Anartificial cell,synthetic cell orminimal cell is an engineered particle that mimics one or many functions of abiological cell. Often, artificial cells are biological or polymeric membranes which enclose biologically active materials.^[1] As such,liposomes,polymersomes,nanoparticles, microcapsules and a number of other particles can qualify as artificial cells.

The terms "artificial cell" and "synthetic cell" are used in a variety of different fields and can have different meanings, as it is also reflected in the different sections of this article. Some stricter definitions are based on the assumption that the term "cell" directly relates tobiological cells and that these structures therefore have to be alive (or part of a living organism) and, further, that the term "artificial" implies that these structures are artificially built from the bottom-up, i.e. from basic components. As such, in the area ofsynthetic biology, an artificial cell can be understood as a completely synthetically made cell that can captureenergy, maintainion gradients, containmacromolecules as well as store information and have the ability toreplicate.^[2] This kind of artificial cell has not yet been made.

However, in other cases, the term "artificial" does not imply that the entire structure is man-made, but instead, it can refer to the idea that certain functions or structures of biological cells can be modified, simplified, replaced or supplemented with a synthetic entity.

In other fields, the term "artificial cell" can refer to any compartment that somewhat resembles a biological cell in size or structure, but is synthetically made, or even fully made from non-biological components. The term "artificial cell" is also used for structures with direct applications such as compartments for drug delivery.Micro-encapsulation allows formetabolism within the membrane, exchange of small molecules and prevention of passage of large substances across it.^[3][4] The main advantages of encapsulation include improved mimicry in the body, increasedsolubility of the cargo and decreasedimmune responses. Notably, artificial cells have been clinically successful inhemoperfusion.^[5]

The German pathologistRudolf Virchow brought forward the idea that not only does life arise from cells, but every cell comes from another cell; "Omnis cellula e cellula".^[6] Until now, most attempts to create an artificial cell have engineered modules that can mimic certain functions of living cells. Advances in cell-freetranscription andtranslation reactions allow the expression of manygenes as well as interdependent genetic and metabolic networks,^[7] but these efforts are still far from producing a fully operational cell.

Abottom-up approach to build an artificial cell would involve creating aprotocellde novo, entirely from non-living materials. As the term "cell" implies, one prerequisite is the generation of some sort of compartment that defines an individual, cellular unit.Phospholipid membranes are an obvious choice as compartmentalizing boundaries,^[8] as they act as selective barriers in all living biological cells. Scientists can encapsulate biomolecules in cell-sized phospholipidvesicles and by doing so, observe these molecules to act similarly as in biological cells and thereby recreate certain cell functions.^[9] In a similar way, functional biological building blocks can be encapsulated in these lipid compartments to achieve the synthesis of (however rudimentary) artificial cells.

It is proposed to create a phospholipid bilayer vesicle with DNA capable of self-reproducing using synthetic genetic information. The three primary elements of such artificial cells are the formation of alipid membrane,DNA andRNA replication through a template process and the harvesting of chemical energy foractive transport across the membrane.^[10][11] The main hurdles foreseen and encountered with this proposed protocell are the creation of a minimal synthetic DNA that holds all sufficient information for life, and the reproduction of non-genetic components that are integral in cell development such as molecular self-organization.^[12] However, it is hoped that this kind of bottom-up approach would provide insight into the fundamental questions of organizations at the cellular level and the origins of biological life. So far, no completely artificial cell capable of self-reproduction has been synthesized using the molecules of life, and this objective is still in a distant future although various groups are currently working towards this goal.^[13]

Another method proposed to create a protocell more closely resembles theconditions believed to have been present during evolution known as the primordial soup. Various RNA polymers could be encapsulated in vesicles and in such small boundary conditions, chemical reactions would be tested for.^[14]

Protocell research has created controversy and opposing opinions, including critics of the vague definition of "artificial life".^[15] The creation of a basic unit of life is the most pressing ethical concern.^[16] Synthetic organisms could escape and cause damage to human health and ecosystems, or the technology could be used to make abiological weapon.^[17] Cells with certain non-standard biochemistries, such asmirror life, could also have a competitive advantage over natural organisms.^[18]

In the mid-2010s the research community started recognising the need to unify the field of synthetic cell research, acknowledging that the task of constructing an entire living organism from non-living components was beyond the resources of a single country.^[19]

In 2017 the NSF-funded internationalBuild-a-Cell large-scale research collaboration for the construction of synthetic living cell was started,.^[20] Build-a-Cell has conducted nine interdisciplinary workshopping events, open to all interested, to discuss and guide the future of the synthetic cell community. Build-a-Cell was followed by national synthetic cell organizations in several other countries. Those national organizations include FabriCell,^[21] MaxSynBio^[22] and BaSyC.^[23] The European synthetic cell efforts were unified in 2019 as SynCellEU initiative.^[24]

Members from theJ. Craig Venter Institute have used atop-down computational approach to knock out genes in a living organism to a minimum set of genes.^[25] In 2010, the team succeeded in creating a replicating strain (namedMycoplasma laboratorium) ofMycoplasma mycoides using synthetically created DNA deemed to be the minimum requirement for life which was inserted into a genomically empty bacterium.^[25] It is hoped that the process of top-down biosynthesis will enable the insertion of new genes that would perform profitable functions such as generation of hydrogen for fuel or capturing excess carbon dioxide in the atmosphere.^[16] The myriad regulatory, metabolic, and signaling networks are not completely characterized. Thesetop-down approaches have limitations for the understanding of fundamental molecular regulation, since the host organisms have a complex and incompletely defined molecular composition.^[26] In 2019 a complete computational model of all pathways in Mycoplasma Syn3.0 cell was published, representing the first completein silico model for a living minimal organism.^[27]

Heavy investing in biology has been done by large companies such asExxonMobil, who has partnered withSynthetic Genomics Inc; Craig Venter's own biosynthetics company in the development of fuel from algae.^[28]

As of 2016,Mycoplasma genitalium is the only organism used as a starting point for engineering a minimal cell, since it has the smallest known genome that can be cultivated under laboratory conditions; the wild-type variety has 482, and removing exactly 100 genes deemed non-essential resulted in a viable strain with improved growth rates. Reduced-genomeEscherichia coli is considered more useful, and viable strains have been developed with 15% of the genome removed.^{[29]: 29–30}

A variation of an artificial cell has been created in which a completely syntheticgenome was introduced to genomically emptied host cells.^[25] Although not completely artificial because thecytoplasmic components as well as themembrane from the host cell are kept, the engineered cell is under control of a synthetic genome and is able toreplicate.

In the 1960sThomas Chang developed microcapsules which he would later call "artificial cells", as they were cell-sized compartments made from artificial materials.^[30] These cells consisted of ultrathin membranes of nylon, collodion or crosslinked protein whosesemipermeable properties alloweddiffusion of small molecules in and out of the cell. These cells were micron-sized and containedcells,enzymes,hemoglobin, magnetic materials,adsorbents andproteins.^[3]

Later artificial cells have ranged from hundred-micrometer to nanometer dimensions and can carry microorganisms,vaccines,genes, drugs,hormones andpeptides.^[3] The first clinical use of artificial cells was inhemoperfusion by the encapsulation ofactivated charcoal.^[31]

In the 1970s, researchers were able to introduce enzymes, proteins and hormones to biodegradable microcapsules, later leading to clinical use in diseases such asLesch–Nyhan syndrome.^[32] Although Chang's initial research focused on artificialred blood cells, only in the mid-1990s were biodegradable artificial red blood cells developed.^[33] Artificial cells in biological cell encapsulation were first used in the clinic in 1994 for treatment in a diabetic patient^[34] and since then other types of cells such ashepatocytes, adultstem cells and genetically engineered cells have been encapsulated and are under study for use in tissue regeneration.^[35][36]

Membranes for artificial cells can be made of simplepolymers, crosslinked proteins,lipid membranes or polymer-lipid complexes. Further, membranes can be engineered to present surfaceproteins such asalbumin,antigens,Na/K-ATPase carriers, or pores such asion channels. Commonly used materials for the production of membranes include hydrogel polymers such asalginate,cellulose andthermoplastic polymers such as hydroxyethyl methacrylate-methyl methacrylate (HEMA- MMA), polyacrylonitrile-polyvinyl chloride (PAN-PVC), as well as variations of the above-mentioned.^[4] The material used determines the permeability of the cell membrane, which for polymer depends on the is important in determining adequatediffusion of nutrients, waste and other critical molecules. Hydrophilic polymers have the potential to bebiocompatible and can be fabricated into a variety of forms which include polymermicelles,sol-gel mixtures, physical blends and crosslinked particles and nanoparticles.^[4] Of special interest are stimuli-responsive polymers that respond topH or temperature changes for the use in targeted delivery. These polymers may be administered in the liquid form through a macroscopic injection and solidify or gelin situ because of the difference in pH or temperature.Nanoparticle andliposome preparations are also routinely used for material encapsulation and delivery. A major advantage of liposomes is their ability tofuse to cell andorganelle membranes.

Many variations for artificial cell preparation and encapsulation have been developed. Typically, vesicles such as ananoparticle,polymersome orliposome are synthesized. An emulsion is typically made through the use of high pressure equipment such as a high pressurehomogenizer or aMicrofluidizer. Twomicro-encapsulation methods for nitrocellulose are also described below.

In a high-pressure homogenizer, two liquids in oil/liquid suspension are forced through a small orifice under very high pressure. This process divides the products and allows the creation of extremely fine particles, as small as 1 nm.

This technique uses a patented Microfluidizer to obtain a greater amount of homogenous suspensions that can create smaller particles than homogenizers. A homogenizer is first used to create a coarse suspension which is then pumped into the microfluidizer under high pressure. The flow is then split into two streams which will react at very high velocities in an interaction chamber until desired particle size is obtained.^[37] This technique allows for large scale production of phospholipid liposomes and subsequent material nanoencapsulations.

In this method, a cell solution is incorporated dropwise into acollodion solution of cellulose nitrate. As the drop travels through the collodion, it is coated with a membrane thanks to the interfacial polymerization properties of the collodion. The cell later settles into paraffin, where the membrane sets, which is then suspended using a saline solution. The drop method is used for the creation of large artificial cells which encapsulate biological cells, stem cells and genetically engineered stem cells.

Theemulsion method differs in that the material to be encapsulated is usually smaller and is placed in the bottom of a reaction chamber where the collodion is added on top and centrifuged, or otherwise disturbed in order to create an emulsion. The encapsulated material is then dispersed and suspended in saline solution.

Artificial cells used fordrug delivery differ from other artificial cells since their contents are intended to diffuse out of the membrane, or be engulfed and digested by a host target cell. Often used are submicron, lipid membrane artificial cells that may be referred to as nanocapsules, nanoparticles, polymersomes, or other variations of the term.^[38]

A temperature-responsive system has been developed to useRNA thermometers to control the timing and location of cargo release from artificial cells.^[39] This is done by having artificial cells express a pore forming protein -alpha hemolysin - under the control of an RNA thermometer, allowing for cargo release to be coupled to temperature changes.^[39]

Enzyme therapy is being actively studied forgenetic metabolic diseases where an enzyme is over-expressed, under-expressed, defective, or not at all there. In the case of under-expression or expression of a defectiveenzyme, an active form of the enzyme is introduced in the body to compensate for the deficit. On the other hand, an enzymatic over-expression may be counteracted by introduction of a competing non-functional enzyme; that is, an enzyme whichmetabolizes the substrate into non-active products. When placed within an artificial cell, enzymes can carry out their function for a much longer period compared to free enzymes^[3] and can be further optimized by polymer conjugation.^[40]

The first enzyme studied under artificial cell encapsulation wasasparaginase for the treatment oflymphosarcoma in mice. This treatment delayed the onset and growth of thetumor.^[41] These initial findings led to further research in the use of artificial cells for enzyme delivery intyrosine dependentmelanomas.^[42] These tumors have a higher dependency ontyrosine than normal cells for growth, and research has shown that lowering systemic levels of tyrosine in mice can inhibit growth of melanomas.^[43] The use of artificial cells in the delivery oftyrosinase; and enzyme that digests tyrosine, allows for better enzyme stability and is shown effective in the removal of tyrosine without the severe side-effects associated with tyrosine deprivation in the diet.^[44]

Artificial cell enzyme therapy is also of interest for the activation ofprodrugs such asifosfamide in certain cancers. Artificial cells encapsulating thecytochrome p450 enzyme which converts this prodrug into the active drug can be tailored to accumulate in the pancreatic carcinoma or implanting the artificial cells close to the tumor site. Here, the local concentration of the activated ifosfamide will be much higher than in the rest of the body thus preventing systemictoxicity.^[45] The treatment was successful in animals^[46] and showed a doubling in median survivals amongst patients with advanced-stagepancreatic cancer in phase I/II clinical trials, and a tripling in one-year survival rate.^[45]

In treatment of genetic diseases,gene therapy aims to insert, alter or removegenes within an afflicted individual's cells. The technology relies heavily on viralvectors which raises concerns about insertionalmutagenesis and systemicimmune response that have led to human deaths^[47][48] and development ofleukemia^[49][50] in clinical trials. Circumventing the need for vectors by using naked or plasmid DNA as its own delivery system also encounters problems such as lowtransduction efficiency and poor tissue targeting when given systemically.^[4]

Artificial cells have been proposed as a non-viral vector by which genetically modified non-autologous cells are encapsulated and implanted to deliver recombinant proteinsin vivo.^[51] This type ofimmuno-isolation has been proven efficient in mice through delivery of artificial cells containing mousegrowth hormone which rescued a growth-retardation in mutant mice.^[52] A few strategies have advanced to human clinical trials for the treatment ofpancreatic cancer, lateral sclerosis and pain control.^[4]

The first clinical use of artificial cells was inhemoperfusion by the encapsulation ofactivated charcoal.^[31] Activated charcoal has the capability of adsorbing many large molecules and has for a long time been known for its ability to remove toxic substances from the blood in accidental poisoning or overdose. However,perfusion through direct charcoal administration is toxic as it leads toembolisms and damage of blood cells followed by removal by platelets.^[53] Artificial cells allow toxins to diffuse into the cell while keeping the dangerous cargo within their ultrathin membrane.^[31]

Artificial cellhemoperfusion has been proposed as a less costly and more efficient detoxifying option thanhemodialysis,^[3] in which blood filtering takes place only through size separation by a physical membrane. In hemoperfusion, thousands of adsorbent artificial cells are retained inside a small container through the use of two screens on either end through which patient bloodperfuses. As the blood circulates,toxins or drugs diffuse into the cells and are retained by the absorbing material. The membranes of artificial cells are much thinner those used in dialysis and their small size means that they have a high membranesurface area. This means that a portion of cell can have a theoretical mass transfer that is a hundredfold higher than that of a whole artificial kidney machine.^[3] The device has been established as a routine clinical method for patients treated for accidental or suicidal poisoning but has also been introduced as therapy inliver failure andkidney failure by carrying out part of the function of these organs.^[3]Artificial cell hemoperfusion has also been proposed for use in immunoadsorption through which antibodies can be removed from the body by attaching an immunoadsorbing material such asalbumin on the surface of the artificial cells. This principle has been used to removeblood group antibodies from plasma for bone marrow transplantation^[54] and for the treatment ofhypercholesterolemia throughmonoclonal antibodies to remove low-densitylipoproteins.^[55] Hemoperfusion is especially useful in countries with a weak hemodialysis manufacturing industry as the devices tend to be cheaper there and used inkidney failure patients.

The most common method of preparation of artificial cells is throughcell encapsulation. Encapsulated cells are typically achieved through the generation of controlled-size droplets from a liquid cellsuspension which are then rapidly solidified or gelated to provide added stability. The stabilization may be achieved through a change in temperature or via material crosslinking.^[4] The microenvironment that a cell sees changes upon encapsulation. It typically goes from being on amonolayer to a suspension in a polymer scaffold within a polymeric membrane. A drawback of the technique is that encapsulating a cell decreases its viability and ability to proliferate and differentiate.^[56] Further, after some time within the microcapsule, cells form clusters that inhibit the exchange of oxygen and metabolic waste,^[57] leading toapoptosis andnecrosis thus limiting the efficacy of the cells and activating the host'simmune system.Artificial cells have been successful for transplanting a number of cells includingislets of Langerhans fordiabetes treatment,^[58]parathyroid cells and adrenal cortex cells.

Shortage of organ donors make artificial cells key players in alternative therapies forliver failure. The use of artificial cells forhepatocyte transplantation has demonstrated feasibility and efficacy in providing liver function in models of animal liver disease andbioartificial liver devices.^[59] Research stemmed off experiments in which the hepatocytes were attached to the surface of a micro-carriers^[60] and has evolved into hepatocytes which are encapsulated in a three-dimensional matrix inalginate microdroplets covered by an outer skin ofpolylysine. A key advantage to this delivery method is the circumvention ofimmunosuppression therapy for the duration of the treatment. Hepatocyte encapsulations have been proposed for use in abioartificial liver. The device consists of a cylindrical chamber imbedded with isolated hepatocytes through which patient plasma is circulated extra-corporeally in a type ofhemoperfusion. Because microcapsules have a highsurface area tovolume ratio, they provide large surface for substrate diffusion and can accommodate a large number of hepatocytes. Treatment to induced liver failure mice showed a significant increase in the rate of survival.^[59] Artificial liver systems are still in early development but show potential for patients waiting fororgan transplant or while a patient's own liver regenerates sufficiently to resume normal function. So far, clinical trials using artificial liver systems and hepatocyte transplantation in end-stage liver diseases have shown improvement of health markers but have not yet improved survival.^[61] The short longevity and aggregation of artificial hepatocytes after transplantation are the main obstacles encountered.Hepatocytes co-encapsulated withstem cells show greater viability in culture and after implantation^[62] and implantation of artificial stem cells alone have also shown liver regeneration.^[63] As such interest has arisen in the use of stem cells for encapsulation inregenerative medicine.

The oral ingestion of live bacterial cellcolonies has been proposed and is currently in therapy for the modulation of intestinalmicroflora,^[64] prevention ofdiarrheal diseases,^[65] treatment ofH. Pylori infections, atopic inflammations,^[66]lactose intolerance^[67] andimmune modulation,^[68] amongst others. The proposed mechanism of action is not fully understood but is believed to have two main effects. The first is the nutritional effect, in which the bacteria compete with toxin producing bacteria. The second is the sanitary effect, which stimulates resistance to colonization and stimulatesimmune response.^[4] The oral delivery of bacterial cultures is often a problem because they are targeted by the immune system and often destroyed when taken orally. Artificial cells help address these issues by providing mimicry into the body and selective or long term release thus increasing the viability of bacteria reaching thegastrointestinal system.^[4] In addition, live bacterial cell encapsulation can be engineered to allow diffusion of small molecules including peptides into the body for therapeutic purposes.^[4] Membranes that have proven successful for bacterial delivery includecellulose acetate and variants ofalginate.^[4] Additional uses that have arosen from encapsulation of bacterial cells include protection against challenge fromM. Tuberculosis^[69] and upregulation of Ig secreting cells from the immune system.^[70] The technology is limited by the risk of systemic infections, adverse metabolic activities and the risk of gene transfer.^[4] However, the greater challenge remains the delivery of sufficient viable bacteria to the site of interest.^[4]

Nano sized oxygen carriers are used as a type ofred blood cell substitutes, although they lack other components of red blood cells. They are composed of a syntheticpolymersome or an artificial membrane surrounding purified animal, human or recombinanthemoglobin.^[71]Overall, hemoglobin delivery continues to be a challenge because it is highly toxic when delivered without any modifications. In some clinical trials, vasopressor effects have been observed.^[72][73]

Research interest in the use of artificial cells for blood arose after theAIDS scare of the 1980s. Besides bypassing the potential for disease transmission, artificial red blood cells are desired because they eliminate drawbacks associated with allogenic blood transfusions such as blood typing, immune reactions and its short storage life of 42 days. Ahemoglobin substitute may be stored at room temperature and not under refrigeration for more than a year.^[3] Attempts have been made to develop a complete working red blood cell which comprises carbonic not only an oxygen carrier but also the enzymes associated with the cell. The first attempt was made in 1957 by replacing the red blood cell membrane by an ultrathin polymeric membrane^[74] which was followed by encapsulation through alipid membrane^[75] and more recently a biodegradable polymeric membrane.^[3]A biological red blood cell membrane includinglipids and associated proteins can also be used to encapsulate nanoparticles and increase residence time in vivo by bypassingmacrophage uptake and systemic clearance.^[76]

A leuko-polymersome is apolymersome engineered to have the adhesive properties of aleukocyte.^[77] Polymersomes are vesicles composed of a bilayer sheet that can encapsulate many active molecules such as drugs orenzymes. By adding the adhesive properties of a leukocyte to their membranes, they can be made to slow down, or roll along epithelial walls within the quickly flowingcirculatory system.

The concept of an Electronic Artificial Cell has been expanded in a series of 3 EU projects coordinated by John McCaskill from 2004 to 2015.

TheEuropean Commission sponsored the development of the Programmable Artificial Cell Evolution (PACE) program^[78] from 2004 to 2008 whose goal was to lay the foundation for the creation of "microscopic self-organizing, self-replicating, and evolvable autonomous entities built from simple organic and inorganic substances that can be genetically programmed to perform specific functions"^[78] for the eventual integration into information systems. The PACE project developed the first Omega Machine, a microfluidic life support system for artificial cells that could complement chemically missing functionalities (as originally proposed by Norman Packard, Steen Rasmussen, Mark Beadau and John McCaskill). The ultimate aim was to attain an evolvable hybrid cell in a complex microscale programmable environment. The functions of the Omega Machine could then be removed stepwise, posing a series of solvable evolution challenges to the artificial cell chemistry. The project achieved chemical integration up to the level of pairs of the three core functions of artificial cells (a genetic subsystem, a containment system and a metabolic system), and generated novel spatially resolved programmable microfluidic environments for the integration of containment and genetic amplification.^[78] The project led to the creation of the European center for living technology.^[79]

Following this research, in 2007, John McCaskill proposed to concentrate on an electronically complemented artificial cell, called the Electronic Chemical Cell. The key idea was to use a massively parallel array of electrodes coupled to locally dedicated electronic circuitry, in a two-dimensional thin film, to complement emerging chemical cellular functionality. Local electronic information defining the electrode switching and sensing circuits could serve as an electronic genome, complementing the molecular sequential information in the emerging protocols. A research proposal was successful with theEuropean Commission and an international team of scientists partially overlapping with the PACE consortium commenced work 2008–2012 on the project Electronic Chemical Cells. The project demonstrated among other things that electronically controlled local transport of specific sequences could be used as an artificial spatial control system for the genetic proliferation of future artificial cells, and that core processes of metabolism could be delivered by suitably coated electrode arrays.

The major limitation of this approach, apart from the initial difficulties in mastering microscale electrochemistry and electrokinetics, is that the electronic system is interconnected as a rigid non-autonomous piece of macroscopic hardware. In 2011, McCaskill proposed to invert the geometry of electronics and chemistry : instead of placing chemicals in an active electronic medium, to place microscopic autonomous electronics in a chemical medium. He organized a project to tackle a third generation of Electronic Artificial Cells at the 100 μm scale that could self-assemble from two half-cells "lablets" to enclose an internal chemical space, and function with the aid of active electronics powered by the medium they are immersed in. Such cells can copy both their electronic and chemical contents and will be capable of evolution within the constraints provided by their special pre-synthesized microscopic building blocks. In September 2012 work commenced on this project.^[80]

Jeewanu protocells are synthetic chemical particles that possesscell-like structure and seem to have some functional living properties.^[92] First synthesized in 1963 from simple minerals and basic organics while exposed tosunlight, it is still reported to have some metabolic capabilities, the presence ofsemipermeable membrane,amino acids,phospholipids,carbohydrates and RNA-like molecules.^[92] However, the nature and properties of the Jeewanu remains to be clarified.^[92][93]

• Protocell
• Synthetic biology
• Artificial life
• Targeted drug delivery
• Respirocyte
• Chemoton
• Jeewanu

• Cell biology
• Synthetic biology

• Wikipedia articles needing page number citations from April 2022
• CS1 German-language sources (de)
• All articles with incomplete citations
• Articles with incomplete citations from April 2022
• Articles with short description
• Short description is different from Wikidata
• Articles with excerpts