Smart materials are materials that can change some of their properties in response to external stimuli like temperature, stress, or electric/magnetic fields. Some examples include piezoelectric materials that generate electricity from stress, shape memory alloys that can return to their original shape after deformation, and hydrogels that can swell/shrink in response to temperature or pH changes. These materials find applications in biomedical areas like drug delivery, wound dressings, sensors, and prosthetics due to their responsive properties. Hydrogels in particular are cross-linked polymer networks that can absorb large amounts of water and are biocompatible, making them promising for tissue engineering applications.

1. Smart Materials
By Mohaned Walied

2. What are Smart Materials?
Smart materials are materials that are manipulated to respond in a controllable and reversible way, modifying
some of their properties as a result of external stimuli such as certain mechanical stress or a certain temperature,
among others. Because of their responsiveness, smart materials are also known as responsive materials. These are
usually translated as "active" materials although it would be more accurate to say "reactive" materials.

3. Types of Smart Materials
1-Piezoelectrics
2-Shape Memory Alloys
3-Magnetostrictive Materials
4-Hydrogels
5-Electroactive polymers
6-pH- sensitive materials
7-Chromo Active materials

4. Hydrogels
-Hydrogel is a 3D interconnected network of macromolecules
-Contains hydrophilic or amphiphilic groups
-Swells in water
-Insoluble in water due to cross-linking
-Water must be at least 10% of total weight

5. History of hydrogels
The existence of hydrogels can be dated back to the 19th century, where they were colloidal gels made of
inorganic salts. They were also one of the earliest biomaterials to be used inside the patient ,research on
hydrogels as tissue engineering materials first began in the 20th century

6. Hydrogels Structure

7. Cross-linking - Prevents dissolution in an
aqueous medium
Chemical Cross-linking
-Covalent Bonding
Physical Cross-Linking
-Intermolecular forces
-Physical entanglement

8. Characteristics of hydrogels
-Polymer Volume in swollen state (Degree of water intakes)
-Mesh size (Degree of cross-linking),
-Average molecular weight
-Sensitivity towards environmental external factors (Temperature, Ph , Pressure etc..)

9. Degree of water intake
S= Percentage of swelling
mt= Wet weight
mo = Dry weight

10. Classification of hydrogels
1- Preparation (polymeric composition)
2- Source
3- Response
4-Type of cross-linking (Physical or chemical)
5-network electrical charge
6- Configuration

11. Preparation of hydrogels
-Homo-polymeric - Single monomer
-Co-polymeric- Two types of monomers
Interpenetrating - Alloy of cross-linked polymers

12. Different Sources
-Natural
● Cationic
● Anionic
● Neutral
-Synthetic - PLA - PEG
-Hybrid - Collagen acrylate

13. Network electrical Charge
Hydrogels can be categorized into four groups on the basic of presence or absence of electrical charge
located on the cross-linked chains
-Nonionic (Neutral)
-Ionic (Anionic or cationic)
-Amphoteric electrolyte (Contains both acidic and basic groups)
-Zwitterionic (contains both anionic and cationic in each structural repeating unit)

14. Based on Configuration
Hydrogels can be classified depending on their physical structure and chemical composition
-Amorphous (Non-crystalline)
-Semi-Crystalline
-Crystalline

15. Sensitivity of hydrogel to environmental
conditions
This is perhaps the most important characteristic of hydrogels that we can base our applications on.

16. The way each hydrogel reacts to the environmental changes will determine its use and application

17. Hydrogels Used in biomedical engineering
-Contact lenses
-Wound dressings
-Drug Delivery (Not there yet)
-Tissue engineering (not there yet)
-Hygiene products (Ex; Diapers)
Bio-compatibility is very important for a hydrogel to make them favorable to the immune system

19. Wound Dressings

20. Tissue engineering

21. Fabrication of hydrogel scaffolds in tissue
engineering
-Emulsification
-Freeze-Drying
-Porogen Leaching
-Gas foaming
-Electrospinning
-3d printing
-Photolithography
-Sol-gel Technique

22. Chromogenic Materials
Materials that change color reversibly as a response to changes in the environment (temperature, pressure
etc..). These materials are one type of smart material. “Chromo” originates from the Greek language and
means that something is colored.

23. History of Chromogenic materials
Photochromic materials are the first chromogenic materials observed from ancient times. Warriors painted photochromic
materials on their bracelets to change the color under sunlight. Photochromism was discovered in the late 1880s, including
work by Markwald, who studied the reversible change of color the solid state. He labeled this phenomenon “phototropy”
The potential of oxidized developers in a color photographic process, however, was first realized by another German
chemist, Rudolf Fischer, who, in 1912, filed a patent describing a chromogenic process to develop both positives and
negatives using indoxyl, and thio-indoxyl-based color developers as dye couplers in a light-sensitive silver halide emulsion

25. Uses of chromogenic materials in biomedical
field
-Photochromic materials are used in sunglasses
-Potentially use thermochromic pigments in baby products to distinguish between hot and cold products.
-Electrochromic glasses for anti glare
-Use of gasochromic materials to detect some gas leakage (ex:hydrogen) to monitor algae mutations and select the most suitable
one
-Chemochromic materials are used in pregnancy tests on the urine
-Biochromic materials are used to detect the presence of certain pathogens
-Radiochromic materials used in testing CT scanners

26. Some chromogenic materials applications

27. Smart fluid
A smart fluid is a fluid whose properties (e.g., the viscosity) can be changed by
applying the stimuli (electric field or a magnetic field). Rheological materials
are one type of smart fluid.

28. Types of Smart fluids
1-Electro-Rheological fluids
2-Magnetorheological fluids
3-Magnetorheological elastomer fluids
4-Electro-conjugate liquids
5-Photo-rheological fluids

29. History of smart fluids
The properties of smart fluids have been known for the last 60 years, but were subject to only sporadic
investigations up until the 1990s, when they were
suddenly the subject of renewed interest, notably culminating with the use of an MR fluid on the suspension
of the 2002 model of the Cadillac Seville STS automobile.
The word rheology comes from greek words (rheo means flow and logia means study)

31. Applications of smart fluids in biomedical field
-MR device can be used with prosthetics for shock absorption
-Ferrofluids are used as contrast agent for MRIs
-Ferro fluids can be used to encapsulate medications and once injected into the body
the drug can be held in the target area by applying a magnetic field

32. Magnetostrictive materials
Magnetostriction is a property of ferromagnetic materials that causes them to change their shape or
dimensions (expand or contract) during the process of magnetization.

33. History of magnetostrictive materials
Magnetostriction was initially reported by James P. Joule in the mid-1840s. He detected there is a change in the
dimension of iron particles with a change in magnetization. He also discovered magnetostriction behavior in nickel in
1842.
In 1865, Villari discovered the reverse effect i.e. by the application of stress correspondingly change the
magnetization in magnetostrictive material.
Anisotropy behavior by magnetostriction is discovered in single-crystal iron in 1926. In the mid of World War II, Ni-
based magnetostrictive composites were used in transducers for sonar applications. Incorporation of rare earth
metals such as terbium (Tb) and dysprosium (Dy) for magnetostrictive materials was reported in 1965 by Clark, due to
their substantial magnetostriction at low temperatures.

37. All ferromagnetic materials show Magnetostriction to different degree.
Free strain which denoted by lambda or epsilon phi. is the free strain that is in 14 parts
per million in the case of iron. And for cobalt it is 50 parts per million; permalloy 27 parts
per million. The last a new materials are actually designed. And, in these materials the
effect of magnetostriction is actually much-much higher, you know sometimes it is even
100 times more than what we used see in the old magnetostrictive materials.
The other interesting point here is the Curie temperature. As we discussed for
piezoelectric materials, a Curie temperature which beyond that temperature the
material will not show the piezoelectric property. Similar for the magnetostrictive
material also if you exceed this temperatures it would not actually show the
magnetostrictive effect.

40. Actuator - Joule effect (Magnetic field >>Mechanical
strain)

41. Sensor -Villari effect

43. Wiedemann Effect
The physical background to this effect is similar to that of the Joule effect, but instead of a purely
tensile or compressive strain forming as a result of the magnetic field, there is a shear strain which
results in a torsional displacement of the ferromagnetic material.
J =Current density
h15 = magnetoelastic parameter that is proportional to the longitudinal magnetic field value
G = Shear modulus

44. Matteucci Effect
The inverse Wiedemann effect, known as the Matteucci effect, is the change in axial
magnetization of a current carrying wire when it is twisted.

45. Magnetostrictive materials in biomedical field
-Magnetostrictive sensors are commonly used in biomedical applications.
-Ultrasonic waves created by magnetostriction can be used in multiple biomedical
applications:
● Remove kidney stones and brain tumours
● Remove broken teeth
● Ultrasonic cavitation (flush fat from the body)
● Sterilising milk
● Study blood flow
● Detect tumours and cancers and examine foetus growth

46. A copper coil coupled with a microscale Galfenol-silicon film is used to track the bone fracture
healing. The load fracture decreases gradually from the osteosynthesis plate to the bone
tissue as the bone healing process starts which generally creates stress on thin Galfenol film.
The stress generated on Galfenol film affects the inductance of the copper coil. The
inductance in turn can be measured by the electrical network analyzer. The Galfenol film is
very cheap and provides an accurate and wireless method to measure stress and strain.

47. Piezoelectric materials
The word “piezo” stems from the ancient Greek word piezein, which means “to press”
or “to squeeze”. “Piezoelectricity” thus literally means “pressure-induced
electricity”
.

48. History of piezoelectric materials
In 1880 brothers Pierre Curie and Jacques
Curie were working as laboratory assistants at
the Faculty of Sciences of Paris. They
discovered that applying pressure to crystals
such as quartz, tourmaline and Rochelle salt
generates electrical charges on the surface of
these materials. This conversion of mechanical
energy into electrical energy is called the direct
piezoelectric effect.

49. Principle and concept of operation
The main principle of a piezoelectric transducer is that a force when applied on the
quartz crystal, produces electric charges on the crystal surface. The charge thus
produced can be called as piezoelectricity.
A piezoelectric actuator converts an electrical signal into a precisely controlled physical
displacement

50. Molecular Structure of a piezoelectric material

51. Mathematical model of piezoelectric materials
Direct Effect
d= Piezoelectric coefficient
ε^T = permittivity under constant stress T
T= Stress
D= Electric displacement per flux density
E= Electric field intensity
Stress >>> Electric potential (Sensor)

52. Converse Effect

55. Piezoelectric application in biomedical
field
-Can be used in smart shoes to reserve energy by charging devices
-Targeted drug delivery
-Ultrasound waves can be used as a diagnostic and therapeutic tool
-Piezoelectric transducers are also used for ultrasonic dental scalers for removal of scaling

57. Electrostrictive Materials
Both electrostrictive and piezoelectrics
belong to the ferroelectric family.
Piezoelectricity is a first-order effect;
however, electrostriction is a second-
order (quadratic) effect, that is, the
induced strain is proportional to the
square of the applied electric field.
Thus, the induced strain is
independent of the direction of the
applied field and the same deformation
(direction and magnitude) occurs when
the field is reversed.There are very
limited applications using
electrostrictive materials.

58. Shape Memory Materials
The introduction of shape memory technology (SMT) to the materials community was found in
the 1990s. The shape memory material (SMM) has the exceptional property of being able to
regain its original shape when subjected to a particular stimulus. After severe deformation and
quasi plastically distortion, SMMs can regain their original shape in the interaction of specific
stimulus (such as thermal energy, magnetic energy, electrical energy, intensity of light, PH in
solution, stress, etc.)

59. Historical background of SMA
The discovery of the term martensite in steels in the 1890s by Adolf Martens was an important step toward
the eventual discovery of shape memory alloys.
About 42 years later In 1932, Chang and Read detected the reversibility of the AuCd SMA not only by
metallographic observations but also by observing changes in resistivity. Greninger and Mooradian in 1938
observed the shape memory effect in Cu-Zn and Cu-Sn alloys.

60. Gradual development of various SMMS over the years

62. Different fabrication processes of SMAs
● Vacuum Melting
● Powder metallurgy
● Additive manufacturing
● Thermal Spray
● Plasma melting
● Magnetron sputtering deposition
● Post Fabrication

63. Table taken from “An introduction to Modern Materials Science:”

64. Stress strain Temperature plot for Shape memory
effect (SME) in NiTi

65. Shape Memory effect
The shape memory effect (SME) is a peculiar property of SMM and is utilized in commercial applications in a
broad range of industries. The SME is based on the phase transformation between the martensite phase and
the austenite phase that occurs without diffusion. This phase transformation occurs following stress or
change in temperature. By regaining their shape, alloys can produce displacement or force, or a
combination of the two, depending on the temperature.

66. Superelasticity
It is an elastic (reversible) response to the applied stress. Unlike the SME, this property allows SMA to
withstand large amounts of stress without undergoing permanent deformation.The transformation occurs
without temperature change.

67. Governing equations
Transformation temperatures (Ms, Mf, As, Af) can be determined by measuring
some physical properties as a function of temperature, since many physical
properties often drastically change upon starting or finishing martensitic
transformation.

68. The transformation begins to take place below the critical temperature T0, at which the free enthalpies of both phases are
the same. With decreasing temperature, the transformation continues until the temperature Mf . The (Ms- Mf)
temperature interval is an important parameter in defining the memory behavior. T0 is the thermodynamic equilibrium
temperature between the two phases, Gm is the Gibbs free energy of martensite, and Gp is the Gibbs free energy of
austenite. So, the driving force for the nucleation of martensite is:
Gc= Chemical energy
Gs= Structural energy term
Ge= Elastic energy term

69. Shape memory training

70. Shape memory training

71. Relation between Temperature/Stress and strain

72. Different heating methods

73. Different Types of Shape Memory Alloys
-One way SMA
-Two way SMA

74. One way SME Two Way SME

75. SMA Applications in biomedical field

76. References
-https://www.youtube.com/watch?v=zg-VnYdBWKA
-https://www.youtube.com/watch?v=iWjLbiAM2Ro
-https://www.youtube.com/watch?v=8KI8BcIZJ0s
-https://www.youtube.com/watch?v=uHbn7wLN_3k
-Mantha S, Pillai S, Khayambashi P, et al. Smart Hydrogels in Tissue Engineering and Regenerative Medicine. Materials (Basel).
2019;12(20):3323. Published 2019 Oct 12. doi:10.3390/ma12203323
- Trans Tech Publications; Zurich, Switzerland: 2013. Smart hydrogel-based biochemical microsensor array for medical diagnostics
-An introduction to Modern Materials Sccience (Book)