9.1.1 Overview

Related to both life science and material science, biomaterials are natural or synthetic materials that are used for diagnosis, treatment, replacement, or repair of damaged tissues or to enhance function. Current applications cover a variety of biological materials, such as inert or active implant materials, drug delivery materials, dental materials, sutures, wound membranes, catheters, adhesives, and other materials that may have contact with body fluids. In addition, there are sensor materials, probing materials, and electrode materials that can be used in medical diagnostics and medical equipment.

In the context of this chapter, biological materials largely comprise biopolymers, with the rest including metallic materials for medical purposes, bioceramic materials, and biocomposite materials. Depending on function, biological materials can be divided into bioinert materials, biologically active materials, biodegradable materials, and bioabsorption materials.

Biocompatibility is the key to biological materials in research and applications. It refers to the chemical, physical, and mechanical reactions in the interaction of biological materials and organisms, including immune response, blood reaction, tissue reaction, and biochemical reaction.

There are two major types of applications of nanobiological materials. Novel nanomaterials that are developed with functional biological molecules are exploited for their self-assembly properties to create multifunctional materials. Other nanomaterials are suitable for biomedical applications and may or may not have biological activity without causing adverse reactions. The main materials are polymer nanoparticles, inorganic nanoparticles, and the nanostructured biomaterials in tissue engineering with specific identification and orientation-induced functions.

Polymer nanobiomaterials may include nanoparticles, nanomicrocapsules, nanomicelles, nanofibers, and nanopore structured biological materials. Polymer nanoparticles have particle sizes in the range of 1–1,000 nm, with a large surface area and the emergence of new properties and new features different from common materials. Polymer nanobiological materials can be synthesized by using methods such as microemulsion. Polymer nanobiomaterials exhibit their applications mainly in immunoassays, drug and gene carriers, DNA nanotechnology, gene therapy, nanoliposomes (the drug delivery system for imitated biological cells), biomolecular adsorption separation, invasive diagnosis, and treatment. Immunoassay here refers to the quantitative analysis of protein, antigen, antibody, and the cell as a whole. It can be divided into fluorescence immunoassay, radioactive immunoassay, and enzyme-linked analysis by means of the markers used in the process.

Immunoassay operations can be performed through the following three steps: in a specific carrier, covalent bonding corresponds to the immune affinity molecules of the analysis object; solution containing the analysis object is cultured with the carrier; and by detecting the amount of free carriers through a microscope, accurate quantitative analysis can be performed on the analysis object. Immunoassay carriers are usually selected as the polymer nanoparticles, particularly some of the particles with a hydrophilic surface. They have a very small amount of adsorption of nonspecific protein and are widely used as a marker carrier.

9.1.2 Drug and Gene Carrier Nanomaterials

In nanobiomaterial research, the current hotspots are drug nanocarrier and nanoparticle gene delivery technology. This technique is based on nanoparticles as drug and gene transfer vectors, by which the molecules for gene therapy, such as drugs, DNA, and RNA, are wrapped in the nanoparticles or adsorbed on the surface. At the same time, they couple with specific target molecules on the surface, such as specific ligands and monoclonal antibodies. The target molecules are combined with specific cell surface receptors and can find a way into specific cells for safe and effective targeting of gene therapy.

Drug nanocarriers come with the advantages of a highly accurate degree of targeting and high level of drug release control and can improve the dissolution rate of insoluble drugs and absorption rate, drug efficacy, and lower toxicity. Nanoparticles as gene vector have the following significant advantages: they can be used for wrapping, concentrating, and protecting nucleotides from degradation via nucleases; they have a larger specific surface area plus biocompatibility; they easily couple to specific targeting molecules on the surface to achieve specificity of gene therapy; cycle time in the circulatory system is significantly prolonged as compared with ordinary particles and, in a certain period of time, it will not be rapidly cleared by phagocytic cells like ordinary particles; they allow a slow release of nucleotide and effectively extend the time of action and maintain an effective concentration of the product to enhance the transfection efficiency and bioavailability of the transfected products; they have fewer metabolites, fewer side effects, and no immune rejection reactions.

Biodegradability is one of the features that cannot be ignored for drug carriers or gene vectors. Through degradation, carriers and drug/gene fragments can be directed into the target cell, keeping the surface of the carrier biodegradable, while the drugs contained in the core are released to provide a curative effect. In this manner, drug release in other tissues can be avoided.

Drug nanoparticle carrier (nanoparticle drug delivery) technology, as one of the important development directions of nanobiotechnology, will bring great changes to the treatment of diseases like malignant tumors, diabetes, and Alzheimer’s disease. Nanomaterials as drug carriers are largely divided into the following two categories: synthetic polymer materials, such as poly-alkyl cyanoacrylate ester (PACA, including methyl, ethyl, butyl, iso-ester, hexyl ester, as well as dissidents ester, hexadecyl ester) and polyester (mainly including polylactic acid (PLA), polylactide, polycaprolactone (PCL), PCL ester, poly-hydroxybutyric acid, glycolic acid, etc.) and their derivatives and copolymers. Drug-loaded nanoparticles prepared from these materials mainly include the following types.

9.1.3 Bioceramic Nanomaterials

Bioceramic nanomaterial was developed in the mid 1980s. This type of new materials is composed of microstructures at the nanoscale level, with its grain size, grain boundary width, second phase distribution, pore size, and defect size only limited to the order of 100 nm.

Bioceramics are nontoxic, free from side effects, and have good compatibility with biological tissues. They are applicable in the production of artificial bones, bone nails, artificial teeth, dental implants, and nails within bone marrow, ranging from short-term replacement and filling to permanent transplantation. They have been developed from bioinert materials in bioactive materials. Nanobioceramics can enhance biological properties and mechanical properties, showing a unique superplasticity, whereas the increased chemical activity leads to higher biological activity.

Bone-like nanoapatite crystals and polyamide synthetic polymer can be synthesized into a complex as artificial nanobone, which can bond with the natural bones and grow tightly together with human muscle and blood vessels. In addition, it can induce cartilage formation, showing a variety of features equivalent to those of human bones. Bioceramic nanomaterials can also be used in making artificial eyes; by using nanoceramic material in the making of the shell of the construct, the artificial eye can be controlled by the muscles to move around, or electrical pulses are applied to stimulate the nerves of the brain to achieve the visual effect.

Bioceramic nanomaterials can be used for nanocell separation technologies, cell staining, as well as antibacterial function and sterilization. In nanocell separation, the first step is to produce amorphous SiO₂ nanoparticles, with the size controlled within 15–20 nm. Its coated monomolecular layer works as the adhesion layer, and the finished size of coating is approximately 30 nm; then, in the second step, PVP colloidal solution containing multiple cells is produced by uniformly mixing the two and using centrifuge technology to separate out the desired cells. The intracellular staining technique requires selection of the type of antibody. Nanogold particles are mixed with prepurified antibodies or the monoclonal antibody to prepare a variety of (nanogold antibody) complexes; respectively, composite particles are combined with various intracellular organs and skeletal systems to form different complexes, by which certain featured colors will be rendered in the irradiation of white or monochromatic light, and thus different colored labels are affixed to various combinations.

Once biological materials are used in the human body, the surrounding tissue may be at risk of infection. Therefore, antibacterial operation and sterilization are especially important. In the reaction to synthesize hydroxyapatite (HA) nanoparticles, silver, copper, or another aqueous solution of soluble salts is added in the reaction, so that antibacterial metal ions are able to get into the apatite crystals. A typical representative of nanomaterials with bactericidal or antivirus features is known as photocatalyst titanium dioxide (TiO₂) and works only in ultraviolet radiation, as described in greater detail in Chapter 10.

9.1.4 Magnetic Nanoparticles

Magnetic nanoparticles have a good surface effect, which is characterized by surge of specific surface, larger functional group density, and selective adsorption capacity, so the capacity for carrying drugs or genes is increased. In the physical and biological sense, paramagnetic or superparamagnetic iron oxide nanoparticles may experience a temperature increase up to 40–45°C under the influence of an external magnetic field, which can kill tumor cells.

Practical application of magnetic nanoparticles for biomedicine covers the following aspects: coating magnetic nanoparticles with polymer materials and combining them with targeting moieties. Then, they can be used as drug carriers in the human body. Under the action of an external magnetic field, magnetic navigation is applied to move the carriers to lesions to complete the treatment; 10–50 nm of the Fe₂O₃ surface coated with methacrylic acid may have its size increased to 200 nm and is able to carry proteins, antibodies, or drugs for cancer diagnosis and treatment. Its local treatment effect is good, with few side effects.

9.1.5 Biocomposite Nanomaterials

Inorganic–organic nanocomposite materials are most commonly found in biological organisms. Biomineralization refers to the process of minerals (biominerals) being formed inside a living body. In the biomineralization process, through the interactions on the interface of organic macromolecules and inorganic ions, inorganic mineral precipitation is controlled at the molecular level, so that biominerals have a special multilevel structure and assembly modes. During the process of biological mineralization, cell secretion can complete the auto-assembly of organic matter, which acts as a template for the formation of inorganic compounds, so that inorganic mineral has a certain shape, size, orientation, and structure.

To a large extent, the manufacture of nanobiological materials is inspired by the biomineralization process. In nature, the cell membrane of some bacteria may be mineralized at different levels. Outside the cell membrane, the protein molecules in regular arrangements can be used as a template to induce the synthesis of microstructured nanomaterials. Biomineralization provides an effective means for the design and processing of nanobiomaterials. US scientists have found a gender-peptide molecule; one end is the hydrophilic arginine-glycine-aspartic acid (RGD), and the other end is amino acids containing phosphorylated residues. RGD helps the adhesion of materials and cells, whereas the phosphorylated amino acid residue can interact with calcium ions.

Tissue engineering is the research and development of organism alternatives aimed at restoring, maintaining, and improving organizational functions by the application of the basic principles and methods of engineering and life science. The idea is first to isolate cells in vitro for culturing, and then a certain amount of the cells are grown onto a three-dimensional biological material to be cultured further; eventually, some tissues and organs with a certain structure can be formed. Stent materials play an important role in tissue engineering, because adherent-dependent cells are only adhesive to the materials before they can grow and divide. By imitating the natural extracellular matrix-collagen structure, nanofibers containing biodegradable materials are used in vitro and in animal experiments in tissue engineering; they demonstrate an attractive application potential. In composition, the currently reported nano-HA/collagen composite is an imitation of inorganic and organic components in natural bone matrix, with a nanoscale microstructure similar to natural bone matrix. The 3-D stent formed from porous nano-HA/collagen composite provides a microenvironment similar to the body for osteoblasts. Cells are able to grow well and secrete the bone matrix on such stents. Experiments in vitro and animal experiments show that such an HA/collagen composite is a good nanobiological material for bone repair.

As the biodegradable polymer material for tissue regeneration template, nanostructured tissue engineering stent material has the following properties: (1) good biological compatibility; (2) cells having good adsorption and proliferation at the material surface; (3) capability of inducing cell growth by prefabricated patterns; and (4) after tissues are grown, the materials can be degraded and excreted through metabolism (Table 9.1).

9.2.1 Nanobioinorganic Materials

Nanoinorganic biological materials mainly include ceramic oxides, alumina, titanium dioxide, porous glass granules, nanocarbon materials, nanosilicon oxide (SiO₂), and nanomagnetic particles.

9.2.2 Nanoorganic Biological Material

9.2.3 Nanotechnology in Drugs

Nanomedicine is essentially a nanocomposite material, which is the nanostructural system assembled and synthesized according to the design of the human body. They are made of nanowire or tubes as the basic unit that is assembled and arranged in one-dimensional, two-dimensional, or three-dimensional spaces, and are associated with stability, gastrointestinal irritation, toxic side effects, high drug utilization, targeted drug delivery, and sustained-release effect.

We know that smaller drug particles have a greater dissolution rate than larger drug particles. So, by controlling drug particle size, we can control the dissolution rate of particles. For water-soluble drugs as oral tablets made of granules, particle size is the key to controlling a drug’s pharmacological effect. For nonwater-soluble drugs, stable water suspensions can be made for subcutaneous injection into the body and can travel with the blood circulation. In this case, the size of drug particles should be controlled very strictly to not block the blood circulatory system. For gas sol–spray pharmaceutical agents, particle size is the key factor determining its effectiveness. The current pharmaceutical particles can only reach the micron level.

In applying nanotechnology in pharmacy, QDs and magnetic nanomaterials have enjoyed a high priority in recent years. QDs are nanoscale metals and can be used as novel fluorescent labeling materials because of their light-emitting characteristics. Magnetic nanomaterials are mainly iron oxide nanomaterials with excellent biosecurity and biocompatibility. Because the nanoscale can lead to an improvement of magnetic properties, iron oxide nanomaterial has become an independent development in the field of biomedical testing and treatment.

9.2.4 Biochips

The biochip is an integrated assembly of one or a variety of biological activities on a small surface; by using tiny samples of physical or biological matter, it can simultaneously detect and study different biological cells, biomolecules, and DNA characteristics, as well as the interaction between them to acquire the life patterns of microactivity. The biochip can be approximately divided into a cell chip, protein chip (biological molecules chip), and gene chip (A-chip), all of which have the advantages of integration, parallel and rapid detection. It has become cutting-edge biomedical engineering technology in the twenty-first century. DNA chip, a representative of the biochip, has been developing rapidly recently and is typically symbolized by nanotechnology, integration, and multifunction. The scope of its use covers molecular biology, disease prevention, diagnosis and treatment, drug development, the development of biological weapons, judicial appraisal, and monitoring and supervision of environmental pollution and food hygiene.

The biochip in a narrow sense is a microarray, including DNA microarrays, oligonucleotide arrays, protein microarrays, and microarrays of small-molecule compounds. Addressable elements are fixed by way of a lattice to the surface of a certain size of substrate (silicon, glass, plastic, etc.). Each dot in the lattice can be regarded as a sensor probe. Biochip operation typically includes the following steps: first, the object of detection can be labeled using a chemical fluorescence method, enzyme label method, isotope marking method, or electrochemical method; second, the labeled object of detection getting into the biochip will respond at the appropriate address, and this specific address will be displayed; third, a scanner is used to search for biochip marks on the display for analysis with calculating software.

Biochips, in a general sense, can be used for rapid parallel processing and analysis of biological components or molecules. They are a kind of solid-thin device with a size of less than a cubic centimeter. Typical biochips include the microarray chip, filtration separation chip, dielectrophoresis separation chip, biochemical reaction chip, and capillary electrophoresis chip.

Instruments commonly used for observing and testing the biochip are the transmission electron microscope and scanning probe microscope. Transmission electron microscopy is required to be conducted in a high vacuum to observe dry samples. There are many types of scanning probe microscope, such as scanning tunneling microscope, atomic force microscope, magnetic force microscope, and scanning electrochemical microscope.

9.2.5 Future Development of Nanobiomedical Materials

The development of biomedical materials at the nanoscale is mainly focused on nanorobots, nanotargeted drugs, invasive diagnostic capabilities and intelligence, drug delivery systems, and medical complex material.

9.5.1 QDs in Biological and Medical Analysis

Fluorescence is a widely used analytical method, particularly in the clinical determination of the content of certain elements in biological samples. The traditional fluorescent labels’ absorption and emission wavelength are often restricted to a narrow range, with the intensity in continuous decline. If we use fluorescence spectroscopy directly to study them, then the bases and nucleic acids may have very low fluorescence quantum efficiency. Only tryptophan, tyrosine, and phenylalanine have natural fluorescence, so the best way to detect them is to use a variety of fluorescent probes.

At present, the fluorescent agent in such probes is an organic dye; however, in most cases, their excitation spectra are narrow, so it is difficult to stimulate a variety of components at the same time and the distribution of the spectra is asymmetric. The most serious flaw is the poor photochemical stability. QDs can absorb a wider range of wavelengths of light and emit a single specific wavelength. QDs have a structure of approximately hundreds to thousands of semiconductor atoms, with the size generally within 10 nm. This gives QDs atomic-like discrete energy levels. After QDs absorb the photon irradiation, an electron can skip the energy gap to an excited state. When falling from the excited state back to the ground state, the appropriate wavelength of photons can be launched. It is possible to design the QDs in different light colors. QDs can be created using group II–IV, CdSe, CdTe, CdS, and ZnSe materials, but also can be formed from group III–V, InP and InAs materials. In 1993, Bell Labs developed a highly efficient synthesis of light-emitting semiconductor QDs, thus beginning the utilization of QDs.

Compared with conventional fluorescent probes, the excitation spectra of QDs are wide and distributed continuously; that is, nanocrystals of different sizes can be excited by a single wavelength of light and emit the light in different colors, while the emission spectrum is narrow and distributed symmetrically. Meanwhile, it has higher photochemical stability and is resistant to photolysis. QDs as fluorescent probes have shown broad application prospects in the field of biomarkers and diagnostics.

QDs generally used in the biochemistry and medical fields function via QD bioconjugation. It is based on QDs as the core, followed by the formation of an organic outer layer by way of coating or chemical modification, which then combines with biological molecules to form a conjugate.

Bioconjugated QDs can be made in the following ways: the use of bifunctional base cross-linking agent for modification with a ZnS disulfide chain; hydrophilic adsorption; surface silylation; surface ionization; and micronanospheres.

QDs highlighted in the research are mainly composed of group II–VI or III–V elements (Table 9.2).

A QD is usually a core–shell-type nanobody, with CdSe as the nucleus and CdS or ZnS as the shell. Compared with traditional organic dyes, it has unique properties; QDs feature a large Stokes shift and a narrow, symmetrical fluorescence spectrum (Figure 9.4).

QDs have a broad range of applications, including a variety of fields and instruments, but their greatest application is for cell and disease detection. Compared with ordinary fluorescent substances, QDs are presented with a strong luminescent capacity and a longer light-emitting time. Nontoxic and easy to detect, they have a broad absorption wavelength range but only emit a single wavelength. They are valued as a future hot commodity to supersede fluorescent materials. QDs are available with a variety of colors in light emission. The color depends on the size of QDs. For the same excitation wavelength, there will be a range of excitation lights qualified for simultaneously detecting multiple targets. Other features of QDs include resistance against light-induced bleaching, safety, low toxicity to cells, suitable for living cells and in vivo studies, and long fluorescence (the fluorescence time is thousands of times longer than that of ordinary fluorescent molecules, which facilitates long-term tracing and storage of the results).

QDs can be used for ultra-sensitive detection of nonisotope-labeled biological molecules. For example, the surface of QDs can be connected to mercaptoacetic acid (HS–CH₂COOH), so that QDs not only can be water soluble but also are able to be combined with biological molecules (e.g., proteins, multipeptides, nucleic acids). Then, QDs can be detected by photoluminescence, thus enabling biomolecular recognition of some specific substances (Figure 9.5).

QDs of different fluorescent characteristics can be combined into hollow polymer balls, creating fluorescent nanoparticles with different spectral characteristics and brightness features available to be labeled to biological macromolecules. Taylor and associates once used nanoparticle-labeled protein to determine straightened individual DNA molecules, where EcoRI enzyme could be combined with the fluorescent nanoparticles at a size of 20 nm through amide bonds and identify the specific sequence of double-helix DNA molecule through 12 hydrogen bonds.

Temperature may directly affect the size of QD particles. In general, the higher the T, the smaller the QD particles obtained, and the shorter the fluorescence wavelength. So, QD particles of different sizes can be displayed in different colors (Figure 9.6). This character can be used to track the activity and proliferation of amino acid receptors in nerve cell membranes.

The diversity of QD colors is able to meet the requirements of analyzing biological polymers (protein, DNA) that contain vast amounts of information. Polymers and QDs can be combined to form polymer beads, which are free to carry QDs of different sizes (colors) and begin to emit on irradiation. Transmitted through prism refraction, a variety of spectral lines of specified density (bar codes) can be formed. This type of bar code has a promising application potential in gene chip and protein chip technology.

QDs can also be used to detect the characteristics of DNA and proteins. Researchers have been able to bring a mixture of a variety of QDs encapsulated into rubber balls of millionths of a meter in diameter each, and they can radiate different colors of light. Researchers can use these rubber balls as different markers of gene sequences or antibodies to identify different DNA or antibody proteins. This provides a new approach to further probe the nature of DNA or antibody proteins. In addition, the hydrophobic modified polyacrylic acid used to coat QDs, combined with immunoglobulin G and streptavidin–biotin, can acquire accurate results for marking on the cell surface protein, cytoskeleton proteins, and the proteins in the nucleus. Its antibleaching performance is often of great value for the quantitative detection of fluorescent molecules and biosimulation of living cells.

Detection of tumor cells with QDs is also one of the hotspots in the current studies. Scientists may have the transferrin and QDs covalently cross-linked, so that cervical cancer cells can be “swallowed” into the cell. Then, QDs connected with transferrin still have biological activity to achieve long-term observation of single-color fluorescence labeling. They use two kinds of QDs of different sizes in marking the mouse fibroblasts: one emits green fluorescence and the other emits red fluorescence. Meanwhile, the QDs emitting red fluorescence are specifically marked on the cell actin filament, while QDs emitting green light are combined with urea and acetic acid. Such QDs have high affinity with the nucleus, and simultaneously the red and green fluorescence in cells can be observed to achieve the two-color fluorescent labeling.

Also, other applications of QDs in biological detection have been frequently reported. For instance, QDs were linked with biotin, urea, acetic acid salts, and certain antibodies, and they successfully achieved specific cell structures. A variety of molecules can be used as guidance materials for QDs, including nucleic acids, lipids on the cell membrane, proteins closely connected with the carrier protein or carrier sugar, as well as some drugs, by which QDs can be guided to a specific cellular structure. Researchers are committed to the application of QDs in neurotransmitter studies (for the understanding of neural signal transduction). They marked the QDs on an important neurotransmitter, 5-hydroxytryptamine, and then observed how the transporter protein in the process of promoting neurotransmitters returned to the cell after the intermittent signal was transmitted through adjacent nerve cells. This can be applied to medical imaging technology.

In general, because of its unique marking characteristics, QD technology will certainly develop into a cutting-edge technology for future biomolecular detection, providing a more advanced approach for DNA testing (DNA chips), protein detection (protein chips), and the exploration of the mechanisms of protein–protein reactions (antigen–antibody, ligand–receptor, enzyme–substrate). Furthermore, this technology will greatly enhance the development of bioimaging technology and biopharmaceutical technology, leading to a huge step forward for disease diagnosis and treatment.

9.5.2 QDs for In Vivo Studies

Early detection of cancer is a major issue in the modern-day medical profession. Traditional detection methods required the tumors to reach a certain size before they could be discovered, and the best treatment is often adversely affected by time. The ideal situation is when just a small amount of cancer cells arise in a local area, and they can be detected and treated.

Different CdSe/ZnS core–shell-type QD surfaces modified with different peptides can be injected into mice for postbiopsy analysis. The results showed that different peptide-modified QDs can be specifically applied to the lungs and vascular system of a normal or a tumor-afflicted mouse. These results indicated that QDs can possibly be utilized in studies of disease diagnosis and drug delivery (Figure 9.7).

Recently at the University of Cincinnati, Professor Donglu Shi and colleagues developed a multifunctional nano combination device [8,9]. Such nanoscale structure uses nanotubes as a substrate. After surface treatment, the outer surface of nanotubes can be coupled to connect QDs, which can be used to partially trace cells in vivo. Due to the strong luminance, QDs can be used for the imaging of the depth tissues (in vivo imaging) and characterization. After a special plasma coating, the nanotubes are loaded with an antibody that can indentify cancer cells on the outer surface to complete the so-called targeting role. The interior of the hollow nanotubes can be used to store anticancer drugs, which can be transported to the vicinity of cancer cells for controlled release to kill cancer cells, thus achieving a local treatment effect. This new method is far superior to conventional chemotherapy.

9.6.1 Background of Hyperthermia

After surgery, radiotherapy, and chemotherapy, the hyperthermic treatment of tumors has become a novel “green therapy” in treating cancer. Hyperthermia used as a treatment modality has a long history that can be dated back to 5,000 BC. It is said that breast tumors treated with heat were documented in the manuscripts of a Danish doctor named Edwin Smith. Legend has it that Hippocrates, the famous ancient Greek doctor, had used heat therapy in treating tumors and he had a proverb: “The disease which cannot be cured with iron can be cured with fire; the disease that fire cannot cure is incurable.”

In 1884, Bruns reported that a case of advanced melanoma infected with erysipelas was accompanied by high fever of 40°C; a few days later, the tumor disappeared. This contributed to survival of 8 more years. Later, people began to use artificial bacterial infection or the injection of a chemical-induced heat source to cause the patients to have a high fever. The most renowned research was performed by Coley, who published a number of articles in 1893 that described his artificial methods of infection-induced fever of 38–42°C. As a result, 12 of the reported 38 cases of advanced cancer were cured, which caused a sensation.

Nineteenth-century German scholars have presented many reports and literature on hyperthermia. Heating technology was then rudimentary and even used inferior methods such as hot needles, burning surface lumps with a small hot iron, or soaking limbs with hot water and local infusion with hot water for heating, and so on. With the application of the electric knife since the nineteenth century, Westermark took the initiative to use radiofrequency coils as radiators for hyperthermia in cervical cancer. After World War II, the microwave technology developed rapidly. In the 1960s, more systematic studies were performed regarding the heating technology in electromagnetic hyperthermia. Despite a long history of hyperthermia, its development was limited as a result of underdeveloped science and technology and backward heating methods and equipment. Until the 1970s, with the rapid development of electronic technology and multidisciplinary involvement and coordination, modern hyperthermic oncology gradually formed its own disciplinary system on the basis of a large amount of basic and clinical research. Ultimately, it developed itself into an emerging discipline integrating oncology, biothermal methods, thermal physics, and electrical and mechanical disciplines with computer and other technologies.

In recent years, the United States, Russia, and other European countries have performed a wide range of research on tumor hyperthermia, showing that hyperthermia is indeed effective against cancer. Hyperthermia in China began in the late 1970s and is currently available in forms of microwave hyperthermia, focused ultrasound hyperthermia, radiofrequency hyperthermia, and the latest generation of endogenous field hyperthermia. Microwave heating can only be applied to superficial cancer treatment because of its shallow penetration depth. Focused high temperature of ultrasound hyperthermia may be 90°C or more. With a higher penetrating power, it can be used to treat deep tumors. However, the limited nature of ultrasound prevents it from either penetrating the gas-bearing tissues or passing through bones, and therefore it is not a candidate for the treatment of lung and esophagus cancers or liver cancer that is blocked by ribs. Moreover, ultrasound must also rely on water as the mediator, because it cannot reach the tumor without going through water on the surface of the focus. Therefore, the location of the disease plays a decisive role and the cost of this kind of therapy is high. Radiofrequency hyperthermia can also be used to heat deep tumors, but it has to be applied through local water cooling. Meanwhile, subcutaneous fat is prone to overheating and pain, resulting in rapid development and difficulties in operation. Endogenous field hyperthermia is the latest generation of the hyperthermia system that is a combination of the advantages of various types of hyperthermia at home and abroad, without the need of water cooling. Patients can be safe and comfortable in a supine position, and this therapy is applicable to the treatment of tumors in the chest, abdomen, pelvis, and other deep positions. Its significant effect has greatly enhanced the patient’s quality of life.

Hyperthermia is the use of a variety of physical energy (e.g., microwave, radiofrequency, ultrasound) to produce thermal effects, so that the tissue temperature increases to the treatment temperature of 43°C and above to accelerate cell death. The role of hyperthermia in tumors includes the destruction of tumor blood vessels, tumor blood clots, and the inhibition of tumor angiogenesis. Statistics show the following: between 39°C and 40°C, the growth of most tumor cells is significantly inhibited; between 40°C and 41.5°C, tumor cell survival rate decreased rapidly; between 41.5°C and 43°C, tumor cells will die within a short period; and between 70°C and 120°C, the tumor cells burst instantaneously. Therefore, hyperthermia is a highly effective means of cancer treatment; it is safe, noninvasive, and is not associated with toxic side effects and infection. Currently, hyperthermia can be divided into two categories: whole body hyperthermia and local hyperthermia. The operation of whole body hyperthermia is less used because it is complicated and would cause strong systemic reactions, producing a certain degree of risk. Currently, local hyperthermia is the most applicable clinical tumor therapy; the most frequently used heat sources include IR, hot water bath, hot bath, ultrasonic waves, radiofrequency, microwave, and so on. Some experimental studies have shown that local hyperthermia not only can directly kill tumor cells but also can enhance the immune function. This possibly results from the high temperature applied locally that may lead to tumor degeneration and necrosis, and the absorption of necrotic tumor products will stimulate the body’s immune function.

There is a need for comprehensive treatment strategies rather than a single treatment of cancer, whereas surgery, radiotherapy, and hyperthermia are all local treatments. Systemic treatment with drugs, for example chemotherapy with traditional medicine, can provide better efficacy. Cancer is a systemic disease. If a local tumor has grown to a volume of 1 cm³, then other parts of the body may have small metastases that cannot be seen with the naked eye. Even with surgical excision, radiotherapy, or hyperthermia, there is still a need for coupling with systemic drugs before residual cancer cells in vivo can be wiped out, so that they cannot be left behind as the scourge of metastasis or have any recurrence in some parts of the body. Because hyperthermia is not a radical means and needs to be combined with systemic chemotherapy or traditional Chinese medicine treatment, efficacy could be enhanced by several times or even dozens of times. Here, the drug amounts are only one-third or one-half of the commonly used dose; therefore, the toxic side effects of the drugs are significantly reduced, whereas the treatment efficacy can be improved greatly. Patients with cancer are happy to accept such treatment because it can greatly improve their quality of life and allow new hope.

In this section, we highlight the use of nanomagnetic materials for hyperthermic treatment (Figure 9.8).

9.6.2 Magnetic Hyperthermia [10–13]

In magnetic hyperthermia, magnetic particles are transported to the treatment area; in an external alternating magnetic field, a hyperthermic effect is generated by the heat due to magnetic loss of the magnetic particles.

Magnetic hyperthermia has the following advantages. First is targeting within the tissues, and second is the bystander effect of heat treatment. A very good biocompatibility of materials can work to improve the efficacy of cancer chemotherapy or radiotherapy.

The main types of magnetic hyperthermia include embolism magnetic hyperthermia, liposome magnetic hyperthermia, intracellular hyperthermia, and whole body hyperthermia.

9.6.3 Magnetic Materials for Hyperthermia

Magnetic materials for hyperthermia are required to have a high specific absorption rate (SAR). Used in the human body, their biological safety must be guaranteed. In addition, the product performance should maintain certain stability. At present, common magnetic materials for magnetic hyperthermia include the following three kinds: Fe₃O₄; γ-Fe₂O₃; and carbonized iron, iron powder, and glass–ceramic iron oxide. These magnetic materials can be synthesized using physical or chemical methods. Physical methods are the colloid mill, jetting, impact, and tear-ultrasonic methods, whereas chemical methods include coprecipitation, precipitation oxidation, sol–gel, and metal organic decomposition.

Magnetic materials for magnetic hyperthermia have SAR related to the particle size. The SAR is high when the particle diameter is approximately 10 nm; if the particle size is between 100 nm and 1 μm, then the SAR is low. To improve the SAR, surface modification is typically needed for the magnetic materials to be prepared in magnetic fluids. There are two common surfactants used for the surface modification: water-based magnetic fluids (dextran, polyvinyl alcohol, PEG) and oil-based magnetic fluids (oleic acid, polyester).

9.6.4 Thermogenesis Mechanism of Magnetic Materials for Magnetic Hyperthermia

In the magnetization process and the magnetization reversal process, part of the energy in magnetic materials would be irreversibly transformed into heat and the loss of energy is called magnetic loss. Therefore, for magnetic materials for magnetic hyperthermia, the heat is sourced from the magnetic loss. Magnetic loss W_m includes the eddy current loss W_e, hysteresis loss W_h, as well as other loss arising from the residual magnetic relaxation or magnetic after-effect W_r, that is, W_m=W_e+W_h+W_r. Under normal circumstances, magnetic losses in ferrites are mainly residual loss and hysteresis loss; in metallic magnetic materials, mainly eddy current loss and hysteresis loss occur.

For a magnetic conductor in an alternating magnetic field, electromagnetic induction may generate eddy currents. This causes magnetic field strength H and magnetic induction intensity B to have an uneven distribution of amplitude and phase inside the materials. Meanwhile, this will make the phase of B lag behind that of H, thus increasing part of the energy loss, known as eddy current loss. Experimental studies show that for some metallic magnetic materials, the measured magnetic loss is much greater than the sum of theoretical calculations of the eddy current loss and quasi-static losses. The difference between experiment and theory causing the additional loss is called abnormal loss. Abnormal loss comes as part of the micro-eddy current induced in the vicinity of the domain wall by electromagnetic induction in the domain wall movement; it also partially results from the deformation of domain walls or domain wall pinning. It is noteworthy that abnormal loss accounts for a large part of the total loss in a number of metallic magnetic materials (e.g., silicon steel sheet).

Hysteresis loss comes from the irreversible magnetization process existing in the magnetic material (the irreversible displacement of the domain wall and irreversible dynamic behavior of the magnetic domain). In the quasi-static magnetic case, the hysteresis loss is proportional to the area of the hysteresis loop. In medium and strong alternating magnetic fields, the hysteresis loss of some metal-based magnetic materials is suitable for empirical formula Steinmetz $W_{\mathrm{h}}=\eta B_{\mathrm{m}}^{n}f$, where f is the frequency and η and n are material-related constants. For example, for 3%Si–Fe alloy, $n\approx 1.6$ and $\eta \approx 1.2\mathrm{J}/({\mathrm{m}}^3·\mathrm{T})$.

Residual loss means all other losses apart from eddy current loss and hysteresis loss. It is caused by the magnetic relaxation processes in different mechanisms. In the low-frequency and weak magnetic fields, the residual loss is mainly magnetic after-effect loss and has nothing to do with the frequency. High-frequency residual losses include losses caused by size resonance, domain wall resonance, and the natural resonance. The residual loss dominates in ferrites.

Residual loss caused by magnetic after-effect is proportional to the frequency, domain wall displacement, and rotation damping of the magnetization vector. This loss is divided into two categories: Richter type and Jordan type. The former is related to temperature and frequency; the latter is slightly dependent on temperature and frequency. Richter-type loss is mainly caused by the induced anisotropy generated by the proliferation of impurities. Jordan-type losses are mainly caused by thermal fluctuations. Ferrite Richter loss is due to the proliferation of valence electrons between the ions.

In the high-frequency and ultrahigh-frequency areas at 10⁴ Hz and above, the imaginary component of permeability on the ferrite spectrum related to magnetic loss μ″ may present several absorption peaks in different frequency regions. They correspond to the resonance loss, which is also a kind of relaxation loss. As the frequency increases, these absorption peaks will be caused by size resonance, domain wall resonance, natural resonance, and natural exchange resonance.

Studies have shown that magnetic particles approximately 10 nm in diameter will mainly have relaxation loss, showing a higher SAR; and hysteresis loss mainly occurs in magnetic particles of 100 nm–1 μm diameter, showing a lower SAR rate.

Two examples of the application of magnetic hyperthermia are described. Shinkai and colleagues used cationic liposomes (including Fe₃O₄ with a particle size of 10 nm) in the hyperthermia of rat glioma. Tumors in more than 80% of the rats disappeared completely. Brusentsov and associates used dextran-coated γ-Fe₂O₃ magnetic fluid (including γ-Fe₂O₃ with a diameter of 10 nm) in the hyperthermia of MX2 tumors in rats. Tumors in 33% of the rats showed complete recession, with the term of survival being extended by 150%.

Although the use of nanomaterials promoted the development of magnetic hyperthermia, there are still many problems in the application of magnetic hyperthermia. First is the need to improve the targeting of hyperthermia, the SAR of the materials, and the degree of biosecurity. It must be combined with other tumor treatment methods to achieve a satisfactory effect. Also, we must consider that the magnetic fluid should have the minimal residue in the body after treatment is completed. In general, nanomagnetic materials can be naturally excreted by the body. Furthermore, we can take advantage of the so-called magnetic field guidance technology to exclude the potential residual material. In addition, after the magnetic fluid enters the body, part of the fluid will inevitably find a way into normal tissue cells. We also need to conduct further studies on the impact of nanomagnetic materials on normal tissues, that is, their side effects.