It is estimated that by 2025, the UK will generate 6 million tonnes of waste per day, with high-tech disposable waste being a contributor to rapid growth. End-of-life appliances contain valuable substances that are not easily recyclable, as well as toxic substances that can be easily released into nature through landfills or improper disposal. Biomaterials, on the other hand, are self-healing and easily degradable, which gives them more room for application. Humans have developed a range of bionic systems, including soft machines and electronic skins, that combine soft-to-hard materials to achieve a high level of functionality by introducing self-healing, stretchable properties. However, introducing stretchability into degradable devices remains challenging.
Gelatin-based gels are a promising option because this biopolymer can be easily obtained without the need for synthesis, can be added with water-soluble additives, and degrades quickly, is environmentally friendly, and can even be edible. However, gels cause hardening and limit the stability and durability of wearable devices or soft robotic components when stretched and dried quickly in air. Moreover, due to the difficulty of controlling material properties and limited tensile capacity, performance is degraded with only a few drive cycles.
Here, the paper presents a widely applicable manufacturing method, design rules, and a set of concepts for biogels that unify the challenging needs of resilient and sustainable (soft) robotics and electronics on a single platform. Inspired by complex organisms from cephalopods, biogels are combined with naturally derived materials such as cellulose and zinc to enable fully biodegradable devices ranging from robotic elements to stretchable electronic components. The gel is based on natural materials and serves as a degradable substrate, including durable, self-adhesive, stretchable, mechanically adjustable and self-healing. Although completely degradable in wastewater, the gel retains its mechanical properties for more than a year under ambient conditions and enables the soft actuator to operate more than 330,000 times without failure. Its scalable production, low material costs and safety in all manufacturing steps will enable its wide range of applications from industry to healthcare to education.
Cellulose, as a ubiquitous structural polysaccharide, can act as an exoskeleton designed for soft pneumatic biogel actuators in dynamic environments (Figure 1a). Combining the biogel with a zinc electrode results in a fully degradable sensor skin (Figure 1b). This mechanically elastic biogel is an ideal material for temporary installation or frequent updates of instantaneous devices for applications as it can be decomposed in a matter of days (Figure 1c). Wastewater bacteria enzymatically digest the biogel within 5 days without causing damage to the environment.
Figure 1 - A biogel with elasticity and complete degradation.
A combination of components – each with a different purpose – is combined to achieve both biodegradable and high mechanical properties. The main polymer network of the gel is gelatin, which determines the Young's modulus (E) of the material, and the addition of sugars (cellulose) improves the ductility of the material. By adjusting the ratio of water and glycerol in the biogel, stable mechanical properties and process conditions can be obtained, and citric acid can prevent the growth of bacteria.
A wide range of emerging soft electromechanical systems – from electronic skins to robotics – require a set of reliable and durable materials and easily adjustable mechanical properties. The paper adjusts the E and limit (engineering) stresses (σu) of the biogel by adjusting the amount of gel, 30 to 300 kpa and 10 to 140 kpa, respectively, both of which increase exponentially with the gel concentration (Figure 2a). At the same time, the ultimate strain (εu) increased linearly from 180% strain to 325% strain (Figure 2b). Further increase in gelatin concentration can yield a higher E value (3.1 MPa) and an increase in σu value (1.86 MPa), but a reduced εu value (254%) to some extent. Add 28% wt% sugar to give the biogel a high ductility, with εu and σu increasing simultaneously (Figures 2c, d). The addition of sugars (and glycerol) as co-solvents promotes helix-helix binding in gelatin, enhances gelatinization, and thermodynamically stabilizes the gel. At the same time, the unraveling of these helix-helix combinations under high deformation leads to increased stretchability.
Figure 2 - Adjustability, stability and mechanical properties of gelatin biogels.
To overcome the dehydration of hydrogels due to loss of free water when used under environmental conditions – replace large amounts of water with a non-volatile food additive glycerol to reduce free water in biogels. Reducing the water-glycerol ratio prolongs storage time and stability without affecting mechanical properties (Figures 2e, f, g). Optimization of these conditions results in the preparation of a shape-stable device (Figure 2h). No signs of material degradation were observed over a year's time, but these experiments were stopped simply because of time constraints, suggesting that the biogel was stable over a longer period of time.
The addition of citric acid to lower the pH inhibits bacterial growth but does not affect its tensile properties. By selecting the composition concentration, a deformable and elastic biogel can be designed that does not dry out and is suitable for soft robotic applications and can operate in the 20-80% RH humidity range. Perform a biaxial tensile test by aerating the biogel into balloon shape (Figure 2j) with a strain of more than 1000%.
Figure 3 - Elastic biogel for soft actuators.
This biogel uniquely combines high performance and degradability, which makes it suitable for applications of soft (biological) robotics, medical devices and industrial robots. The potential of a soft pneumatic actuator inspired by an elephant's nose is demonstrated here. The s-shaped movement of the pneumatically driven actuator reaches a maximum displacement of 10 cm at its tip (Figure 3a). The paper selected two modes of S-shaped and U-shaped motion to simulate the lifting and grabbing movements of the crane. The number of drive cycles is monitored by applying a constant pressure to the plate connected to the force transducer to inflate the u-actuator. Actuators made of high water/glycerol ratio biogel (G2420) can perform 10,000 cycles continuously (about 10 hours of continuous execution), with sufficient application prospects in medium durability applications. Actuators with a lower water/glycerol ratio (G2430) are highly durable and do not fail even after 60,000 cycles. Use force adjustment settings (adjusting drive pressure and frequency) to increase cycle life by more than 330,000 times without failure (Figure 3c). Even underwater after applying cooking oil, the actuator can work for 1.5 hours and 2000 cycles.
The actuator can grasp and lift a variety of flat or curved shapes up to 120g at a distance of 16mm. The u-shaped design makes the bending angle 286° (Fig. 3), which corresponds to a maximum linear strain of 232%. By applying a pressure of 50 kPa, a full range of bending angles of 281° can be obtained (Figure 3g). The use of food additives , mono and diglycerides of fatty acids ( E471 ) , creates stable bubbles within the volume of the biogel. The foam sandwiched between the two zinc electrodes acts as a deformable capacitor (Figure 3h), allowing the actuator to react to obstacles such as rose thorns. Applying a load compresses the void in the soft foam (Figure 3i) and approaches both electrodes, resulting in a change in resistance (Figure 3j).
The biogel provides a self-adhesive platform where biodegradable soft electrons adhere to various surfaces without the need for additional adhesives. Figure 4 shows an autonomous sensor patch that is rapidly assembled using the thermoplastic properties of a tunable biogel.
Figure 4 - Degradable electronic sensor patch.
The healing or assembly of the biogel is done by a brief local melting of an infrared laser near a crack or incision (Figure 4a). The biogel was laser-repaired in less than 10 min of treatment time, completely restoring its original mechanical properties (Figure 4b). Use laser-assisted rapid healing (LARH) to assemble biogels with different E values (1.4, 0.4, and 0.2 MPa) into component-level modulus gels. The uniaxial stretch of this mold-grade biogel reduces the strain of the hardest part by 90% at an overall strain of 50% (Figure 4c). Rapid assembly of the LARH enables complex 3D shapes such as trilobites (Figure 4d) or custom complex substrates for stretchable electronics. Using this biogel, biodegradable, stretchable, multimodal electronic skin with a temperature, humidity, and strain response sensor was prepared (Figure 4e). Constructed from zinc foil or temperature-sensitive paste, the sensor uses a zinc elbow that stretches to 50% uniaxial strain, and when stretched to 20%, its resistance does not change and it withstands more than 1000 cycles. Sensor data is recorded, analyzed, and transmitted wirelessly via a reusable flexible printed circuit board (PCB) mounted on a biogel.
The temperature sensor, made from a mixture of carnauba wax and graphite powder, shows a near-linear resistive response over a temperature range of 10 to 40 °C (Figure 4f). The humidity sensor is designed as a digital electrode whose impedance decreases exponentially as the RH increases (Figure 4g). By increasing the finger spacing, etc., it is possible to have a large displacement of the electrodes on the gel, so that the strain transducer has a linear response and no hysteresis under cyclic stretching (Figure 4h). The sensor skin monitors temperature changes near the hot cup (Figure 4i), changes in humidity (Figure 4j), or skin deformation (Figure 4k). During exercise and sweating, the biogel adheres to the human skin even when the intensity of exercise increases. No signs of skin irritation were seen after 7 hours of wear. The prolonged contact time with the skin did not change the mechanical properties of the biogel, and the mechanical properties of the biogel remained unchanged even after 4 days of accumulated wear and tear for 38h during daily activities. If applied with talcum powder, it can be achieved without stickiness.
Based on the concept of a capacitive biodegradable pressure sensor, a pressure-sensitive electronic skin array consisting of a 1 mm thick biogel foam and a zinc foil electrode of a 4×4 matrix can be stretched (Figures 4m, n). The array of stretchable pressure sensors is recorded and read by a flexible PCB (Figure 4O). In addition to being able to quantify the load on a single specific sensor, the array of pressure sensors can also detect objects with complex shapes (Figure 4p, q). When stored under environmental conditions, the electronic skin remains functional for more than a year.
In summary, the paper introduces a set of degradable, elastic soft robots, electronic skin and health care materials that are curable, deformable, self-adhesive, and resistant to dehydration. Applying the thesis's design methods, a new set of durable, biodegradable soft actuators and autonomous electronic platforms with multimodal sensing capabilities were prepared.
Links to papers
https://www.nature.com/articles/s41563-020-0699-3