Fascination with Technology Smart artificial muscles improve bone healing

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In our "Fascination with Technology" section, we present impressive projects from research and development to designers every week. Today: how a smart implant with "artificial muscles" monitors and even promotes the healing process of bone fractures via smartphone.

Bones are stable and elastic at the same time, they grow, are constantly being remodeled, and can withstand a lot. If they break, they can heal, provided the fragments are aligned properly. But sometimes this doesn't go as planned and the bone doesn't heal properly even after surgery. This happens more frequently with fractures of the lower leg - in about fourteen out of a hundred patients. As doctors can't look into the leg after surgery and watch the bone heal, it remains unnoticed for a long time what is developing there. Only after weeks does an X-ray show whether new bone tissue is doing what it should be doing in the right place. If it doesn't, it leads to pain, disability, and high costs.

Implant with new capabilities

A new implant is expected to provide a permanent insight into the leg: it is intended to constantly monitor, control and even actively promote the healing process. A large research team at the intersection of medicine, engineering, and informatics is working on this at the University of Saarland/Germany. "We are jointly developing a smart implant that does not require additional interventions or devices. To do this, we give the implant, which is needed anyway to hold the bone pieces together, completely new capabilities," explains Professor Stefan Seelecke, who carries out research with his team at the University of Saarland and the Saarbrücken Center for Mechatronics and Automation Technology (ZeMA).

Micro-massage promotes healing

The smart implant will have it all: as soon as the surgical wound is stitched up, the implant plate itself is to continuously provide information about how the fracture is healing. If the patient puts strain on the fracture in an unfavorable way, it should issue a warning. At the fracture gap, where the bone fragments lie against each other, the implant should become stiff or soft as needed, and - this is the particular coup - it should perform a micro-massage through small movements there: this actively promotes bone healing through growth incentives. All this should take place automatically and be controllable via smartphone from the outside. The implant incorporates know-how from various disciplines.

Shape memory wires as drive and sensor

A central role is played by hair-thin nickel-titanium wires, also known as shape memory wires. They are part of the project team of experts for smart material systems Stefan Seelecke and Paul Motzki: The engineers thus lend their "intelligent muscles" to the implant: "We use shape memory wires as drives: they ensure in the implant that it can become stiff or soft, can move and exert force. On the other hand, we use the wires as sensors to keep an eye on the processes at the fracture gap," explains Paul Motzki, who holds a bridge professorship at the University of Saarland and ZeMA called "Smart Material Systems for Innovative Production".

We use shape memory wires as drives: they ensure in the implant that it can become stiff or soft, move and exert force. On the other hand, we use the wires as a sensor to keep an eye on the processes at the fracture gap.

Paul Motzki

The nickel-titanium wires can shorten and lengthen again, similar to muscles, depending on whether electricity flows through them. The reason lies in the crystal lattice of the alloy: "Nickel-titanium has a shape memory. The alloy has two phases at the crystal lattice level that can convert into each other," explains Paul Motzki. "When the current flows, the wire heats up, its crystal structure transforms and shortens. When the current is switched off, it cools down, changes the phase and becomes long as before."

By alternately tensing and relaxing the wires and letting them work together as players and opponents of a flexor and extensor muscle, movement is created: bundles of fine wires become muscle fibers of technology.

Artificial muscles serve as sensors

The muscles themselves serve as sensors. "When the wires deform, the electrical resistance changes. We can assign a precise measurement value to even the smallest deformation of the wire. This makes it possible to read all sensory data from the numbers," says doctoral student Susanne-Marie Kirsch, who is researching this.

Based on the measured values, the tiniest changes at the fracture gap can be read. In cooperation with the trauma surgeon of the Saarland University, Professor Tim Pohlemann, and the Professor for Innovative Implant Development, Bergita Ganse, who are leading the overall project, this will allow conclusions to be drawn about the healing process: whether, for example, the stiffness in the bone fracture is increasing. These information will in future be transmitted - in this case wirelessly - to the smartphone.

What can be seen at the Hanover Fair

In their prototype, which they present at the Hannover Fair, the engineers demonstrate how they use their artificial muscles in the implant: these lie across the fracture gap. Through electrical impulses, the wire muscle strands become longer, shorter or remain stationary as needed, ensuring that the plate at the fracture gap becomes softer or stiffer. The researchers can drive the artificial muscles at the fracture gap to perform slow or fast lifting movements. The best healing promoting success is expected with a stroke of 100 to 500 micrometers. Thanks to their automatically integrated sensor properties, the wires also serve as nerves of the implant: when they move at the fracture gap, the researchers notice whether the bone is becoming firmer, i.e. healing, because the wires simply have to pull more.

At the Hanover Fair, the Saarbrücken experts for intelligent material systems are also showing smart small drives, energy-efficient gripping systems and soft robotic arms in the form of elephant trunks, as well as a new cooling and heating process.

University of Saarland at the Hanover Fair: Hall 2, Booth B10