Artificial skeletal muscles are particularly important for the development of in vitro models for disease research or for use in biohybrid robots. However, there are some difficulties in the traditional production of these muscles, as 2D muscle monolayers often peel off after a few days and are not suitable for long-term use or live cell imaging – a technique in which cells are examined in a living state under a microscope or other imaging technique. For this reason, scientists at the Massachusetts Institute of Technology (MIT) have now applied a new technique to artificial muscle tissue that contracts in multiple directions and mimics movements of natural muscles. In the study, they presented a microtopographic stamping method that precisely controls the alignment of muscle fibers, and additive manufacturing played a particularly important role in this. The muscles could potentially be used in regenerative medicine, muscle disease research and robotics.
The new process “Simple Templating of Actuators via Micro-Topographical Patterning” (STAMP) uses 3D-printed stamps to structure microscopic grooves directly in natural hydrogels. The 3D-printed stamp is used to incorporate structured grooves into a soft hydrogel. These grooves guide the muscle cells as they grow and ensure that they align into functional fibers that can contract in different directions.
Top left: CAD model of the micro stamp and holder; top right and bottom: resulting 3D-printed parts.
But how exactly does the procedure work? The first step is to produce the 3D-printed stamps. These fit into 24-well plates and have fine vertical grooves (12.5-125 μm). The stamps are then fixed in a holder with bubble release and a liquid consisting of fibrinogen and thrombin is poured into the wells. The whole process is incubated at 37 °C for about an hour so that the fibrin polymerizes to form a cross-linked hydrogel. After removing the plunger, the hydrogel shows a precise, regular groove structure and a smoother surface, which improves the conditions for cell culture. The stamps are sterilized before use and can be reused several times after cleaning.
To test the method, the researchers developed an artificial iris resembling a biohybrid actuator designed to mimic how the pupil of the human eye expands and contracts. The structure had two muscle fiber orientations: one formed concentric circles, and the other radiated outward. Both worked together to produce contractions in response to light stimulation, demonstrating a degree of coordination that is normally difficult to find in man-made muscle tissue.
It is important to note that natural muscle fibers do not grow in perfectly straight lines, but vary their orientation in the body, giving them a greater range of motion. Artificial muscles, on the other hand, are often constrained by traction in one direction – a constraint that makes complex movements in biohybrid actuators difficult. With STAMP, however, muscle growth can be specifically controlled so that the artificial tissue comes functionally closer to the real biological model. In addition, computer-aided modeling confirmed that the muscle fibres grown with STAMP contract in a coordinated and multidirectional manner – a prediction that was confirmed by the experimental tests.
The use of 3D printing offers several benefits, but above all it ensures the accessibility of the stamping process, because thanks to additive manufacturing, microscopically small grooves can be printed that correspond exactly to the dimensions of the muscle cells. The process is also cost-effective, can be used in a single step and enables precise alignment. The 3D-printed stamps can also be reused by simple ultrasonic cleaning, which makes the process more sustainable. Although the study focused on skeletal muscle, the method is not limited to one cell type and the researchers believe that the process can be adapted for neurons, cardiac muscle cells and other tissues.
(a) CAD-Design eines Mikrostempels mit 25 μm breiten Rillen zur Nachbildung der Iris-Muskeln; (b) Maus C2C12-Zellen auf Fibrin mit irisähnlichem Muster; (c) Schematische Darstellung der Iris-Regionen und Muskelkontraktionen.
In the future, the researchers also see applications outside of medicine, for example to develop energy-efficient alternatives to mechanical components in soft robotics, i.e. in environments where flexibility is important and robots can thus be made more adaptable.You can find out more about this HERE.
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*All Photo Credits: MIT and Biomater. Sci