Humans have always watched and used animals and plants to detect danger and warning signs for climate changes and natural disasters. One of typical and popular of these had been the use of Canaries in coal mining even up until 1980’s as an early warning system of toxic gases such carbon monoxide, as the gas would kill the bird before affecting the miners. (1) This is the basis of “biosensors” that uses a biological element in an organism such as humans to detect and sense abnormality and diseases, or measuring certain functions. A common old such example is the “artificial cardiac pacemaker” that is implanted in the individual when the natural heart pacemaker for control of the heart beat is not working properly. Cardiac defibrillator also separately or at the same time could be implanted for the individuals with arrhythmia and at risk of cardiac failure. In the case of the pacemaker, when it does not detect normal heartbeat through its sensor, it will stimulate the ventricle of the heart with a short low voltage pulse that will normalize the heart beat. (2-3) Another popular type of biosensor in common use is the blood glucometer in diabetes. This biosensor uses the enzyme glucose oxidase that breaks down the glucose as a measure of blood glucose level. In brief, the biosensors through their transducer or detecting element, detects a measurable change in chemical, physiological or electrical system of the organism such as humans. (4) The blood glucose biosensor monitors blood sugar chemically, and the pacemaker detect abnormal heart beats physiologically and electrically.
The application of biosensors since the early use of cardiac pacemaker has evolved over the past half a century, and with more advancement in digitalism, the idea and research is extending to make the biosensors personalized and available to everyone into their digital gadgets such as their cell phones. While the personal gadgets such as cell phones already carry health monitor applications, including vital signs monitoring, the biosensors research’s goal is to bring to the hands of the individuals diagnostic and treatment tools. In this article, I will explore this area of research and advancement in medicine and will show the path towards the future of medicine that will be personalized. Since this area of medicine research is aggressively progressing and a detailed review of the subject is beyond the scope and space of this site, a concise review of a few areas of the biosensor development in a few fields of medicine will be conducted and presented.(5)
Beyond pacemaker for our Hearts:
Since the heart is one of the most vital organs of the body and mortality due to the cardiac diseases such as heart attacks are still the leading cause, and since the invention of pacemaker saved so many lives in the past, the area of biosensors in cardiology is worth of advancement and consideration. Heart other than being the most vital organ of the body, is also the most active, working 24/7 even in sleep, so is a unique organ functioning physiologically, dynamically, electrically and magnetically. The artificial pacemaker that was adapted in invention from the natural heart pacemaker itself, was based on the electrical currency within the heart. To go beyond and use the biosensors for further diagnostic and treatment tools of the heart diseases, the research needed to invent more sophisticated devices to detect and control other functional parameters of the heart, e.g. physiological, dynamic and magnetics field of the heart beyond electrical conduction. Therefore these parameters such as oxygen saturation, blood flow pressure, its pH, cardiac output, temperature, and the sound of the heart beat all needed to be counted and put into the invention of these devices. (5)
One of the earliest of cardiac biosensors was a piezopolymer pressure sensor developed in 1990’s as a portable fetal heart rate monitor to permit an expectant mother to perform the fetal non-stress test, a standard pre-delivery test at home. This sensor was designed to detect the distinctive features of the fetal heart tone, namely, the acoustic signature, frequency spectrum, signal amplitude, and localization. The components of the sensor served to fulfill signal detection, acceleration cancellation, acoustical isolation, electrical shielding, and electrical isolation of the mother. The theoretical analysis of the sensor response transformed to numerical values presented to the mother. (6) Soon still in 1990’s the different biosensors for detection and measurement of oxygen saturation, ventricular pressures and more in patients with cardiac disease specially heart failure was developed. (7)
Following the footstep of glucose monitoring biosensors from endocrinology, the cardiac biosensors research applied the similar enymatic detection and measurement to detect extra-cellular lactate accumulation to predict, hence prevent ischemic heart diseases, initially in lab animals, and later on in humans. (8) By the outset of the new millennium, the direct myocardial revascularization (DMR) using biosensor detecting map of the heart ventricles, had been examined as an alternative treatment for patients with chronic refractory myocardial ischemic syndromes who are not candidates for conventional coronary revascularization. (9) Using other cardiovascular system such as the minute capillaries of the body to implant cellular biosensors for the functional characterization and monitoring of molecular alterations in tissues in response to various stimuli were developed. These cellular biosensors provide information about cell and tissue internal transduction pathways, and such bio-hybrid sensors were also used to detect and record any tissue aggregates in the heart muscles, or any impedance measurement even in 3D and also for the detection of the effectiveness of drugs and therapies. (10)
Tang and colleagues from Stanford University in California in 2004 invented smart gene therapy for early treatment of ischemic heart disease so to prevent the afflicted heart from myocardial infarction (MI) or heart attack. Applying an ischemia-sensing biosensor to activate the expression of a therapeutic gene (heme oxygenase-1 [HO-1]), which acts to limit the extent of ischemic injury, these researcher were able to have the expression of the HO-1 gene under the control of the ischemia biosensor. So this smart biosensor is able to turn on and remove the ischemia and turn itself off when the repair is done. (11)
A frontier in the personal biosensor is “Holter Monitor” invented by the experimental physicist, Norman Holter in a primal version in 1949, and was available for commercial use since 1962. Holter monitor that could be worn comfortably by the individual, measures and monitors electrocardiac activity or ECG for a long period of time, at least 24 hours and is able to detect any abnormal heart activities such as arrhythmias. (12) The next developmental device to holter monitor is the wearable physiological monitoring system or “smart vest” as a washable shirt, which uses an array of sensors connected to a central processing unit with firmware for continuously monitoring physiological signals. The data collected can be correlated to produce an overall picture of the wearer’s health. The wearable data acquisition system is designed using microcontroller and interfaced with wireless communication and global positioning system (GPS) modules with a remote monitoring. The physiological signals monitored are electrocardiogram (ECG), photoplethysmogram (PPG), body temperature, blood pressure, galvanic skin response (GSR) and heart rate. (13)
The next step has been the development of more such personal biosensor devices through the cell phones and personal gadgets, so to make it possible for popular and easy use. One of these devices is a wrist-worn integrated health monitoring device (WIHMD) which performs the measurements of blood pressure, pulse, ECG, respiration rate, heart rate, and body surface temperature and the detection of falls to determine the onset of emergency situation with remote monitoring and control healthcare service center through cell phones were invented. IT-based blood glucometers have also been developed using cell phones. More health monitoring devices over the past decade have been developed such as one to use on bed while the person is sleeping; or an a chair while the person is sitting or on a toilet seat, measuring ECG, body temperature, body fat ratio, weight, estimate the degree of activity by motion analysis using a camera. Some of these devices already are capable, based on the diagnostic information and the result of the biosensors, to provide an appropriate medical consultation. (14)
While the field of cardiac biosensor is advancing, perhaps the next step in optimizing it would be going beyond detection and diagnosis of basic cardiac physiological abnormalities to provide treatment strategies. Combining the heroic smart gene therapy for early treatment of ischemic heart disease by Tang and colleagues with capability of the use of cell phones or personal gadgets for control and monitoring would be an ultimate goal.
From the heart to the brain: Biosensors possibilities
Every cells in the body has electrical action potentials including the brain cells or neurons. That is how overstimulation of the neurons can cause seizures that could be measured on the surface of the scalp by EEG (Elecrtoencephalography). Also causing seizure to the brain or ECT (Electroconvulsivetherapy) has been in use for long to treat refractory and severe mental conditions such as Schizophrenia or Depression. These methods and tools have been gross, measuring and treating the neurons from the surface and externally. Inside the brain and internally, chemical signaling of the neurotransmitters underlies every function of the nervous system to coordinate and control our emotions, behaviours and thoughts. Until the very recent time, the means of studying the neurotransmitters and neuromodulators production, release and signaling have all been through indirect measurement or clinical signs and symptoms manifestations. So we have been thus far limited temporally and spatially in real-time measurement and understanding of this myriad of complex neuronal functions. The advent of the new microelectrode biosensors has given us the unprecedented spatial and temporal resolution, enabling us for the first time in direct measurement in real time of the chemical conversations between cells in the nervous system. (15)
The urgent need in this era of biosensors advancement in neuroscience is well known to the scientists and clinicians in the field, as the neurochemical signaling in the brain constantly changes based on the requirement and environmental (external) or even internal stimuli. A basic example of this is the tonic (basic) neurotransmitters release and signaling at usual times, and phasic release and signaling in reaction to the temporal stimuli and demands. (16) Moreover many neurotransmitters, particularly the most abundant and generic ones such as GABA and glutamate, and neuromodulators such as Adenosine have not a local area of release and influence, but a vast diffuse area to distant synapses. (17-18) This importance would be possible through implantation of biosensors directly inside the brain tissue.
Although biosensor design strategies are rapidly becoming highly diverse, enzymatic biosensors remain an important vehicle of study of the brain. Enzymatic biosensors exploit an analyte-dependent step to convert an inert substrate to an active product. The accumulation of this active product in the biosensor is a measure of the concentration of analyte. Most biosensors in the brain research use oxidase enzymes with amperometric detection methods. There are related (non-biosensor) electrochemical techniques that have proved particularly useful in detecting, with carbon fibre microelectrodes, the release of transmitters such as dopamine or nitric oxide in the brain. Since the neurotransmitters and neuromodulators in the brain are sophisticated substrates and are not produced or limited by one single enzyme to detect easily, multi-enzyme sensors have been designed for some such as adenosine, ATP and acetylcholine. Also much smaller microelectrode and even ultra- microelectrode amperometric biosensors as tiny as 7 μm in diameter with better spatial and temporal resolution and less invasive than microdialysis electrodes are needed for studying chemical signalling in the brain. (19-21)
One area of the clinical application of the neuro-biosenors has already started in the case of Alzheimer’s disease. The known biomarker of the accumulation of Beta-amyloid (β-A) peptides, resulting in neurodegeneration and progression of this dementia has been used for diagnosis and monitoring the progress of the disease. Up until recently, no sensitive and inexpensive method has been available for β-A detection, and the available methods such as neuroimaging, enzyme-linked immunosorbent assay (ELISA), and polymerase chain reaction (PCR) to detect β-A, need sophisticated equipments, high expertise and complicated operations, and still are challenged with their low detection limit. Recently, β-A antibody based electrochemical immuno-sensing approach has been explored to detect β-A at pM levels within 30–40 minutes compared to 6–8 hours of ELISA test. The introduction of nano-enabling electrochemical sensing technology could enable rapid detection of β-A at point-of-care and may facilitate fast personalized health care delivery. (22)
Chasing the microbial invasions with the biosensors:
The use of biosensors in detection of microbes started in food industry to safeguard the unspoiled food products by such invasions and contaminations. Immunodiagnostics and enzyme biosensors are two of the leading technologies that have had the greatest impact on the food industry. The use of these two systems has reduced the time for detection of pathogens such as Salmonella in hours and has provided detection of biological compounds such as cholesterol or chymotrypsin. (23) By the end of 1990’s, a new type of biosensor for pathogens had been developed, that produced a spectral fingerprints of biological systems by using surface-enhanced infrared absorption (SEIRA) spectroscopy. The sensor was developed to detect Salmonella by immobilizing and detecting its antibodies. (24)
The biosensor detection of microbes soon by the start of the millennium became a possibility through multi-analyte biosensors using DNA hybridization, magnetic microbeads, and giant magnetoresistive (GMR) sensors to detect and identify biological warfare agents. (25) One of the earliest method of biosensor detection of pathogens, was modeled from the immune system such as antibodies that how they bind to the pathogens for defense. So the biosensors were modeled as such to bind similarly to the invading microbes, thus detect them first for diagnosis then treatment and ultimately prevention. One of the earliest of this was modeling the human pulmonary surfactant protein A (hSP-A), that plays a key defensive role against airborne invading pulmonary pathogens, such as Mycobacterium tuberculosis. (26) Along the same path, and with more sophisticated methods, the specific DNA sequences of the pathogens themselves were used to detect them. One of such designed bioelectric chips successfully detected 86% of the human papillomavirus (HPV) contained in clinical samples. (27)
E.coli bacteria with its many varied strains is a major cause of food and water contamination and a major cause of infectious diseases in humans specially Urinary Tract Infection (UTI), but often undetected due to its low numbers to be detected by the conventional methods. (28) By early 2000 nano-technology had been designed for the development of biosensors using nano-fabricated structures coated with elements such as gold that have affinity for biomolecules, so to detect this virulent pathogen. With this technology, rapid detection of this pathogen in a very low bacteria concentration by detecting Anti-E. coli antibody was feasible. (29) Soon by applying the array biosensor, the capability for simultaneously measuring titers of antibody against multiple pathogenic antigens became a reality. In an early study, Moreno-Bondi and colleagues were able to detect human antibodies against four different targets, tetanus toxin, diphtheria toxin, staphylococcal enterotoxin B (SEB) and hepatitis B, simultaneously in sera from eight different donors in a single assay with titers as low as approximately 100 fg by this technology. (30) Liao and colleagues were also able to detect several pathogens in human clinical fluid samples by targeting the pathogens’ DNAs and RNAs using a microfabricated electrochemical sensor array. In this amperometric detection system, the sensor array had 100% sensitivity for direct detection of gram-negative bacteria without nucleic acid purification or amplification, and could serve as a point-of-care device for rapid diagnosis of urinary tract infections. (31)
UTI as a very common infection specially in women with mortality in children became one of the first infection and its pathogens, one of the first target of biosensors to move from detection in lab to the bed side. Modeling the blood glucose biosensor detection in diabetes, the researchers in the field took the UTI biosensors by targeting DNAs and RNAs of pathogens to the point of care application by the clinicians in their offices and out of the labs. In addition these researches with the application of similar biosensors have been able to perform antimicrobial susceptibility testing taking days to be performed in the labs, to only 1-3 hours at the bedside. This made feasible the prompt treatment of the afflicted patients with the right antibiotics instead of initiating random treatments until the lab results arrive. (32)
Early detection and diagnosis of other infectious agents such as tuberculosis (TB) and Pseudomonas aeruginosa that by conventional methods is time-consuming, operator dependent and lacks sensitivity, has become a reality with biosensors with close to perfect sensitivity and specificity. (33) The application of biosensors in detection of pathogens and diagnosis of infectious diseases is not only yielding early detection and diagnosis, from laboring and long conventional lab tests to the bed side rapid tests, but they have also been more economical and possible in poor developing countries that suffer the most from infectious diseases. (34) An area of rapid need of detection of pathogens to save lives is in emergency blood donations. In emergency situations and when resources are depleted, on-site blood donations require the rapid and accurate detection of blood-borne pathogens, including human immunodeficiency virus. The conventional currently used techniques such as PCR and antibody capture by an enzyme-linked immunosorbent assay (ELISA) while precise, are time-consuming and require sophisticated equipment that is not compatible with emergency point-of-care requirements. In these urgent situations, the rapid and accurate detection of these pathogens is bedside possible by the biosensors that can filter out other cross-confounding pathogens as well. (35)
In addition to the rapid pathogens detection, rapid antimicrobial susceptibility testing (AST) for proper antibiotic treatment is also possible nowadays by specific biosensors such as the electrokinetics enhanced biosensors. (36) Most recently on an attempt to make biosensors detection of pathogens to the bedside, Phaneuf and colleagues developed a portable, microfluidic biosensor platform capable of simultaneous, multiplexed detection of several of the bacterial pathogens that cause very common enteric and diarrheal diseases, in less than 20 min. (37) Biosensors incorporated with photonic crystal (PC) structures hold promise to be available at the point of care and also inexpensive, reliable, sensitive and highly specific. Most recently the applications of PC-based biosensors has been incorporated with emerging technologies, including telemedicine, flexible and wearable sensing, smart materials and meta-materials. (38)
Biosensors in detecting cancers:
One of the earliest use of biosensors in cancer detection in early 1990’s was their experimental use in cell metabolic activities, including the cells divisions and perfusions. (39) Of the first tumors to be detected with biosensors was the neural crest and liver tumors. (40, 41) Biosensors are designed to detect biomarkers of cancer cells such as their specific proteins, DNAs and RNAs, transducing them into an electrical signal for early detection, analysis and even anti-tumour drug delivery and effective treatment at various target sites. Biosensor technology has the potential to provide fast and accurate detection, reliable imaging of cancer cells, and monitoring of angiogenesis and cancer metastasis, and the ability to determine the effectiveness of anticancer chemotherapy agents. (42)
Cancer biomarkers are potentially one of the most valuable tools for early cancer detection, accurate pretreatment staging, determining the response of cancer to chemotherapy treatment, and monitoring disease progression. (43) Prostate-specific antigen (PSA) was one of the first tumor biomarkers to be identified and put into routine clinical use for screening and diagnosis of prostate cancer. (44) Elevated cancer antigen (CA) 125 is most commonly associated with ovarian cancer and is also linked to cancers of the uterus, cervix, pancreas, liver, colon, breast, lung, and digestive tract. (45) CA 15-3 is an important biomarker analyzed in breast cancer patients. Other biomarkers that are linked to breast cancer are carcinoembryonic antigen (CEA), BRCA1, BRCA2, and CA 27.29. (46) Cancer-testis (CT) antigens are a unique class of cancer biomarker that are highly expressed in many tumors, but not in normal cells, except for germ cells of the testis. Thus, they have been heavily pursued as potential immunogenic targets for cancer immunotherapies (i.e. cancer vaccines), and autoantibodies to CT antigens have been pursued as cancer biomarkers. (47)
The advent of nanotechnology or nano-sensors, using nanomaterials or nano-particles such as Liposomes, dendrimers, buckyballs, carbon, silicon nanowires, and graphene nanotubes allows more sensitive, specific and precise measurement of cancerous tissues through binding of these biomolecules to the surface of the nanostructure. Additionally, the use of nanotechnology means smaller sensors, which translates into better access to and detection of cancer markers, as well as more powerful and specific signal enhancements, reduced cost, and high throughput detection. (48, 49)The nano-sensors has also made possible the rapid screens of telomerase activity of cells that are elevated in many malignancies. (50) Electrochemical biosensors with multi-array sensors up to 100 have been used in breast cancer screening and detection of cancer susceptibility genes. (51) Also antibodies raised against cancer cells such as monoclonal antibody, have been used by biosensors to detect cancer cells such as breast cancer. (52)
By late 1990’s, the biosensors’ research were able in scanning and screening of cell mutations leading to cancers through DNA analysis by techniques such as polymerase chain reaction in real-time. Applied on the human tumour suppressor p53 gene, this technique allowed the detection of point mutations in exon 6 PCR products, derived from a breast tumour biopsy sample. (53) Rapid and effective differentiation between normal, cancerous and metastatic cells through their differential interactions with functionalized nanoparticles, transduced through multivalent polymer sensor has been effectively possible within seconds to minutes. (54)
Cisplatin an anti-tumour agent used in chemotherapy of many cancers since late 1970’s recently with the application of a novel cell biosensor chip system have been allowing continuous monitoring of different cancer cell deaths in real-time. (55) Highly sensitive and specific biosensors based on fluorescence resonant energy transfer (FRET) have been able visualizing lively cancer cells progression and metastasis. (56) Ligand-directed targeting and capturing of cancer cells by biosensors is another new approach for detecting circulating tumor cells. Ligands such as antibodies and cancers specific peptides have been successfully used by biosensors for real-time capturing circulating cancer cells in the blood. (57)
Zhang and colleagues have developed a nanowire-based biosensor for the detection of micro-RNAs that are important regulators of gene expression and are associated with cancer development. (58) The use of single-walled carbon nanotubes has greatly enhanced the detection capabilities of electrochemical biosensors, providing increased sensitivity to enzymatic reactions, and have been used in nucleic acid-based sensors and immunosensors for cancer biomarkers to enhance signal detection and transduction. Nanotechnology has also enabled advances in optical biosensor technology in the form of surface-enhanced Raman scattering, achieving a degree of multiplicity, measuring up to 20 biomarkers at one time without any interference. (59-62)
The development of microfluidic “laboratory on a chip” (LOC) devices is a classic example of how nanotechnology can improve patient care. LOC technology takes the complexity of a laboratory and simplifies it into a low-cost, easy-to-use, portable system that can be used by physicians or patients. LOC methods that incorporate immune-assays and DNA hybridization arrays have been tested for their ability to identify individuals who may be at a high cancer risk. (63) Another major application of nanotechnology is the use of quantum dots, that are luminescent nanocrystals that have many of the same properties as optical biosensors. (64) Quantum dots can emit light of different wavelengths, intensity, and spectral width, allowing for the diagnosis and detection of multiple unique molecular elements, able to track molecules and entire cells as they move through an environment (65). As such, they can be very important in monitoring cancer development by tracking the migration of cancer cells, cancer metastasis, and drug therapy effectiveness. The allure of quantum dots is their high stability, multimodality, and small size (~50–100 units in diameter for biological applications). Quantum dots are also capable of delivering therapeutic agents to specific target sites to improve pharmaceutical effectiveness while minimizing side effects. (66) Similarly, the use of dendrimers as drug delivery systems and for effective targeting strategies has also emerged from the field of nanotechnology. (67)
Immunosensors in detecting antibodies:
In addition to classical immunochemical techniques such as ELISA for detection of antibodies in diagnosis of autoimmune disorders and infections, various novel antibody-based sensor technologies have been developed since 1980’s to further increase the utility of antibody-based detection methodologies. (68-72) Various nanomaterials such as colloidal gold/silver, semiconductor nanoparticles, and markers loaded nanocarriers (carbon nanotubes, apoferritin, silica nanoparticles, and liposome beads) that have revolutionized biosensors’ detection and diagnosis of different biological substrates have also advanced the power of immunosensors. The enormous signal enhancement associated with the use of nanomaterial labels and with the formation of nanomaterial-antibody-antigen assemblies provides the basis for ultrasensitive electrochemical detection of disease-related protein biomarkers, biothreat agents, or infectious agents. (73)
The post-genomics era has realized that the sequenced genome is not enough to understand the pathophysiology, diagnose and manage the biological and pathological processes fully at a practical level. Other new “omics” fields such as “proteomics” and “metabolomics” characterized by data-intensive research and biotechnologies enabling to narrow the existing gaps between discovery science and the clinical applications. (74-75) Moreover chasing the genes as if every condition is hereditary and ignoring the epigenetics, or the impact of environment on the genetic make up of the beings has proved insufficient and not solving more than a quarter of any pathological puzzle. Specifically when studying autoimmune disorders and cancers, the genomics are very shortcoming. On the other side, the genes are not anything more than DNAs and RNAs or sequences of protein chains, and not only antigens, antibodies, cytokines, MHC and other inflammatory and immune proteins, but a whole host of biomarker proteins that are in need of their specific studies of their own. That is where biosensors and immunosensors with the help of post-genomics of proteomics and metabolomics come to play an important part in solving the mystery of life in health and disease.
The application of the biosensors through nanotechnology has created the new science field of “Nanoproteomics” that is used in many clinical conditions such as infectious, endocrine, autoimmune, and neurodegenerative diseases, as well as brain injury and several types of tumors. (76) While the conventional diagnostic techniques in autoimmune disorders such as ELISA and microarray are low in sensitivity, the nanoproteomics techniques could be promising. For example peptide-coated nanotubes detected 12 out of 32 Rheumatoid Arthritic(RA) patients missed by ELISA and microarray. (77) Several other studies have also reported the use of nanoproteomics for the detection of RA-related immunoglobulins. (78) Carbon nanotubes has also been able to detect anti-proteinase 3 autoantibodies that are markers of Wagner’s granulomatosis, three-fold more compared to the conventional and recent fluorescent detection techniques. (79). In addition, nanoproteomics techniques have also been assessed for utilization in celiac disease through the detection of anti-gliadin autoantibodies using different techniques where high sensitivity detection potentials have been demonstrated. (80). Lastly and most recently more advanced nanoproteomics techniques such as magnetic and optical sensors such as quantum dots, have been capable of detecting the tinniest and complicated protein particles in different diseases such as autoimmune disorders, unreachable before even with other predecessor biosensors. (81-84)
Sky is the limit: Molecular Motors
It took biomedical engineers many years until late 1980’s to recognize the molecular motors of biological beings such as microbes, known already by microbiologist. The first to be recognized was the cilia and flagella for their intracellular and extracellular motilities, including mitosis and organelle movement, through their microtubules. These motilities were observed to be highly ordered, fast and efficient, operating in both the anterograde (outwards from the cell body) and retrograde (from the periphery towards the cell body) directions. Similar microtubule-associated movements were also observed to be involved in secretion, endocytosis and the positioning of organelles within the cell. The multi-functions of these molecular motors are possible through mechano-chemical proteins such as Kinesin, MAP 1C, a microtubule-activated ATPase, dynein ATPase. All these also regulated from inside and outside by various diffusible factors such as Ca2+, cyclic adenosine monophosphate (cAMP), polypeptides and so on. Also other constituents of the cytoskeleton are involved in cellular motility, such as actin microfilaments and their motor myosin, intermediate filaments, and non-actin filaments. (85-86)
It was not until the advent and advancement of the biosensors and bio-informatics that engineers of hard bioscience started to ponder of modeling the molecular motors.(87) Thereby the minute molecular machines offered exciting possibilities for a new generation of hybrid biomotor sensing and actuation systems with applications ranging from microfluidic mixers and motors, to chemical sensing, to medical diagnoses. (88) These sensing and actuation systems precision had been engineered by nature over millions of years with unquestionable advantages over man-made chemo-mechanical systems, with no “wear-and-tear” (88), but self-regulating and self-healing capabilities (89), and very high almost 100% energy conversion efficiency (90), to name a few. For the first time humans initiated an outstanding process of inventing molecular motor-based biosensing and bioactuation micro and nano systems.(91) Molecular motors have been thus far used to realize microfluidic actuators and sensors, including the nano-propeller system by attaching a nanofabricated nickel propeller to an F1-ATPase, and micro-electro-mechanical-system (MEMS)-based actuator, power generating devices, and sensor system using for example bacterial flagellar motors. Despite all these most recent progress, the field of engineering molecular motors for hybrid living-synthetic engineered systems is still in its infancy.(92-94)
Numerous bacteria species swim around to find favorable conditions for their survival through flagellar motors found in many including Escherichia coli and Salmonella enterica typhimurium that are precision engineered by nature to provide cellular locomotion in response to environmental stimuli, i.e., tactile response. (95) Motor control is regulated through mechanisms that both sense the environment and communicate sensed information to the motor output to actuate the motor’s rotation. A motile E. coli cell typically has four to eight flagellar motors embedded in the cell envelope at random points on the cell body. Each motor, 45 nm in diameter, is constructed from about 20 different proteins that work in tandem and function like a man-made stepping motor. As it is seen in the following diagram, the L and P rings are the bushing for the driveshaft (rod), the MS and C rings constitute the rotor, which is surrounded by a ring of about 11 stator particles embedded in the cytoplasmic membrane. Each flagellar motor is connected to a helical flagellar filament through a universal joint called the hook. Through this structure, the motor spins the filament at a very high speed of over 100 cycles per second. The four rotating filaments on the bacterial cell body allow the cell to swim either in search of nutrients or away from repellants. Each flagellar filament, formed by subunits of the protein flagellin, is about 10 μm long with a diameter of about 20 nm. A flagellar motor can produce a power output of 10−15 and also, it can produce a torque level of 4 nN–nm. (96-97)
The highly efficient sensing and movement of genetically engineered microbial cells present their potential opportunity for an integration of living and engineered systems. This might make real applications in biosensing, micro/nano fluidics, etc., producing novel micro/nano devices for drug delivery, energy conversion, real-time detection of chemical and biological toxins, etc. Specifically, the high power and high torque capability of flagellar motors suggest the possibility of using flagellar motors in a hybrid system for micro-actuation as well as micro power generation. Furthermore, the chemotactic responses of flagellar motors toward repellents, such as nitrate and nitrite, suggest the possibility of using flagellar motors in a hybrid system for the detection of harmful chemicals, such as nitroaromatic and organic nitrate explosives. The challenges are to create a micro/nano system that has a predetermined number of flagellar motors at designed locations and orientation, to sustain the motors’ activity and optimize their sensitivity, which are tailored to specific applications, and to design and fabricate micro/nano systems with integrated modules for sensing and control. (96-97)
A prerequisite to the integration of bacterial flagellar motors with micro/nano-fabricated devices is a controlled interfacing of biological, i.e., bacterial cells, and abiological, i.e., nanoparticles, components at similar scales, allowing control over the number and location of cells in the nano-sensors. However, such controlled interfacing is not trivial and not only should the interplay of biophysicochemical and mechanical properties of both biological and abiological components be considered but careful rational design is also required for the development of the bio-hybrid micro/nano systems with desired functions. These include three parts that dynamically interact each other: (1) the abiological surface, e.g., nano-sensors’ properties of which depend upon its physicochemical characteristics, including charge, roughness, and accessible surface area as well as available functional groups and ligands, (2) the biological component, i.e., bacterial flagella, the properties of which rely upon its biophysicochemical characteristics, including length, charge, hydrophilicity and hydrophobicity, and available functional groups and ligands, and (3) the bio–abio interface where the biological component contacts with the abiological via one or combinations of electrostatic, electrosteric, steric, hydrophilic and hydrophobic, and ligand–receptor binding interactions. (94)
On the other hand, real-time detection of chemical and biological toxins in the environment is challenging because of the number of potential agents to be distinguished, the complex nature of the agents themselves, and the myriad of similar agents that are constantly present in the environment. Thus, chemotactic systems capable of non-specific identification, e.g., determining the presence of chemical and biological toxins by targeting generic factors, could be highly desirable in some applications. Although much still needs to be done, the initial important step toward the development of future hybrid systems of rotational cells and micromachined components has already been taken. (94)
Personalized or mobile biosensors:
All the above and until most recently, the biosensor has been confound to the research arena or labs for faster and more precise detection of diseases. Over the past decade there has been increasing interest in the extension of scientific achievements in bioinformatics and biosensors to the bedside and clinics, to be practically used by clinicians or point-of care. At the same time some biomedical scientists and engineers have been striving to personalize some of these developments and make it available for personal use, as wearable devices or conversion to mobile applications on personal gadgets such as cell phones. Informing or warning some patients of emergency conditions in certain diseases through such mobile and personalized devices could save lives and cost and take treatment and preventing medicine to a higher level of care for this new century. A simple application of such devices at this early stage could be only informative and interactive, making the user more conscious of their own health condition in a way to adjust incorrect lifestyles, to obtain a personalized therapy tuned to their own physiological reactions and on their own environmental condition. On the clinicians and health authorities side, current and prompt knowledge of a citizen’s health status and monitoring them from a distance without frequent visits would be invaluable.
One of such devices is wearable e-textile or clothes with biosensors where conductive and piezoresistive materials, sensors and electrodes are knitted in and connected to an electronic portable unit. (98) One of these wearable early sensor or intelligent devices, has been a wrist-worn integrated health monitoring device connected to a home telehealthcare system in used in the early years of this century. This device performed blood pressure, pulse oximetry, electrocardiogram (ECG), respiration rate, heart rate, blood glucose level and body surface temperature and also detection of falls to determine the onset of emergency situations, and all these bio-signals transmitted to a healthcare service center through a commercial cellular phone. Another such early device was mounted on bed, while the patient is sleeping; and another one on a toilet seat measuring all the above vital signs plus body fat ratio and weight. (99)
Another of these biosensor personalized devices has been used in elderly population suffering from urinary incontinence, especially in nursing home residents with dementia. This common problem in this patients population due to its distressing and cost on both sides affects not only the patients but also the caregivers and the health care system. Effective continence management is required to provide quality care, and to eliminate high labor costs and annoyances to the caregivers resulting from episodes of incontinence. Through a smart wireless continence management system for dementia-impaired elderly or patients in institutional care settings such as nursing homes and hospitals, the wearable biosensor detects wetness and through its intelligent signal relay mechanism notifies the care-givers to attend to the patient in real time promptly. (100)
In cancer diagnostics, since the conventional biopsies are invasive and depending on the site, location and the time of biopsy not very sensitive, the possibility of repeated ongoing sampling through the tumor biomarkers by biosensors is promising. Daniel and colleagues in 2009 have reported an implantable diagnostic device that could be left behind during biopsy, and by using a semi-permeable membrane, containing nanoparticle magnetic relaxation switches, able to detect a model cancer biomarker produced ectopic tumors in mice. Applications of such a device in cancer diagnostic are numerous, including verification of successful continuous monitoring the cancer biomarkers, progress and metastasis of the tumours in vivo, and even real time chemotherapeutic treatment through the device. (101)
During the last decade, there has been a rapidly growing trend toward the use of cellphone-based devices in bioanalytical sciences. For example, they have been used for digital microscopy, cytometry, read-out of immunoassays and lateral flow tests, electrochemical and surface plasmon resonance based bio-sensing, colorimetric detection and healthcare monitoring, among others. Cellphone can be considered as one of the most prospective devices for the development of next-generation point-of-care diagnostics platforms, enabling mobile healthcare delivery and personalized medicine. Many cellphone-based devices, such as those targeted for diabetic management, weight management, monitoring of blood pressure and pulse rate, are already available under health monitoring apps in many cell phones. In addition several of cell phone devices have already introduced other medical applications such as microscopic imaging and sensing applications for medical diagnostics using novel computational algorithms and components already embedded on cellphones. (102)
Most recently a portable system for personalized blood cell counting consisting of a microfluidic impedance cytometer and portable analog readout electronics, feeding into an analog-to-digital converter (ADC), and being transmitted via Bluetooth to a user-accessible mobile application has been reported. This new biosensor device could soon have the utility of wide panels of detection and counting beyond blood cells such as biomarkers including proteins, nucleic acids, and various cell types. (103) Also most recently from Russia there has been a report on monitoring cancer circulating tumor cells (CTCs) in blood soon will be a reality through the application of synthetic DNA or RNA aptamers. Aptamers transform cell isolation technology, by binding to and releasing cells on-demand, detecting cell surface biomarkers in their native state, and conformation without previous knowledge of their biomarkers. Once aptamers are produced, they can be used to identify CTC biomarkers using mass spectrometry. The biomarkers and corresponding aptamers then could be exploited to improve cancer diagnostics and therapies. (104)
The incorporation of mechanical, electrical and other engineers with the chemists, biochemists, electronics and computer engineers and specialists, created the field of biomedical engineering to invent the biosensors. This over time led to the advent of first artificial cardiac pacemaker; holter monitor and glucometer; then over the past couple of decades to cardiac ischemia-sensing biosensor; Beta-amyloid electrochemical nano-sensor in Alzheimer’s disease; immunosensors detecting DNAs and RNAs of pathogens through bioelectric chips and point-of-care rapid diagnosis of common infections; nanotechnology or nano-sensors using nanomaterials in chasing cancer biomarkers, even circulating cancer cells; Lab on chip (LOC) taken all these lab tests to the bedside and physician offices; the post-genomic sciences of proteomics and metabolomics merging with biosensor technology to advance antibodies detection in autoimmune and other disorders; and the unbelievable modeling of molecular motors by hybrid biomotor-sensing machines; and finally the personalized or mobile biosensors.
Still to come will be the application of the molecular biomarkers and their detection by biosensors particularly through nano-technology not only for medical diseases, but changing the current diagnostic paradigms of psychiatric disorders that are still relying on the poor diagnostic system of symptoms counting of DSM. The development of new genomic, proteomic, and metabolomic tools along with the more and more sophisticated and accurate biosensor technologies, and the fusion of clinical biomarker data, electroencephalogram, and MRI data, and else promise the point-of-care testing and proper diagnosis and ultimately real time treatment. All these through the application of mobile communications technology could be personalized and available to the individuals as well. (105)
Furthermore the biological properties of the mitochondria coupled with the relative simplicity of the mitochondrial genome that giving this organelle extraordinary functionality as a biosensor will be soon exploited and adapted in biosensing devices. This will place the field of mitochondrial genomics in a position of strategic advantage to launch significant advances in personalized medicine in many medical conditions for diagnosis and treatment in the near future. (106) Expanding biosensors specially through nanotechnology to real time, fast and on-site detection and diagnosis of urgent, contagious and spreading conditions such as infectious epidemic like Ebola virus (EBOV) is vitally needed in the near future. For example the application of electrochemical EBOV immunosensing can detect the virus within ∼40min-6 hours compared the current diagnostic method of ELISA and RNA of 3-10 days. (107)
Dr. Mostafa Showraki, MD, FRCPC
Lecturer, School of Medicine, University of Toronto
Author: ADHD:Revisited Book
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