linear encoder to verify position. The implementation of an individual force sensor for each sample enables the measurement of Young's modulus while stimulating the sample. Removable and interchangeable Teflon plungers mounted using neodymium magnets contact each sample. Variations in plunger height and design can vary the strain and force type on individual samples. This allows for the evaluation of a myriad of culture conditions and regimens simultaneously. The system was validated using contact accuracy, and Young's modulus measurements range as key parameters. Contact accuracy for the system was excellent within 1.16% error of the construct height in comparison to measurements made with a micrometer. Biomaterials ranging from bioceramics (cancellous bone, 123 MPa) to soft gels (1% agarose, 20 KPa) can be measured without any modification to the device. The accuracy of measurements in conjunction with the wide range of moduli tested demonstrate the unique characteristics of the device and the feasibility of using this device in mapping real-time changes to Young's modulus of tissue constructs (cartilage, bone) through the developmental phases in ex vivo culture conditions.

A high throughput mechanical screening device for cartilage tissue engineering

“…The step protocol implemented here also takes into account differences in sample height, and allows the user to select a “first step” after completing the test, which is then used to calculate the equilibrium modulus. In contrast to previous HTS devices that used individual actuators (Lujan, Wirtz et al 2011) or varying height plungers (Salvetti, Pino et al 2012) to account for differences in sample heights, this protocol requires no additional parts and is easily tuned to apply a range strain magnitudes. Additional advantages of this system include individual force sensing capability, a semi-automated MATLAB program for data analysis, and the ability to fabricate and assemble the device at a reasonable cost (~$6000 circa 2013).…”

“…The MATE system incorporated real-time measures of load during dynamic stimulation of engineered cartilage, using a six-sample actuating system (Lujan, Wirtz et al 2011). Still more recently, a 12-sample tissue stimulator was developed that recorded load from each sample via individual force sensitive resistors (Salvetti, Pino et al 2012). These devices illustrate how real-time and multi-sample mechanical analysis can be incorporated into tissue systems.…”

“…These devices illustrate how real-time and multi-sample mechanical analysis can be incorporated into tissue systems. While promising, throughput in these devices is restricted to a relatively small sample capacity, and expansion to higher throughput formats might be limited by sensor technology (Lujan, Wirtz et al 2011; Salvetti, Pino et al 2012). Additional development is needed to make such devices compatible with HTS of chemical libraries.…”

Engineering and Monitoring 3D Cell Constructs with Time-Evolving Viscoelasticity for the Study of Liver Fibrosis In Vitro

“…In this direction, different bioreactors with integrated mechanical actuation and sensing have been developed [ 32 , 33 , 34 , 35 , 36 ]. The systems able to perform unconfined bulk compression (i.e., the most suitable testing strategy for characterising hepatic cell constructs) are mainly based on electromagnetic actuators [ 37 , 38 , 39 , 40 , 41 , 42 ], hydraulic and vacuum pumps [ 43 , 44 , 45 ] and on air pressure regulators [ 46 ]. Sensing can be performed thanks to load cells or thin film force sensor [ 37 , 39 , 40 , 41 , 42 ], optical encoders [ 38 ], including laser sensors [ 43 ], ultrasound imaging [ 44 ], linear variable displacement transducers (LVDTs) [ 45 ] or Hall effect sensors [ 46 ].…”

“…The systems able to perform unconfined bulk compression (i.e., the most suitable testing strategy for characterising hepatic cell constructs) are mainly based on electromagnetic actuators [ 37 , 38 , 39 , 40 , 41 , 42 ], hydraulic and vacuum pumps [ 43 , 44 , 45 ] and on air pressure regulators [ 46 ]. Sensing can be performed thanks to load cells or thin film force sensor [ 37 , 39 , 40 , 41 , 42 ], optical encoders [ 38 ], including laser sensors [ 43 ], ultrasound imaging [ 44 ], linear variable displacement transducers (LVDTs) [ 45 ] or Hall effect sensors [ 46 ]. These systems generally allow for testing multiple samples; however, many of them are unsuitable for the real-time testing of soft tissue constructs, as required for liver IVMs.…”