Forget about little green men, the future of life might be a lot closer to home than we think. Silicon, the element that powers our computers, has long been the subject of science fiction fantasies about silicon-based life. But could these fantasies become reality?
The Carbon vs. Silicon Showdown:
Life on Earth is built on carbon. It’s the backbone of our DNA, proteins, and everything that makes us, well, us. But silicon, its neighbor on the periodic table, is surprisingly similar. It’s abundant in the Earth’s crust and can form four bonds, just like carbon. So why didn’t life choose silicon?
Silicon’s Challenges:
Turns out, silicon has some major drawbacks. Its bonds are weaker and less stable than carbon’s, especially in water. Silicon also struggles to form double and triple bonds, limiting the complexity of silicon-based molecules. And let’s not forget about silicon dioxide, the equivalent of carbon dioxide, which is a solid, making it difficult for silicon-based life to metabolize.
The Rise of Silicon Intelligence:
Despite these challenges, silicon has become the foundation of modern technology. It’s the heart of our computers, smartphones, and the entire digital world. Silicon’s ability to control the flow of electricity has led to the creation of transistors and integrated circuits, the building blocks of modern computing.
Silicon Brains: The Dawn of a New Intelligence?
With the rapid development of AI, silicon-based intelligence is rapidly evolving. AI systems are now capable of complex tasks, from medical diagnosis to financial analysis, even surpassing human capabilities in some areas. This raises a fascinating question: Could silicon-based intelligence eventually develop its own form of consciousness?
The Future of Silicon and Life:
While the possibility of silicon-based life in the traditional sense remains uncertain, the rise of silicon intelligence challenges our understanding of life itself. We are creating a new form of intelligence, powered by silicon, and it’s a force that will undoubtedly shape the future. Whether it’s a new form of life or a powerful tool for humanity, silicon is poised to play a critical role in the evolution of our world.
Bonus Fun Fact: The first transistors were actually made from germanium, not silicon. But germanium is much rarer and less stable than silicon, making silicon the clear winner for powering our digital world.
Silicon and carbon, both in Group 14 of the periodic table, play very different roles in life on Earth. All life forms on Earth have chosen to be based on carbon, while silicon, which is extremely abundant, has not played a role in the evolution of life.
In recent years, with the rise of artificial intelligence, silicon-based intelligence is gradually showing its potential. Is it possible to surpass carbon-based intelligence in the future?“
“Whether the brain is made of carbon or silicon, if its complexity enables it to show signs of consciousness, then this also gives it a subjective inner life.”
——Raymond Kurzweil (American author and inventor, current director of engineering at Google)”
Silicon is the second most abundant element in the Earth’s crust, second only to oxygen, accounting for 27.7% of the Earth’s crust. If there are alien life forms with basic knowledge of chemistry, when they observe the Earth from a distant place in the universe, even if they cannot detect the existence of humans, they should easily find that the Earth is a planet occupied by silicon oxide and water.
Human beings have a long history of using silicon-containing materials. The ancient Egyptians used quartz sand to make crystal clear glass, and the Chinese used clay to make exquisite porcelain. The wisdom left by our ancestors witnessed the indissoluble bond between humans and silicon. However, since silicon is very difficult to purify, the road to the discovery of silicon was full of ups and downs. It was not until 1823 that the Swedish chemist Berzelius heated potassium fluorosilicate and potassium together, and cleverly used water to remove the impurity potassium silicide, that he obtained high-purity silicon for the first time. At this time, rare and precious metal elements such as iridium, platinum, and rhodium had already been discovered by humans.
High-purity silicon has a silvery luster, which made Berzelius mistakenly believe that silicon was a metal, so he created the word silicium to name silicon according to the Latin rules, where -ium is usually a root word for metal elements. Later, people realized that the physical and chemical properties of silicon were closer to carbon and did not belong to metal, so they removed the -ium root and used the word silicon proposed by British mineralogist Thomas Thomson instead.
The evolution of silicon’s naming reflects the changes in human cognition of silicon, and the similarity between silicon and carbon has also inspired many rich imaginations about silicon-based life.
Why didn’t life on Earth choose silicon?
Looking at all life forms on Earth, from tiny bacteria to giant blue whales, the key molecules in all life forms – including nucleic acids, proteins and lipids – are built with carbon atoms as the basic skeleton .
Silicon and carbon belong to the same group 14 in the periodic table and should have similar physical and chemical properties. Moreover, the abundance of silicon in the earth’s crust is significantly higher than that of carbon (the latter only accounts for about 0.04% of the mass of the earth’s crust). This raises a natural question: Why didn’t life on Earth choose silicon? Is it possible for silicon-based life to exist?
As early as 1891, Julius Scheiner, an astrophysicist at the University of Potsdam in Germany, first proposed the concept of silicon-based life in a paper. Subsequently, British chemist James Emerson Reynolds further developed this theory in a scientific lecture in 1893. He specifically pointed out that the excellent thermal stability of silicon compounds may enable silicon-based life to survive in high temperature environments.
These hypotheses inspired science fiction writers, and silicon-based life became a classic theme in science fiction literature.
Among the many works, Stanley G. Weinbaum’s “Mars Odyssey” published in 1934 is a milestone. The novel describes a strange Martian creature called Tweel, which is mainly composed of silicon and has an amazing lifespan of 500,000 years. In particular, the author designed an imaginative metabolic model for this creature: every ten minutes, Tweel will excrete a brick. This seemingly bizarre setting contains reasonable speculation based on the knowledge of elemental chemistry: since the metabolic product of carbon-based life (such as humans) is gaseous carbon dioxide, then the metabolic product of silicon-based life is likely to be solid silicon dioxide.
However, as modern chemical research deepens, scientists have gradually realized that the early conception of silicon-based life may be too idealistic. The limitations of silicon in chemical properties make it difficult for it to play an important role as a basic element of life.
Life is essentially a delicate chemical system that requires a delicate balance between molecular stability and reactivity. Although silicon atoms have four valence electrons like carbon atoms and can theoretically form four covalent bonds, silicon atoms have one more electron layer than carbon atoms, which significantly weakens their control over the outermost electrons. This difference in electronic structure makes the bond energy of the Si-Si bond much lower than that of the CC bond, and the long chain structure formed by silicon atoms is unstable, especially in aqueous solution, where water is the cradle of carbon-based life on Earth. Not only that, silicon atoms also find it difficult to form stable double and triple bond structures like carbon atoms. These have severely limited the diversity and complexity of silicon compounds.
On the other hand, the combination of silicon and oxygen goes to another extreme: the Si-O bond is extremely strong (the bond energy is as high as 452 kJ/mol), which makes silicon dioxide a highly stable solid substance that is difficult to participate in the metabolic cycle of organisms like gaseous carbon dioxide. Therefore, although the “creatures that exhale bricks or sand” depicted in science fiction works are very imaginative, from a chemical point of view, the possibility of such life forms existing in reality is very small.
If we look beyond Earth, is there a silicon-based life form in interstellar space? This question remains unresolved. However, existing astronomical observation data does not seem to support this hypothesis. So far, in addition to simple compounds such as ubiquitous silicon dioxide and silicates, astronomers have not discovered any complex or advanced silicon-based molecular structures in the universe.
In contrast, the existence of carbon-based organic molecules in the universe has been widely confirmed. Through spectral analysis and meteorite research, scientists have detected a variety of carbon-containing organic molecules in the interstellar medium and comets, including amino acids that form the basis of life. These discoveries further confirm the unique position of carbon in the origin of life.
Is silicon-based life possible?
Although it seems that building a life system based on silicon by imitating existing carbon-based organic molecules faces insurmountable obstacles, it is unexpected that with the rapid development of computer technology based on silicon semiconductors, especially after the booming research on artificial intelligence (AI), silicon and intelligent life have produced many intriguing intersections in the contemporary era, rekindling people’s exploration of the possibility of non-carbon-based life. Silicon has become the basic material of the modern electronics industry, which is closely related to the physical and chemical properties of silicon.
Silicon is a semiconductor. Pure silicon crystals are almost non-conductive, but as long as trace amounts of specific impurity elements are introduced, their conductivity will change significantly. From the perspective of atomic structure, the valence layer of silicon atoms has four electrons. If a trace amount of trivalent elements such as boron, gallium, and indium with fewer valence layer electrons is added, the silicon crystal will have fewer negatively charged electrons. Physicists regard this as the equivalent of adding some positively charged “holes.” This type of semiconductor with holes as the main carrier is called a P-type semiconductor (P stands for positive, i.e., positive charge). In contrast, if elements such as phosphorus, arsenic, and antimony with five valence electrons are added to the valence layer, excess free electrons will form in the crystal. This type of semiconductor with electrons as the main carrier is called an N-type semiconductor (N stands for negative, i.e., negative charge).
Through precise manufacturing processes, P-type and N-type semiconductors are combined and connected in a specific way to create basic electronic components such as transistors. A transistor is essentially a micro electronic switch that controls the on and off of current to achieve the “0” and “1” states in binary operations. It is the coordinated work of these hundreds of millions of micro switches that constitute the core of modern computer chips and realize complex computing and information processing functions.
It is worth mentioning that the material that initially opened a new chapter in the semiconductor industry was not silicon, but germanium (Ge), which is in the same group as silicon in the periodic table. On December 16, 1947, the first point-contact semiconductor transistor made of germanium components came out. However, the content of germanium is limited, and its content in the earth’s crust is only one-hundred-thousandth of that of silicon. In the long run, stable supply will be a problem. Secondly, germanium has poor chemical stability, especially its oxide germanium dioxide is soluble in water and cannot provide a reliable surface protection layer for semiconductor devices like silicon dioxide. However, the silicon used to manufacture semiconductor devices must have ultra-high purity (more than 99.999999999%), and this technical bottleneck was not overcome by Bell Laboratories until the late 1950s. They developed a process for purifying and preparing single-crystal silicon by regional melting, paving the way for the large-scale application of silicon in semiconductors.
Today, silicon-based chips have become the cornerstone of the modern information society. From the smartphones in our pockets to the data centers in the cloud, from smart home devices to self-driving cars, silicon-based chips are everywhere. The computing power of these fingernail-sized chips has far surpassed the early computers that occupied the entire room size in the last century.
This exponential technological progress has also set off an unprecedented technological revolution. In the late 1980s, when I first entered elementary school, even calculators that could only perform simple arithmetic were rare. I remember that the elders in my family were still using abacus to keep accounts. Today, my children can skillfully unlock smartphones and watch online videos before they start school. In just a few decades, the rapid development of technology has completely changed our living habits.
If the previous industrial revolutions – through breakthroughs in steam engines, electricity, automation and other technologies – have greatly improved production efficiency and greatly liberated human physical labor , then the current core technology of artificial intelligence (AI), represented by deep learning, has changed the way humans work at a deeper level: it is gradually taking over human mental labor and taking on more and more work that originally required deep thinking by humans. For example, in the medical field, AI can assist doctors in disease diagnosis by analyzing massive amounts of medical images and medical records; in the financial field, AI can quickly process market data and provide investment advice; even in artistic creation, AI large models have begun to imitate human artists in writing poems, painting and composing music. Thinking and creativity are the core abilities that humans believe are different from other creatures and are proud of. This is why we feel that today’s intelligent robots are becoming more and more similar to humans.
Of course, from the perspective of scientific rigor, we should realize that silicon materials are mainly used for chips and storage units, and defining them as “silicon brain” or “silicon-based intelligence” may be more accurate than “silicon-based life”. With the continuous upgrading of artificial intelligence systems, “silicon brain” is gradually surpassing human “carbon brain” in more and more fields, which makes us worry about how the value and uniqueness of human beings will be reflected in the future? Will these intelligent robots equipped with “silicon brain” really develop autonomous consciousness and emotional experience similar to humans?
Although “silicon-based intelligence” is not a life form in the traditional sense, it has at least partially realized the concept of non-carbon-based life in terms of function. Looking back a hundred years ago, those pioneering scholars who first proposed the concept of “silicon-based life” probably could not have foreseen that such powerful silicon-based intelligence is not a product of natural evolution, but a crystallization of the wisdom of carbon-based life.
Humans are not only the creators of silicon-based intelligence, but are also likely to become its dependents, collaborators, and even competitors in the future. On one side is the human brain that has evolved naturally over millions of years, and on the other side is silicon-based intelligence that is developing at an exponential rate. How humans and silicon-based intelligence will get along will undoubtedly be a far-reaching topic, and the story of life and silicon is far from over.
Interaction and comprehensive effects of colorants, antioxidants and light stabilizers in plastic processing
In the plastic processing industry, colorants, antioxidants and light stabilizers are indispensable additives, each of which plays an important role in improving product appearance and extending service life. However, the interaction between these three is complex and subtle, and their coordination not only affects the final appearance of plastic products, but also directly affects the physical properties, chemical stability and service life of the product.
This article aims to explore in depth the effects of colorants on antioxidants and light stabilizers, and how to achieve effective synergy between the three through scientific and reasonable formulation design and process control.
1. Basic characteristics and effects of colorants
Colorants give plastics rich colors by absorbing or reflecting light of specific wavelengths. There are many types of colorants, including organic pigments, inorganic pigments, dyes, etc. Different types of colorants have significant differences in chemical structure, stability, weather resistance, etc. These characteristics directly determine their impact on plastic substrates and additives.
1.1 Chemical stability of colorants
The chemical stability of colorants is one of the key factors affecting their interaction with antioxidants and light stabilizers. Some colorants may decompose or react chemically with other additives under high temperature or light conditions, thereby weakening the effectiveness of antioxidants and light stabilizers. For example, colorants containing heavy metals such as chrome yellow may react with sulfur-containing antioxidants at high temperatures to form sulfide precipitation, which not only affects the appearance of the product, but also reduces the antioxidant effect.
1.2 Optical properties of colorants
The optical properties of the colorant determine its ability to absorb and reflect light. Some colorants are photoactive or photosensitized, absorbing UV light and initiating photochemical reactions that accelerate the photoaging process of plastics. In addition, the hiding power and tinting power of the colorant will also affect the distribution and effect of the light stabilizer on the plastic surface.
2. Effect of colorants on antioxidants
The main function of antioxidants is to prevent plastics from degradation due to oxidation during processing, storage and use. However, the interaction between colorants and antioxidants may weaken the effectiveness of antioxidants.
2.1 Chemical reaction weakens performance
As mentioned above, some colorants and antioxidants will react chemically at high temperatures to produce ineffective or harmful products. These products not only fail to provide antioxidant protection, but may also accelerate the degradation of plastics. Therefore, when choosing colorants and antioxidants, it is necessary to avoid using combinations that may react chemically.
2.2 Physical adsorption reduces efficiency
The dispersion state of the colorant particles in the plastic will also affect the effectiveness of the antioxidant. If the colorant particles are too large or unevenly dispersed, they may adsorb some antioxidant molecules, resulting in a decrease in the effective concentration of the antioxidant in the plastic. In addition, the colorant particles may also become active centers for oxidation reactions, accelerating the oxidative degradation of the plastic.
3. Effect of Colorants on Light Stabilizers
The main function of light stabilizers is to prevent plastics from light aging under light conditions. However, the impact of colorants on light stabilizers cannot be ignored.
3.1 Photoactivity and photosensitivity accelerate aging
Colorants containing heavy metal elements or impurities such as copper, manganese, and nickel are photoactive or photosensitive, and can absorb ultraviolet rays and trigger photochemical reactions. These reactions will accelerate the photoaging process of plastics, causing cracks and fading on the surface of the products. At the same time, photoactive colorants may also catalyze the decomposition reaction of light stabilizers, reducing their light stabilization effect.
3.2 Molecular structure weakens efficacy
Colorants with certain molecular structures can interact with light stabilizers, directly weakening their effectiveness. For example, organic colorants such as azo red may react with hindered amine light stabilizers, causing the light stabilizers to fail. In addition, colorants may also affect the distribution and mobility of light stabilizers on the plastic surface, thereby affecting their light stabilization effect.
IV. Comprehensive Impacts and Countermeasures
4.1 Formulation design
When designing a plastic coloring formula, it is necessary to fully consider the effect of colorants on antioxidants and light stabilizers. Through experiments, the effects and coordination of the three together on the color and anti-aging stability of the product should be determined. At the same time, factors such as the type of plastic substrate, processing conditions and use environment should also be considered to formulate a scientific and reasonable formula.
4.2 Additive Selection
Selecting antioxidants and light stabilizers that are compatible with colorants is the key to reducing mutual influence. When selecting additives, it is necessary to understand the chemical properties, stability, weather resistance and other characteristics of various additives, and avoid using combinations that may cause chemical reactions or interactions. In addition, factors such as the environmental protection and safety of additives also need to be considered.
4.3 Processing condition control
During the processing of plastic products, process parameters such as processing temperature and time should be strictly controlled to avoid high temperature and long-term processing, which may lead to adverse reactions between colorants, antioxidants and light stabilizers. At the same time, it is also necessary to optimize the mixing and dispersion process to ensure that colorants and additives are evenly dispersed in the plastic and improve the quality and performance of the products.
4.4 Environmental protection and safety
With the improvement of environmental awareness and the improvement of regulations, the selection of non-toxic or low-toxic colorants, antioxidants and light stabilizers has become an industry trend. These additives are not only harmless to the human body and the environment, but also can improve the environmental performance and market competitiveness of the products. Therefore, environmental protection and safety factors should be given priority in the formulation design and production process.
Reversibly cross-linked polymers
Reversibly cross-linked polymers
As a basic material, the output of polymer materials has exceeded that of metal materials and inorganic materials, and they play an irreplaceable and important role in production and life. With the extensive use of polymer materials, the waste of resources and environmental pollution caused by them are becoming increasingly serious, and have become a global problem that hinders the development of human society. The main reason for this dilemma is that polymer products that meet service requirements, such as vulcanized rubber, epoxy resin, etc., usually have a stable network structure cross-linked by covalent bonds. Once these materials are damaged during service, it is usually difficult to repair the mechanical damage and lead to scrapping. Scrapped polymer materials are difficult to recycle and can only be discarded or incinerated.
Therefore, the development of sustainable polymer materials with long-term service stability, the ability to repair mechanical damage and be recycled is particularly important and urgent to promote the construction of a sustainable society. In recent years, the construction of reversibly cross-linked polymers by cross-linking polymer segments with reversible forces such as supramolecular forces and dynamic covalent bonds has received increasing attention. Based on the dynamics of reversible forces, reversibly cross-linked polymers can often exhibit good self-healing properties, recyclable processing properties, and closed-loop recycling properties, and are considered to be a type of sustainable polymer material. However, while reversible forces give materials sustainability, they also make polymer materials exhibit poor mechanical properties. Therefore, improving the mechanical properties of reversibly cross-linked polymer materials is the key point in the development of this type of material and also a difficulty in the field of polymer science.
1. Classification of reversibly cross-linked polymers
Reversibly cross-linked polymer networks connected by reversible forces have attracted extensive attention due to their unique network dynamics. At present, the reversible forces introduced into polymer networks are mainly divided into two categories: one is non-covalent interactions, such as hydrogen bonds, coordination bonds, hydrophilic-hydrophobic interactions, host-guest interactions, and π-π stacking; the other is dynamic covalent bonds, mainly including D-A reactions, Schiff base reactions, alkoxyamine reactions, ester exchange reactions, boric acid ester bonds, boro-oxygen hexacyclic rings, disulfide bonds, etc. According to the type of reversible forces, reversibly cross-linked polymers can be divided into reversibly cross-linked polymers based on non-covalent forces and reversibly cross-linked polymers based on dynamic covalent bonds.
1.1 Reversible cross-linking polymers based on non-covalent forces
Reversibly cross-linked polymers based on non-covalent forces are also called supramolecular polymers. They are polymers formed by monomer molecules connected by non-covalent forces (also called supramolecular forces), such as hydrogen bonds, hydrophilic/hydrophobic interactions, metal coordination bonds, host-guest interactions, and π-π stacking. In polymer networks, there is often more than one supramolecular force, and these supramolecular forces work together to keep the material in a stable topological structure. By changing the type, quantity, and chemical structure of the functional groups of supramolecular forces, we can prepare supramolecular polymer networks with different chemical or physical properties.
1.2 Reversible cross-linking polymers based on dynamic covalent bonds
Reversible cross-linked networks based on dynamic covalent bonds are also called covalent adaptable networks (CANs). They are formed by reversible chemical reactions of monomers with suitable reactive groups under the condition of functional recognition. Dynamic covalent bonds have stronger stability than non-covalent forces such as electrostatic and hydrogen bonds. At the same time, they can be stimulated by specific stimuli (such as light, heat, pH or redox agents). Therefore, the use of dynamic covalent bonds to prepare reversible cross-linked polymer networks can effectively improve the stability of the material. At present, a variety of dynamic covalent bonds have been reported to be used to synthesize various reversible cross-linked polymer materials, including imine bonds, disulfide bonds, borate bonds, hexacyclic boranes, Diels-Alder reactions, etc.
2. Preparation of Dynamically Reversible Cross-linked Polymers
Diels-Alder reaction
The Diels-Alder (DA) reaction is one of the most widely studied dynamic covalent reversible cross-linking reactions. It is a [4+2] cycloaddition reaction between an electron-rich conjugated diene (such as furan) and an electron-poor dienophile (such as maleimide). It is a thermally reversible reaction that can undergo a dissociation reaction at a certain temperature. Most of the dynamic reversible polymers based on the DA reaction utilize the temperature sensitivity of the DA reaction to control the formation and breaking of covalent bonds by changing the temperature.
Disulfide exchange
The disulfide bond is also one of the most widely studied dynamic chemical covalent bonds. The disulfide bond is unstable and its covalent bond is easily broken. It undergoes chemical bond reorganization with other sulfur atoms. The disulfide bond exchange reaction can be triggered by a variety of stimuli (heat, light, external free radicals, etc.).
Transesterification
The ester exchange reaction is a reversible equilibrium reaction and one of the earliest reactions used to construct dynamic cross-linked polymers. Its mechanism is shown in Figure 3. However, the rate of the ester exchange reaction is usually slow. Currently, the rate of the ester exchange reaction is increased by introducing catalysts, increasing the reaction temperature, and using solvents as an aid.
Transamidation reaction
DuPrez et al. studied the use of transamidation reaction to replace the transesterification reaction, because compared with the ester group, the amide group is easier to form thermodynamically, the polyamide is easier to synthesize, and the hydrolysis stability is higher and it is not easy to decompose. They synthesized the polyvinyl carbamate network through the polycondensation reaction of diacetoacetate, diamine and triamine.
Imine and acylhydrazone bond exchange
Imine bonds are obtained by reversible condensation of amines and aldehydes, and usually participate in three equilibrium reactions: hydrolysis, displacement, and exchange. Similar to imine bonds, acylhydrazone bonds can be obtained by condensation of hydrazine and aldehyde or ketone groups.
Siloxane balance
Siloxane balance refers to the dynamic balance of the siloxane bond breaking and segment rearrangement reaction to form new siloxane bonds under acid or base catalysis. Zheng et al. used silane alcohol salt of silyl tetramethylammonium to initiate the ring-opening copolymerization of octamethylcyclotetrasiloxane and bis(heptylmethylcyclotetrasiloxane)ethane to prepare polydimethylsiloxane. The crosslinking density of the polymer can be controlled by the ratio of octamethylcyclotetrasiloxane and bis(heptylmethylcyclotetrasiloxane)ethane. Siloxane balance gives the polymer self-healing properties, but it takes a long time of high temperature to fully restore its mechanical strength.
Transalkylation
Obadia et al. used 1,6-dibromohexane bifunctional crosslinker to cure self-condensed α-azide-ω-alkyne monomers to prepare poly-1,2,3-triazolium ionic liquids. The polymer exhibited typical Arrhenian stress relaxation. Studies have confirmed that the cause of stress relaxation is the C-N alkyl transfer reaction in the crosslinked network. The relaxation time of the polymer is as short as a few seconds at 200°C, and it may be used in the field of solid electrolytes in the future. Huang et al. used (3-bromopropyl) trimethoxysilane to modify silica to crosslink butadiene-styrene-vinyl pyridine rubber to form pyridinium at the interface, and directly mechanically mixed and then hot-pressed to prepare a composite material. The C-N alkyl transfer reaction of pyridinium at high temperature gives the material reprocessability and ductility.
Olefin metathesis
Olefin metathesis refers to the [2+2] addition reaction of metal carbene with carbon-carbon double bonds in olefins to generate metal heterotetracyclic intermediates, which undergo [2+2] reverse reaction to generate new olefins and new carbene. The new carbene undergoes similar reaction to generate another new olefin and regenerate the original metal carbene. Guan et al. used transition metal-catalyzed olefin metathesis with strong carbon-carbon double bond dynamic exchange to prepare dynamic cross-linked polymers. They introduced Grubbs second-generation Ru metathesis catalyst into cross-linked polybutadiene. The Ru catalyst was grafted onto the polymer network through metathesis reaction with double bonds in polybutadiene. Under the catalysis of Ru catalyst, the polybutadiene network showed significant stress relaxation, was ductile at room temperature, and could self-repair. A small amount of the polymer sample was placed at the crack of ordinary polybutadiene, which could also effectively repair the crack. They also introduced sacrificial hydrogen bonds into the polymer network by introducing secondary amide side chains, which significantly enhanced the mechanical properties of the polymer.
3. Application
3.1 Application of Dynamic Covalently Cross-linked Polymers
Introducing dynamic covalent bonds into polymer materials has a wide range of applications, especially in flexible sensing, electronic technology, biomedicine, etc. Bai et al. used aminopolysiloxane and 1,4-diformylbenzene (DFB) to generate Schiff base dynamic cross-linking network bonds and added conductive carbon black to the matrix material to prepare a conductive composite material with high mechanical strength, healable and recyclable, contributing to the development of sustainable electronic materials; Hou et al. used the disulfide bond in lipoic acid as a dynamic bond and introduced polyaniline as a conductive material to prepare flexible electronic skin with good flexibility, puncture resistance and rapid self-healing ability, and achieved multifunctional sensing characteristics in the human body, showing great advantages in the field of wearable electronics and human signal monitoring. Not only that, healable materials also have potential applications in 3D printing. Qu et al. prepared high-strain, self-healing elastomers based on dynamic boron-oxygen bonds. Because the dynamic boron-oxygen bonds give it the characteristics of “solid-liquid” conversion, the material’s 3D printing at room temperature shows its versatility. In addition, dynamic covalent bonds are also widely used in biomedicine. Zhao et al. synthesized an injectable self-healing hydrogel with rapid shape adaptability, good skin adhesion, and antioxidant activity, which can be used to treat drug-resistant bacterial infections and wound healing.
3.2 Repair and recycling of reversibly cross-linked supramolecular plastics
The repair and recycling performance of reversibly cross-linked polymer materials depends largely on the dynamic reversibility of the reversible forces and the mobility of the polymer chains. Supramolecular forces with high dynamicity (such as hydrogen bonds) and dynamic covalent bonds are suitable for the preparation of polymer materials with good repair and recycling performance. However, compared with traditional covalent bond polymer materials, the mechanical strength and stability of reversibly cross-linked polymer materials constructed by highly reversible supramolecular forces are lower. Based on the synergy of high-density reversible hydrogen bonds in polymer composites and the in-situ generated microstructures during the composite process, the author’s research group prepared high-strength, repairable and recyclable reversible cross-linked polymer plastics. Furthermore, based on highly reversible dynamic covalent bonds and polymer composites, supramolecular plastics with efficient repair at room temperature were constructed. High-strength and high-stability supramolecular thermosetting plastics were prepared by cross-linking rigid polymer chains with dynamic covalent bonds.
3.3 Repair and recycling of reversibly cross-linked supramolecular elastomers
Polymer elastomers with high strength, high elasticity, high tensile properties and high toughness are widely used in tires, shock absorbers and seals. Introducing reversible supramolecular forces and dynamic covalent bonds in elastomers can play a role in energy dissipation. At the same time, the micro-nanostructures in the elastomers can play the role of cross-linking sites, improving the mechanical strength and elastic recovery properties of the elastomers.
4. Outlook
The increasing consumption of oil resources and the continuous increase of environmental pollution have gradually aroused people’s attention to environmental issues. In recent years, stringent plastic bans and restrictions have been introduced around the world, forcing people to start looking for degradable and recyclable plastic substitutes. Thermosetting materials are widely used due to their good chemical stability and mechanical properties, but their permanent cross-linking is not conducive to recycling, while the strength and thermal stability of thermoplastic materials limit their application.
Reversibly cross-linked polymers have both the stable covalent cross-linked network structure of thermosetting polymers and the reprocessability of thermoplastic polymers, making the repeated processing and utilization of thermosetting polymers possible. This is of great significance for extending the service life of thermosetting polymers, reducing waste, and recycling them, and has become a hot topic in current material research.
On Earth, tens of millions of species are jointly composing the magnificent chapter of life, and humans and all other species are carbon-based life forms.
On the stage of life on Earth, carbon serves as the basic skeleton, supporting the complex structure and diverse functions of life. Life on Earth shows tenacious vitality and strong adaptability, but all this splendor is limited to the scope of the blue planet Earth. Once you cross the boundaries of the Earth and head into the vast outer space, the fragile nature of life will be exposed without reservation.
When we expand our vision to the entire universe, the many limitations of carbon-based life gradually become apparent. It is neither perfect nor extremely powerful.
Therefore, scientists want to know: Is there a life form more powerful than carbon-based life in some corner of the universe? Among many speculations, silicon-based life, which has certain similarities with carbon-based life, has become the focus of heated discussion in the scientific and science fiction fields.
What exactly is silicon-based life?
In simple terms, since carbon-based life is built with carbon as the basic skeleton, silicon-based life is a form of life that uses silicon as the basis to build a life structure. Why do scientists focus on silicon? This is because carbon and silicon have many similarities. They belong to the fourth main group elements and have similar chemical properties, which provides a certain chemical basis for the conception of silicon-based life.
In the fantastic world of science fiction novels and movies, silicon-based life is often portrayed as a desirable existence.
Not only do they have highly developed intelligence, as if they can understand all the mysteries of the universe, they also have an “indestructible body” and show extremely tenacious vitality. In some settings, the lifespan of silicon-based life is as long as a million years, and they can even break through the constraints of time and achieve immortality.
In comparison, humans, the carbon-based life that dominates the earth, are quite vulnerable despite standing at the top of the earth’s food chain. Humans face the threat of many natural enemies. Deadly bacteria and viruses are like hidden killers that may attack at any time; ubiquitous radiation is like an invisible blade that quietly erodes human health; high and low temperature environments are also like severe tests that can cause fatal damage to human life if you are not careful.
In contrast, silicon has a more stable internal structure and can theoretically adapt to harsher environments, such as areas of space filled with high-energy cosmic radiation.
Perhaps the most striking advantage of silicon-based life is its potential for super-high intelligence. In today’s era, the field of artificial intelligence is booming, and electronic components such as semiconductors are the key to the realization of artificial intelligence. Silicon happens to be the core raw material for manufacturing semiconductors.
With the continuous advancement of science and technology, the artificial intelligence created by humans is becoming more and more advanced. It is hard not to imagine that one day in the future, when these artificial intelligences have human-like thinking and consciousness, perhaps they will become the “silicon-based life” we have always dreamed of. Such silicon-based life does not need to carry out complex metabolism like carbon-based life, and only needs basic energy supply to survive.
However, does silicon-based life really exist in the universe?
From an in-depth theoretical analysis, the probability of the existence of silicon-based life does not seem to be high.
In nature, silicon is difficult to exist independently in the form of a single substance, and mostly appears in the form of oxides. This is because silicon and oxygen have a very strong ability to bind. At the same time, silicon-based compounds have very poor stability. In laboratory environments, they can only exist for a few seconds before decomposing. More importantly, silicon is difficult to form complex biological macromolecules, and silicon chains are very easy to break in liquid water, which undoubtedly poses a huge challenge to the formation of silicon-based life.
Despite many theoretical obstacles, the diversity of the cosmic environment is beyond imagination.
Perhaps those extremely harsh environments that are completely unsuitable for the birth and survival of life in the eyes of humans are precisely the cradles for other forms of life to be nurtured. Some scientists have put forward a unique view that based on the chemical properties of silicon, it is unlikely that silicon-based life will appear on a planet like Earth.
Because for silicon-based life, the earth’s environment is too “harsh” and the temperature is too low to meet their survival needs. Silicon-based life may only exist in an extremely high temperature environment. Humans often admire the beauty of the earth’s environment and its near-perfect habitability, but they never thought that in the eyes of other life forms, the earth’s environment may be full of challenges. For example, for silicon-based life, the large amount of liquid water and oxygen on the earth may be like the deadly carbon monoxide in the eyes of humans, and is a big enemy that hinders their survival.
The celestial environments in the universe vary greatly, from the hot surfaces of stars to the cold interstellar dust clouds, from high-pressure gas planets to barren rocky planets. Such rich and diverse environmental conditions provide unlimited possibilities for the birth of different life forms.
Perhaps, the legendary silicon-based life does not only exist in the fantasy world of novels and movies. On a planet in the distant starry sky, they are reproducing in their own unique way of life. Like humans, they are full of desire for the unknown and are exploring the “aliens” in the universe that belong to them.
Silicon, this amazing element, plays diverse and critical roles in our lives.
It is the core material of solar panels, capturing the sun’s energy for us; it is also the basis of computer chips, like the heart of the computer, driving the operation of the digital world.
Not only that, silicon is everywhere, from daily shampoo to industrial silicone rubber. This makes us spread our imagination: In the depths of the distant universe, is there a world whose skeleton is completely built by silicon, or even life based on silicon?
On Earth as we know it, life is built on the basis of carbon. From macroscopic animals, plants, and fungi to microscopic bacteria and viruses that are invisible to the naked eye, their constituent substances, such as amino acids, sugars, fats, and nucleic acids, have a common carbon chain skeleton despite their diverse forms and functions. Therefore, we call life on Earth carbon-based life.
In the periodic table, carbon and silicon are in the same main group, which makes them have many similarities in chemical properties. Carbon can form methane with hydrogen, and silicon can also form silane with hydrogen; the structure and properties of carbonates and silicates are quite similar; carbon can connect itself to form long carbon chains, while silicon can combine with oxygen to build more flexible and soft silicon-oxygen chains.
Based on these characteristics, people can’t help but wonder if there is such a creature whose body parts are composed of chemicals with silicon-oxygen chains as the skeleton? This hypothetical creature is given the name of “silicon-based life”.
So, are the diatoms we commonly see silicon-based organisms? The answer is no. Although diatoms have a shell made of silicon dioxide, their internal core material is still composed of DNA and protein, and they are essentially carbon-based life.
The discussion on silicon-based life has a long history.
As early as 1891, Julius Shenner, an astrophysicist at the University of Potsdam, took the lead in studying the possibility of silicon-based life and became the first person to mention silicon-based life. Later, in 1893, British chemist James Reynolds pointed out in a speech at the British Association for the Advancement of Science that silicon compounds have high thermal stability, a property that makes it possible for life based on them to survive in high temperature environments.
British geneticist John Burton Sanderson Haldane even boldly imagined that there might be life based on semi-molten silicates deep in the planet, which relies on the oxidation of iron to obtain energy.
Science fiction has always been at the forefront of exploring unknown life forms. Stanley Weisbaum described a strange silicon-based life in “Mars Odyssey”: this life form has a lifespan of up to one million years, and deposits a silica brick every ten minutes. Unlike carbon-based life that inhales oxygen and exhales carbon dioxide, silicon-based life exhales silica. Moreover, their lifespan is extremely long, but their reaction speed is extremely slow, even slower than that of sloths.
Asimov, a master of science fiction and popular science, boldly proposed six possible forms of life in the universe in his article “Not as We Know It: On the Chemistry of Life”:
- Fluorinated silicone organisms using fluorinated silicone as a medium;
- Sulfur-mediated fluorocarbon organisms;
- Nucleic acid/protein (oxygen-based) organisms using liquid water as a medium;
- Nucleic acid/protein (nitrogen-based) organisms using liquid ammonia as a medium;
- Lipid-based organisms that use liquid methane as a medium;
- Lipid compound organisms using liquid hydrogen as a medium.
Among them, the carbon-based life we are familiar with is only one of them.
It is worth noting that Asimov ranked silicon-based life (using fluorinated silicone as a medium) first, mainly based on the following three reasons:
First, silicon is relatively abundant in the universe, especially on terrestrial planets, where the mass ratio of silicon to carbon is as high as 925: 1. During the formation of planets, carbon mostly dissipates in the form of methane, while silicon is tightly bound to oxygen.
Second, silicon can form various, stable and inactive organic silicon compounds through silicon-oxygen chains. In the vast universe, as long as there is a possibility, it is likely to become a reality in some corner.
Third, Asimov particularly emphasized that silicon-based life is more likely to be born when combined with fluorine. This is because fluorine-containing silicon compounds can be liquid, and compared to silicon dioxide and silicates with extremely high melting points, this liquid property provides more favorable conditions for the chemical reactions of life. We can compare the silicon-oxygen chain to the carbon chain, and regard fluorine as the oxygen or hydrogen element in carbon-based life. The life form constructed in this way is full of mystery and unknown.
Silicon-based life often appears in modern science fiction films and TV shows. For example, in the science fiction blockbuster “Star Trek”, there is a silicon-based life called Horta. They have a unique life cycle. Every 50,000 years, almost all Horta will die, leaving only one individual to guard the eggs that are about to hatch the next generation.
Liu Cixin, a famous Chinese science fiction writer, also showed his unique imagination of silicon-based life in his works.
In the work “The Mountain”, the protagonist Feng Fan encountered an alien civilization, which introduced itself like this: “We are mechanical life forms, with muscles and bones made of metal, brains that are ultra-highly integrated chips, and electric currents and magnetic fields as our blood. We feed on radioactive rocks in the core of the earth and rely on the energy they provide to survive. We were not created by anyone, but are the result of natural evolution, evolving from the simplest single-celled machinery, which evolved from PN knots accidentally formed on rocks under the action of radioactivity. Our primitive ancestors first discovered and used electromagnetic energy, and we have never discovered fire, as you know it.”
Here, the chip can be seen as a silicon-based brain, and the radioactive rock is a silicon-based food. The silicon-based life described by Liu Cixin was first born inside the earth-like planets, which makes us wonder whether similar silicon-based life may exist deep inside the earth?
In another well-known work by Liu Cixin, “The Rural Teacher”, the imagination was expanded to the vast universe, constructing a cosmic war between carbon-based life and silicon-based life.
In this war, all kinds of unimaginable weapons and means of warfare have appeared: “leapfrog” time and space jump, laser evaporation of the ocean, detonation of supernovas, antimatter cloud barriers, four-dimensional scanning, data mirror combination, singularity bombs, etc., which are a feast for the eyes of science fiction fans. Since the novel is based on the perspective of the supreme commander of the Carbon-based Empire, there is no direct and detailed description of the Silicon-based Empire, but we can also get a glimpse from the commander’s inner monologue: “After the war, the most urgent thing to rebuild in the Milky Way is respect for life. This respect is not only for carbon-based life, but also for silicon-based life. It is based on this respect that the Carbon-based Federation did not completely eliminate silicon-based civilization. However, the Silicon-based Empire lacks this kind of emotion for life.
If before the Carbon-Silicon War, war and conquest were just an instinct and pleasure for them, now, this concept has been deeply rooted in every gene and every line of code, becoming the ultimate goal of their survival. Since silicon-based creatures have much higher information storage and processing capabilities than us, it can be predicted that the recovery and development of the Silicon-based Empire at the top of the First Spiral Arm will be extremely rapid. Therefore, we must establish a wide enough isolation zone between the Carbon-based Federation and the Silicon-based Empire. “
Silicon-based life in Western science fiction works is often portrayed in more specific physical images, but Liu Cixin adheres to his consistent style and creates in a freehand manner from the side, giving readers a broad space for imagination.
However, there are also many doubts in the scientific community about the existence of silicon-based life. Some scientists pointed out that silicon dioxide, as a possible excrement of silicon-based life, has an extremely high melting point. This means that if silicon-based life exists, they either need to survive in a high temperature environment of more than 2000 degrees, or their life activities will be extremely slow and sluggish. In addition, the silicon content in the earth’s crust is as high as 28%, which is much higher than the 0.03% of carbon, but it is carbon-based life, not silicon-based life, that is finally born on Earth. By analogy, why must silicon-based life appear in other parts of the universe?
Despite these controversies, humans always have the right to fantasize. As long as a hypothesis does not conflict with existing scientific knowledge and has not been falsified by scientific discoveries, it is worth proposing and exploring, which is itself a manifestation of the scientific spirit.
Looking back at the silicon-based life forms in Liu Cixin’s novels, most of them use ultra-high-integration chips as their brains, with superb information storage and processing capabilities. This setting can’t help but remind us of the development of science and technology in the real world.
In March 2016, AlphaGO, an artificial intelligence Go program developed by Google, defeated top human Go player Lee Sedol. This event was a milestone in the field of artificial intelligence.
Before this, many Go professionals believed that there were a lot of “virtual” elements in Go, and that human perceptual cognition had a natural advantage in these aspects, making it difficult for artificial intelligence to conquer this area. However, AlphaGO’s victory broke this perception. As a program, it demonstrated the powerful computing power of silicon-based processors. A computer can be installed with multiple different programs to deal with various complex problems. In terms of the accuracy of problem solving, computing often has an incomparable advantage.
With the continuous advancement of technology, we can even imagine that computers may be able to write programs independently in the future. Does this mean that they will gain a special kind of “life”?
Perhaps in the near future, we will be surprised to find that silicon-based life has already quietly appeared around us, and it is cultivated by humans themselves. These beings built on cold silicon chips have been given more complex functions and “vitality” by humans in their continuous development and evolution, and have surpassed their creators – humans. Then, will they replace humans as the masters of the world and push humans into an abyss of no return?
We can only hope that Asimov’s three laws of robotics can become universal truths and build a safety barrier between humans and possible silicon-based intelligence in the future.
The first law states: A robot may not injure a human being or, through inaction, allow a human being to come to harm; the second law states: A robot must obey the orders given to it by human beings, except where such orders would conflict with the zeroth law or the first law (the zeroth law was later added: A robot must protect the interests of mankind as a whole; the other three laws are valid only under this premise).
In today’s rapidly developing science and technology, will silicon-based artificial intelligence (AI) become a close friend and powerful tool for mankind, or will it become mankind’s gravedigger? This question is like a sword of Damocles, hanging over the future of mankind, causing us to think deeply.