Glossary

1.3 Shape memory polymers

11 min read•august 20, 2024

Shape memory polymers are smart materials that can remember and recover their original shape when triggered by external stimuli. These versatile materials exhibit unique properties that make them valuable in soft robotics applications, from to self-deploying structures.

SMPs can be thermally, light, electrically, or moisture-activated, offering diverse actuation methods. Their is governed by molecular mechanisms like switches, phase separation, and supramolecular interactions, enabling programmable shape-changing abilities in soft robotic systems.

Properties of shape memory polymers

  • Shape memory polymers (SMPs) are a class of smart materials that can be programmed to memorize and recover their original shape upon exposure to external stimuli
  • SMPs exhibit a shape memory effect, which allows them to be deformed into a temporary shape and then return to their original, permanent shape when triggered by a specific stimulus
  • The shape memory effect in polymers is governed by the material's molecular structure, cross-linking density, and the type of external stimulus applied

Thermally induced shape memory

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  • Thermally induced shape memory is the most common type of shape memory effect in SMPs
  • The shape recovery is triggered by heating the material above its transition temperature (glass transition temperature (Tg) or melting temperature (Tm))
  • When heated, the polymer chains become more mobile, allowing the material to return to its original, permanent shape
  • Examples of thermally induced SMPs include , poly(ε-caprolactone), and polyester-based polymers

Light activated shape memory

  • Light activated shape memory polymers respond to specific wavelengths of light (UV, visible, or near-infrared)
  • Light-responsive SMPs often incorporate photosensitive molecules or nanoparticles that absorb light and convert it into heat, triggering the shape recovery
  • Light activation allows for remote, non-contact control of the shape memory effect
  • Examples of light-activated SMPs include azobenzene-containing polymers and carbon nanotube-reinforced polymers

Electrically triggered shape memory

  • Electrically triggered SMPs can be actuated by applying an electric current or voltage
  • The shape recovery is induced by the Joule heating effect, where the electrical current passing through the polymer generates heat
  • Electrically responsive SMPs often incorporate conductive fillers (carbon nanotubes, graphene) to enhance their electrical conductivity
  • Advantages of electrical actuation include rapid response, precise control, and the ability to integrate with electronic systems

Moisture responsive shape memory

  • Moisture responsive SMPs are sensitive to changes in humidity or water content
  • The shape memory effect is triggered by the absorption or desorption of water molecules, which plasticize the polymer and lower its transition temperature
  • Moisture-responsive SMPs can be designed to respond to changes in environmental humidity or the presence of body fluids (sweat, blood)
  • Potential applications include moisture-activated medical devices, self-tightening textiles, and humidity sensors

Mechanisms of shape memory effect

  • The shape memory effect in polymers is attributed to various molecular mechanisms that enable the material to store and release mechanical energy
  • These mechanisms involve the interplay between the polymer's molecular structure, cross-linking density, and the external stimuli

Molecular switches

  • Molecular switches are functional groups or segments within the polymer chain that can undergo reversible conformational changes in response to external stimuli
  • These conformational changes allow the polymer to switch between a temporary, deformed shape and its original, permanent shape
  • Examples of molecular switches include thermally activated phase transitions (crystallization, melting) and photoisomerization of azobenzene groups
  • Molecular switches play a crucial role in storing and releasing the mechanical energy required for shape memory effect

Phase separation

  • Phase separation is a mechanism that occurs in block copolymers or polymer blends with incompatible components
  • The incompatible components form separate domains with different mechanical properties and transition temperatures
  • The shape memory effect arises from the interplay between the hard, elastic domains that maintain the permanent shape and the soft, switchable domains that enable temporary shape fixing and recovery
  • Phase separation allows for the tuning of shape memory properties by adjusting the composition and morphology of the polymer system

Supramolecular interactions

  • Supramolecular interactions, such as hydrogen bonding, π-π stacking, and metal-ligand coordination, can be exploited to create shape memory polymers
  • These non-covalent interactions form reversible cross-links between polymer chains, enabling the material to fix a temporary shape and recover its permanent shape upon exposure to external stimuli
  • Supramolecular SMPs often exhibit excellent shape recovery and self-healing properties due to the dynamic nature of the non-covalent interactions
  • Examples of supramolecular SMPs include hydrogen-bonded polyurethanes, metal-coordinated polymers, and host-guest systems based on cyclodextrins

Types of shape memory polymers

  • Shape memory polymers can be classified based on their chemical composition, network structure, and shape memory behavior
  • The choice of SMP type depends on the specific application requirements, such as the desired transition temperature, mechanical properties, and shape memory performance

Thermoplastic vs thermoset SMPs

  • Thermoplastic SMPs are composed of linear or branched polymer chains that can be melted and reshaped multiple times
  • They exhibit a reversible shape memory effect, as the temporary shape can be reprogrammed by heating the material above its transition temperature and applying a new deformation
  • Thermoset SMPs, on the other hand, have a permanently cross-linked network structure formed by irreversible chemical reactions (curing)
  • Thermoset SMPs exhibit a one-way shape memory effect, as the permanent shape is fixed during the curing process and cannot be reprogrammed
  • Thermoplastic SMPs offer greater flexibility in terms of shape reprogramming, while thermoset SMPs provide better mechanical stability and shape fixity

Physically vs chemically cross-linked networks

  • SMPs can be categorized based on the nature of the cross-links that hold the polymer network together
  • Physically cross-linked SMPs rely on non-covalent interactions (hydrogen bonding, ionic interactions, crystallization) to form the network structure
  • These cross-links are reversible and can be disrupted by external stimuli (heat, stress), allowing for shape reprogramming
  • Chemically cross-linked SMPs have covalent bonds between the polymer chains, resulting in a permanent network structure
  • Chemical cross-links provide better mechanical stability and shape fixity compared to physical cross-links, but limit the ability to reprogram the permanent shape

One-way vs two-way shape memory

  • One-way shape memory polymers can remember and recover only one permanent shape
  • The shape memory cycle involves deforming the material into a temporary shape, fixing the temporary shape (by cooling or removing the external stress), and then recovering the original, permanent shape upon exposure to the appropriate stimulus
  • Two-way shape memory polymers, also known as reversible SMPs, can alternate between two different shapes in response to external stimuli
  • The material can spontaneously switch between a permanent shape and a temporary shape without the need for manual programming
  • Two-way SMPs are typically achieved by incorporating liquid crystalline segments or by creating asymmetric network structures

Fabrication methods for SMPs

  • Various fabrication techniques can be employed to process shape memory polymers into desired shapes and structures
  • The choice of fabrication method depends on the specific SMP composition, the desired geometry, and the intended application

Solvent casting

  • Solvent casting is a simple and versatile method for fabricating SMP films and sheets
  • The process involves dissolving the polymer in a suitable solvent, casting the solution onto a flat surface or mold, and allowing the solvent to evaporate
  • Solvent casting enables the incorporation of fillers, dyes, or other additives to modify the properties of the SMP
  • The method is particularly useful for creating thin, flexible SMP films for applications such as smart textiles and

Melt processing

  • Melt processing techniques, such as extrusion and , are commonly used for thermoplastic SMPs
  • The polymer is heated above its melting temperature and then shaped by forcing it through a die (extrusion) or injecting it into a mold cavity (injection molding)
  • Melt processing allows for the rapid and large-scale production of complex SMP shapes and structures
  • The method is suitable for fabricating SMP components with high dimensional accuracy and reproducibility, such as gears, springs, and structural elements

3D printing of SMPs

  • , also known as additive manufacturing, has emerged as a powerful tool for fabricating SMP structures with intricate geometries and customized designs
  • Various 3D printing techniques, such as fused deposition modeling (FDM), stereolithography (SLA), and selective laser sintering (SLS), can be adapted for processing SMPs
  • 3D printing enables the creation of complex, multi-material SMP structures with spatially varying properties and functionalities
  • The method offers great design flexibility and rapid prototyping capabilities, making it particularly attractive for developing novel SMP-based soft robotic devices and medical implants

Characterization techniques for SMPs

  • Characterizing the properties and performance of shape memory polymers is essential for understanding their behavior and optimizing their design for specific applications
  • A range of analytical techniques are employed to study the thermal, mechanical, and shape memory characteristics of SMPs

Thermal analysis (DSC, TGA)

  • Differential scanning calorimetry (DSC) is used to investigate the thermal transitions in SMPs, such as the glass transition temperature (Tg) and melting temperature (Tm)
  • DSC measures the heat flow into or out of the sample as a function of temperature, providing insights into the polymer's phase transitions and thermal properties
  • Thermogravimetric analysis (TGA) is employed to study the thermal stability and degradation behavior of SMPs
  • TGA measures the change in sample weight as a function of temperature, revealing information about the polymer's composition, thermal decomposition, and filler content

Dynamic mechanical analysis (DMA)

  • (DMA) is a powerful technique for characterizing the viscoelastic properties of SMPs
  • DMA measures the material's response to an oscillatory mechanical stress or strain as a function of temperature or frequency
  • The technique provides information on the polymer's storage modulus (elastic component), loss modulus (viscous component), and damping behavior
  • DMA is particularly useful for determining the temperature-dependent mechanical properties of SMPs and identifying the optimal temperature range for shape memory actuation

Shape memory performance evaluation

  • Evaluating the shape memory performance of SMPs involves quantifying key parameters such as shape fixity, shape recovery, and recovery speed
  • Shape fixity refers to the ability of the SMP to maintain its temporary, deformed shape after the removal of the external stress
  • Shape recovery describes the extent to which the SMP can return to its original, permanent shape upon exposure to the appropriate stimulus
  • Recovery speed is a measure of how quickly the SMP can recover its permanent shape once the stimulus is applied
  • These shape memory parameters are typically assessed using thermomechanical cyclic tests, where the sample is subjected to repeated deformation and recovery cycles under controlled conditions
  • The results of these tests provide valuable insights into the SMP's shape memory behavior, cyclic stability, and the influence of various processing and environmental factors

Applications of SMPs in soft robotics

  • Shape memory polymers have found numerous applications in the field of soft robotics, where their programmable shape-changing abilities and compliance are highly advantageous
  • SMPs enable the development of adaptive, responsive, and multifunctional soft robotic systems that can interact safely with humans and the environment

Actuators and artificial muscles

  • SMPs can be used to create compact, lightweight, and silent actuators for soft robotic applications
  • SMP actuators can generate large deformations and high actuation forces while maintaining a low profile and simple design
  • By exploiting the shape memory effect, SMP actuators can be programmed to perform complex, multi-step movements in response to external stimuli
  • Examples include SMP-based artificial muscles for prosthetics, miniature robots, and bio-inspired locomotion systems

Self-deploying structures

  • SMPs are ideal for creating self-deploying structures that can autonomously change their shape and size in response to environmental cues
  • Self-deploying SMP structures can be compactly stored in a temporary, folded configuration and then expand to their full, functional shape when triggered by heat, light, or other stimuli
  • This capability is particularly useful for space applications, where structures need to be packed efficiently for launch and then deployed reliably in orbit
  • Other examples include self-expanding stents for medical applications and self-assembling robots for search and rescue operations

Soft grippers and manipulators

  • SMPs can be used to create adaptive, compliant and manipulators for handling delicate objects or interacting with unstructured environments
  • SMP-based soft grippers can conform to the shape of the target object, providing a secure and gentle grasp without the need for complex control systems
  • The shape memory effect allows the gripper to switch between an open, compliant state for grasping and a closed, rigid state for holding and manipulation
  • SMP soft grippers have applications in industrial automation, agricultural harvesting, and minimally invasive surgery

Wearable robotics and exoskeletons

  • SMPs are promising materials for creating lightweight, comfortable, and adaptable wearable robotic devices and exoskeletons
  • SMP-based wearable systems can be designed to provide assistive forces, support, or haptic feedback to the user
  • The shape memory effect enables the device to adapt its shape and stiffness to the user's body and movement patterns, improving comfort and reducing the risk of injury
  • Examples include SMP-based orthoses for rehabilitation, soft exosuits for human power augmentation, and responsive footwear for gait assistance

Challenges and future perspectives

  • Despite the significant progress in SMP research and development, several challenges and opportunities remain in advancing their applications in soft robotics

Improving recovery stress and speed

  • One of the key challenges in SMP actuators is achieving high recovery stress and fast response times
  • Recovery stress refers to the force generated by the SMP during shape recovery, which determines the actuator's load-bearing capacity
  • Improving recovery stress often involves optimizing the polymer composition, cross-linking density, and incorporation of reinforcing fillers
  • Response speed is critical for applications requiring rapid and precise actuation, such as soft robotic grippers and dynamic control systems
  • Strategies to enhance response speed include reducing the SMP's thickness, increasing thermal conductivity, and exploring novel stimuli-responsive mechanisms

Enhancing cyclic stability and durability

  • For long-term and reliable operation, SMP-based soft robotic devices must demonstrate excellent cyclic stability and durability
  • Cyclic stability refers to the ability of the SMP to maintain its shape memory performance over repeated actuation cycles without significant degradation
  • Durability encompasses the material's resistance to mechanical wear, fatigue, and environmental factors (UV radiation, moisture, chemical exposure)
  • Improving cyclic stability and durability may involve optimizing the polymer network structure, incorporating self-healing capabilities, and developing robust protective coatings

Multifunctional and stimuli-responsive SMPs

  • Future research efforts will focus on developing multifunctional and multi-stimuli-responsive SMPs for advanced soft robotic applications
  • Multifunctional SMPs combine shape memory properties with additional functionalities, such as self-sensing, self-healing, or energy harvesting capabilities
  • These materials can enable the creation of smart, autonomous soft robotic systems that can adapt to changing environments and self-monitor their performance
  • Multi-stimuli-responsive SMPs can be actuated by a combination of external stimuli (heat, light, electric fields, magnetic fields), offering greater flexibility and control over the shape memory behavior
  • Exploring novel stimuli-responsive mechanisms, such as ultrasound or chemical triggers, can further expand the application potential of SMPs in soft robotics

Biocompatible and biodegradable SMPs

  • For soft robotic applications in biomedical and environmental fields, the development of biocompatible and biodegradable SMPs is crucial
  • Biocompatible SMPs are materials that can be safely used in contact with living tissues without eliciting adverse immune responses or toxic effects
  • Biodegradable SMPs are designed to degrade naturally in the environment or within the body after fulfilling their intended function
  • These materials are particularly relevant for implantable medical devices, drug delivery systems, and eco-friendly soft robotic components
  • Research efforts will focus on developing SMPs from renewable resources, optimizing their degradation kinetics, and ensuring their biocompatibility through extensive in vitro and in vivo testing

Key Terms to Review (18)

3D Printing: 3D printing is an additive manufacturing process that creates three-dimensional objects by layering materials based on digital models. This technology enables the production of complex shapes and structures that can be tailored for specific applications, leading to innovations across various fields such as engineering, medicine, and art.
A. M. P. De Silva: A. M. P. De Silva is a notable researcher known for his contributions to the field of shape memory polymers (SMPs). His work primarily focuses on the development and understanding of how these materials can be engineered to change their shape in response to external stimuli, such as temperature or stress. This research has significant implications for soft robotics, where adaptive materials are essential for creating flexible and responsive systems.
Actuators: Actuators are devices that convert energy into motion, enabling the movement or control of mechanical systems. They play a crucial role in soft robotics by providing the necessary forces to manipulate and change the shape of robotic structures, thus facilitating interaction with their environment. Actuators can be powered by various energy sources, including electrical, hydraulic, or pneumatic systems, and are often essential for implementing advanced functionalities in robotic designs.
Adaptive Structures: Adaptive structures refer to systems that can change their shape, stiffness, or properties in response to external stimuli or environmental conditions. This flexibility allows them to optimize performance and functionality across various applications, particularly in soft robotics, where the ability to adjust to different situations can significantly enhance usability and effectiveness.
Dual-shape effect: The dual-shape effect refers to the unique ability of certain materials, specifically shape memory polymers, to exhibit two distinct shapes based on temperature changes or external stimuli. This effect allows these materials to switch between a temporary shape and a permanent shape, which can be exploited in various applications, including soft robotics, where adaptable structures are crucial for functionality.
Dynamic Mechanical Analysis: Dynamic Mechanical Analysis (DMA) is a technique used to study the mechanical properties of materials as they are deformed under periodic stress. This method allows for the measurement of material behavior over a range of temperatures and frequencies, helping to understand how materials respond to mechanical forces and their overall performance in various applications.
Grippers: Grippers are mechanical devices used in robotics and automation to grasp, hold, and manipulate objects. They are crucial in the functioning of robotic systems, enabling them to interact with their environment by securely handling various shapes and sizes. Grippers can be designed using different materials and mechanisms, including those that utilize shape memory polymers and shape memory alloys, which enhance their adaptability and performance in diverse applications.
Injection Molding: Injection molding is a manufacturing process used to produce parts by injecting molten material into a mold. This technique allows for high precision and the ability to create complex shapes, making it essential for producing components in various fields, including soft robotics. It’s particularly useful for working with materials like plastics and certain types of elastomers, which can be formulated to exhibit unique properties such as shape memory.
L. H. C. de Lima: L. H. C. de Lima refers to a researcher known for significant contributions to the field of shape memory polymers (SMPs). Their work has focused on understanding the mechanisms of shape memory effects and developing new materials that can remember and return to predetermined shapes upon exposure to specific stimuli, such as temperature changes. This research plays a crucial role in advancing the applications of SMPs in various fields, including soft robotics, where materials can mimic natural movements and adapt to their environments.
Phase Transition: Phase transition refers to the change of a substance from one state of matter to another, such as from solid to liquid or liquid to gas, often involving changes in energy and structure. This phenomenon is crucial in materials science, especially for shape memory polymers, as it allows these materials to 'remember' a specific shape and revert back when triggered by external stimuli like temperature or stress.
Polylactic Acid: Polylactic acid (PLA) is a biodegradable thermoplastic made from renewable resources like cornstarch or sugarcane. It has gained popularity due to its environmentally friendly properties, making it a key material in various applications, including packaging, textiles, and 3D printing. Its connection to shape memory polymers lies in its ability to undergo reversible shape changes under specific thermal conditions.
Polyurethane: Polyurethane is a versatile polymer composed of organic units joined by urethane links, widely used in various applications due to its flexibility, durability, and resilience. This material can be formulated to exhibit different properties such as rigidity or elasticity, making it suitable for a range of uses including coatings, foams, adhesives, and elastomers. Its unique characteristics allow it to play an essential role in the development of advanced materials and devices.
Recovery Strain: Recovery strain refers to the ability of a material, particularly shape memory polymers, to return to its original shape after being deformed. This characteristic is crucial as it enables these materials to 'remember' their initial form, which is often triggered by an external stimulus such as heat. The process of recovery strain is what allows shape memory polymers to be used in various applications, including self-healing materials and soft robotics, where flexibility and adaptability are essential.
Shape Memory Effect: The shape memory effect is a property of certain materials that allows them to return to a predetermined shape when exposed to an external stimulus, such as heat or light. This unique behavior is especially prevalent in shape memory polymers, which can be programmed to switch between different shapes, making them incredibly useful for various applications in soft robotics, biomedical devices, and beyond.
Soft actuators: Soft actuators are devices made from flexible materials that can deform and move in response to external stimuli, such as air, temperature, or electric signals. These actuators mimic biological systems and enable complex, adaptive movements, making them essential in various applications that require safe interaction with humans and delicate objects.
Thermomechanical Analysis: Thermomechanical analysis (TMA) is a technique used to measure the dimensional changes of materials as they are subjected to temperature variations and mechanical stress. This method is crucial for understanding how materials respond under different thermal conditions, revealing insights about their mechanical properties and transitions, such as softening or crystallization. TMA plays a vital role in the characterization of materials like shape memory polymers, providing essential data on their behavior when exposed to heat and stress, which directly influences their functionality in soft robotics applications.
Thermoplastic Shape Memory Polymers: Thermoplastic shape memory polymers are a type of smart material that can change their shape in response to external stimuli such as temperature. These materials can be deformed at a higher temperature and then return to their original shape upon cooling, making them useful in various applications, including soft robotics, medical devices, and self-healing materials. Their unique properties allow for reversible deformation, enabling the creation of complex structures that can adapt to different conditions.
Thermoset shape memory polymers: Thermoset shape memory polymers are a type of polymer that can change their shape in response to external stimuli, such as temperature, while retaining a permanent set once cured. These materials undergo a unique phase transition that allows them to 'remember' a specific shape and return to it when heated above a certain temperature. They combine the properties of thermosetting materials, which harden irreversibly, with the ability to switch shapes, making them useful for various applications in soft robotics and beyond.
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