Optical tweezers

1 Principles of operation

Optical tweezers use the transfer of momentum from light to matter to confine and move microscopic objects. A tightly focused laser beam creates a three-dimensional force field that can hold particles near the focus. The method is especially effective for transparent objects whose refractive index differs from that of the surrounding medium.

The trapping effect depends on the balance between forces that pull a particle toward the region of highest intensity and forces that push it along the beam direction. In practice, the instrument behaves like a very small, tunable spring. This allows precise control over position, displacement, and force.

1.1 Radiation pressure and gradient force

Light carries momentum, and when it is absorbed or scattered by a particle, part of that momentum is transferred. This transfer produces radiation pressure, which tends to push the particle in the direction of beam propagation. If this were the only force present, stable trapping would be difficult.

A focused beam also generates a gradient in light intensity. For particles with a higher refractive index than the surrounding medium, this gradient force pulls them toward the brightest region, usually near the focal point. Stable trapping arises when the gradient force overcomes the scattering force.

1.2 Optical trapping mechanism

The trapping mechanism depends on the interaction between the particle and the electromagnetic field. In the ray-optics regime, large particles can be understood as deflecting many individual light rays, each contributing a small momentum change. For smaller particles, the object behaves more like an induced dipole that is drawn toward the field maximum.

The result is a local potential well around the focus. A trapped particle can move slightly within this region, but the restoring force increases with displacement. This restoring behavior underlies many quantitative measurements in optical tweezing.

1.2.1 Stable equilibrium in the focal region

A stable trap is formed where the net force on the particle is zero and small displacements produce restoring forces. Lateral confinement is usually strong because the beam is tightly concentrated in the focal plane. Axial confinement is often weaker and depends more sensitively on beam shape and optical alignment.

The equilibrium position can shift if the laser power changes or if the surrounding medium is altered. In many experiments, the particle is held close to the beam center but not exactly at a single mathematical point, since thermal motion continually causes small fluctuations.

1.2.2 Influence of particle size and refractive index

Particle size strongly affects trapping behavior. Very small particles interact weakly with the light field, while very large particles may be difficult to hold if the beam cannot provide enough gradient force. The best trapping conditions usually occur when particle dimensions are comparable to the focal region or when the optical system is optimized for the particle scale.

Refractive index contrast is equally important. Particles with a higher refractive index than the medium are generally trapped at the intensity maximum. Objects with a lower refractive index can be repelled or require alternative trapping arrangements. The medium itself also matters, since its optical properties influence scattering, absorption, and focusing.

1.3 Trap stiffness and force calibration

Trap stiffness describes how strongly the optical trap resists displacement. It is commonly modeled as a spring constant over a small range around equilibrium. Accurate calibration is essential when optical tweezers are used as force-measuring tools.

Calibration relates measured position signals to actual forces on the trapped particle. The result depends on laser power, bead size, viscosity, detector response, and optical geometry. Because these factors can vary, calibration is usually performed for each setup and experimental condition.

1.3.1 Brownian motion methods

Brownian motion methods use the random thermal fluctuations of a trapped particle. By analyzing the position distribution or time-dependent motion, researchers can infer the trap’s restoring strength. This approach is widely used because it relies on intrinsic thermal behavior rather than an external pulling mechanism.

The particle’s motion is influenced by viscous drag from the surrounding fluid. Careful analysis of the fluctuation statistics provides a practical estimate of force sensitivity and trap stiffness. These methods are especially useful for small beads in liquid environments.

1.3.2 Power spectrum and equipartition methods

In the power spectrum method, the frequency content of the particle’s motion is measured and fitted to a theoretical response curve. The corner frequency of the spectrum is related to the stiffness of the trap and the drag on the particle. This technique can be highly precise when detector noise is low.

The equipartition method uses the average thermal energy stored in the trapped particle’s displacement. By comparing the measured variance with the thermal energy scale, the stiffness can be computed. This approach is conceptually simple and often serves as a standard calibration check.

2 Instrument design

An optical tweezers system combines a laser, beam-shaping optics, detection hardware, and mechanical control components. The optical path must deliver a stable, well-focused beam to the sample while preserving beam quality. Mechanical and electronic stability are equally important, since small drifts can alter the trap position or sensitivity.

Modern instruments may be integrated with microscopes, microfluidic devices, or spectroscopic modules. This flexibility has made optical tweezers adaptable to many different research settings.

2.1 Laser source

The laser provides the coherent light required to create the optical trap. Continuous-wave sources are commonly used because they supply steady intensity and predictable focusing behavior. The choice of laser determines both the trapping efficiency and the level of heating in the sample.

Power stability and low pointing drift are important. Even minor fluctuations can change the trap strength or produce unwanted motion. For many applications, the laser must be tunable enough to balance trapping performance against possible photodamage.

2.1.1 Wavelength selection

Wavelength selection depends on the sample and the surrounding medium. Longer wavelengths often reduce absorption by biological materials and therefore limit heating. Shorter wavelengths can produce stronger scattering in some cases, but they may also increase damage risk.

In transparent media, wavelengths are typically chosen to minimize interference with fluorescence or absorption-based measurements. The selection also affects the efficiency of the focusing optics and the detector response. As a result, wavelength choice is often a compromise between trapping force, safety, and compatibility with other measurements.

2.1.2 Beam quality and stability

Beam quality affects how tightly the light can be focused. A clean, well-collimated beam produces a smaller focal spot and a more predictable trap. Poor beam quality can broaden the focus and weaken confinement.

Stability is equally essential. Beam pointing drift, mode fluctuations, and intensity noise may shift the trap or introduce measurement artifacts. High-quality laser systems therefore emphasize stable output and careful optical isolation.

2.2 Beam delivery and focusing optics

The beam delivery path directs the laser through mirrors, lenses, and sometimes beam expanders or modulators before it reaches the sample. Each element must preserve alignment and minimize losses. The focusing optics then concentrate the light to the small region where trapping occurs.

In microscope-based systems, the trapping beam is commonly introduced through the objective lens. This arrangement enables precise control of the focus relative to the imaging plane. It also allows simultaneous observation of the trapped object.

2.2.1 High-numerical-aperture objectives

High-numerical-aperture objectives are central to most optical tweezer systems. They gather light over a wide angle and create the steep intensity gradient needed for strong trapping. Their ability to form a tight focus is what makes stable three-dimensional confinement possible.

Such objectives require careful matching to the optical system and sample medium. Immersion type, correction quality, and working distance all influence performance. Inaccurate alignment or incorrect immersion conditions can reduce trapping efficiency.

2.2.2 Alignment and steering systems

Alignment systems keep the beam centered and properly focused. Adjustable mirrors, lenses, and steering stages help place the trap at the desired location in the sample. Precise alignment is necessary both for initial setup and for maintaining long-term stability.

Beam steering may also be used to move the trap actively. Small changes in beam direction translate into particle motion, allowing researchers to reposition objects without mechanically moving the sample stage. This capability is especially useful in manipulation and assembly experiments.

2.3 Detection and feedback

Detection systems measure the position of the trapped particle and the response of the optical trap. These measurements support force estimation, calibration, and active control. Sensitive detectors can resolve nanometer-scale motion under suitable conditions.

Feedback loops may use the detected position to adjust the beam or stage in real time. Such systems improve stability and can hold a particle at a fixed location even when external disturbances are present.

2.3.1 Position sensing

Position sensing often relies on imaging or interferometric detection. Video microscopy can track particle motion directly, while quadrant photodiodes or similar sensors provide faster response for dynamic measurements. The choice depends on the required speed and precision.

High-resolution position sensing is essential for force spectroscopy and calibration. The detector must distinguish true particle motion from background noise, optical drift, and stage movement. Careful signal processing often improves measurement reliability.

2.3.2 Force measurement electronics

Electronics convert detector signals into usable force and position data. Amplifiers, filters, and data acquisition systems must preserve temporal resolution while limiting noise. In high-precision experiments, electronics are designed to match the bandwidth of the particle motion.

The force acting on the particle is inferred indirectly from displacement, drag, or feedback corrections. Reliable electronics therefore play a central role in translating optical trapping into quantitative measurement.

2.3.3 Active stabilization

Active stabilization compensates for drift in the laser, optics, or sample stage. It may use closed-loop control based on position readings or reference markers. This is especially important in long experiments where small thermal or mechanical changes would otherwise degrade trap performance.

Stabilization can also reduce vibration and improve repeatability. In some setups, it enables extended measurements of molecular or cellular mechanics with minimal positional error.

3 Types of optical tweezers

Optical tweezers exist in several forms, each suited to different experimental goals. Some systems emphasize simplicity and robustness, while others are designed for parallel trapping or specialized near-field effects. The choice of configuration depends on sample type, desired control, and available instrumentation.

3.1 Single-beam optical tweezers

Single-beam optical tweezers are the most common design. A single focused laser beam creates one trapping region, usually at the focal point of a high-numerical-aperture objective. This arrangement is versatile and relatively straightforward to implement.

Single-beam traps are well suited for particle manipulation, force measurements, and biological assays. Their main advantage is simplicity, but they can be limited when many traps or complex spatial arrangements are needed.

3.2 Dual-beam optical tweezers

Dual-beam optical tweezers use two opposing beams to hold a particle between them. The counterpropagating geometry can provide strong axial confinement and reduce some alignment challenges associated with single-objective designs. It is often used when a particle must be held in the center of a larger optical path.

This configuration can also be adapted for stretching objects such as cells or fibers. Because the force balance comes from both beams, the trap may be more stable in certain directional measurements.

3.3 Holographic optical tweezers

Holographic optical tweezers create multiple trap sites by shaping the phase of a laser beam. This makes it possible to control many particles at once and to reposition traps rapidly. The method is especially valuable for complex assembly tasks and parallel experimentation.

Because the trap pattern is defined electronically, holographic systems offer a high degree of flexibility. They can generate arrays, dynamic trajectories, or trap shapes tailored to a specific sample.

3.3.1 Spatial light modulators

Spatial light modulators are devices that alter the phase of incoming light across a surface. In holographic systems, they encode the desired trap pattern and direct light into multiple focal points. Their programmable nature allows fast reconfiguration without mechanical changes to the optics.

These modulators are central to advanced optical manipulation. Their performance affects trap efficiency, pattern fidelity, and the ability to form stable arrays.

3.3.2 Multiplexed trapping

Multiplexed trapping refers to holding several particles simultaneously. Each trap can be independently positioned, allowing coordinated motion and arrangement. This capability is useful for studying interactions between particles or for building ordered structures.

The main challenge is distributing laser power among multiple traps without making each one too weak. Efficient multiplexing requires careful balance between trap number, spacing, and intensity.

3.4 Plasmonic and near-field tweezers

Plasmonic and near-field tweezers use strong electromagnetic fields generated near nanostructured surfaces. Instead of relying solely on conventional far-field focusing, they exploit localized field enhancement to trap very small objects. These systems can operate on scales smaller than those accessible to standard optical tweezers.

They are particularly promising for nanoscale manipulation, though they may be more sensitive to heating and surface effects. Their use often requires specialized fabrication and careful control of the local environment.

4 Interaction with trapped objects

The response of a trapped object depends on its optical, mechanical, and thermal properties. In many cases, the particle behaves like a passive probe of the surrounding field. In biological samples, however, the object may also be alive, active, or mechanically complex.

4.1 Forces on dielectric particles

Dielectric particles experience both gradient and scattering forces in the optical field. Their behavior can often be predicted from refractive index contrast, shape, and size. Spherical beads are the simplest case, but irregular objects can also be trapped if the optical forces are favorable.

The surrounding fluid adds viscous drag, which influences movement and the time required to reach equilibrium. For quantitative work, optical forces are usually interpreted together with hydrodynamic effects.

4.2 Trapping of biological samples

Biological samples can often be trapped without direct physical contact, making optical tweezers valuable for gentle manipulation. Cells, organelles, DNA, and proteins have all been studied using this technique. Success depends on minimizing damage while maintaining sufficient trapping strength.

The ability to handle living material has made optical tweezers important in biophysics and molecular biology. They are especially useful when mechanical measurements must be made on individual specimens rather than on bulk populations.

4.2.1 Cells and organelles

Cells can be trapped, moved, oriented, or held against mechanical probes. Organelles and other internal structures may also be manipulated in specialized contexts. These experiments help researchers examine size, deformability, and mechanical response.

Because cells are sensitive to light exposure, trapping conditions must be chosen carefully. Low-absorption wavelengths and controlled laser power are often preferred to preserve viability.

4.2.2 DNA and proteins

DNA and proteins are common targets in force spectroscopy. Individual molecules can be stretched, folded, or unfolded while force and extension are monitored. These experiments provide detailed information about molecular structure and energy landscapes.

Handling such small objects often requires coupling them to larger beads that can be trapped more easily. The bead acts as a mechanical handle, transmitting the optical force to the molecule of interest.

4.3 Heating and photodamage

Laser light can heat the sample through absorption by the particle, the medium, or impurities. Excess heating may alter biological function or change the mechanical behavior of the system. For this reason, thermal effects are a central consideration in experimental design.

Photodamage can arise from direct absorption, reactive chemical processes, or prolonged exposure to intense light. The risk depends on wavelength, power, exposure time, and the nature of the specimen.

4.3.1 Absorption effects

Absorption converts optical energy into heat. Even modest absorption may matter in small volumes where heat cannot dissipate quickly. Local temperature changes can affect viscosity, reaction rates, and molecular stability.

In biological experiments, absorption-related heating can stress cells or interfere with normal function. Researchers therefore often select wavelengths and power levels that minimize this effect.

4.3.2 Thermal management

Thermal management includes power reduction, beam shaping, medium selection, and use of temperature monitoring. Good heat dissipation helps preserve sample integrity and measurement accuracy. In some systems, fluid flow or stage design is adjusted to remove heat more efficiently.

Careful thermal control allows longer observation times and more reproducible results. It also reduces the likelihood that the trap itself alters the process being studied.

5 Experimental methods

Optical tweezers experiments usually follow a sequence of preparation, calibration, manipulation, and analysis. Each step must be controlled to ensure that measured forces reflect the sample rather than instrument drift or misalignment. Methods vary according to whether the goal is basic trapping, force measurement, or dynamic manipulation.

5.1 Sample preparation

Samples are often diluted to isolate individual particles or molecules. The suspension medium must be chosen to match optical, mechanical, and biological requirements. For bead-based assays, surfaces may be chemically treated to attach molecules or reduce unwanted sticking.

Preparation also includes cleaning the optical chamber and controlling buffer conditions. These steps help maintain stable trapping and consistent behavior across measurements.

5.2 Trap calibration procedures

Calibration procedures determine the relationship between detector output, displacement, and force. They may be performed before each experiment or whenever conditions change significantly. Common methods include analysis of thermal motion, drag response, and spectral behavior.

Reliable calibration depends on knowing particle size, viscosity, temperature, and optical power. Inaccurate calibration can lead to systematic errors in the reported force values.

5.3 Manipulation and micromanipulation

Once calibrated, the trap can be used to move objects with high precision. Micromanipulation includes placing particles at specific locations, holding them during observation, and applying controlled mechanical loads. This is one of the defining strengths of the technique.

Manipulation can be manual, semi-automated, or fully automated. Advanced systems allow multiple degrees of freedom and coordinated motion of more than one trapped object.

5.3.1 Translation and rotation of particles

Particles can be translated by moving the beam or the sample stage. In some cases, optical forces can also induce rotation, especially for asymmetric objects or birefringent particles. This enables orientational control as well as positional control.

Such capabilities are useful for aligning cells, studying anisotropic materials, and assembling small components into desired configurations.

5.3.2 Force spectroscopy

Force spectroscopy measures how a system responds to applied load. In optical tweezers experiments, a trapped bead can pull on a molecule or be pushed against a resisting structure. The resulting force-extension or force-time curves reveal mechanical properties of the sample.

This technique is widely used for studying unfolding transitions, binding events, and motor activity. It offers high sensitivity over a range of forces that is especially relevant to soft biological matter.

5.4 Data acquisition and analysis

Data acquisition systems record position, intensity, and timing information at high speed. The resulting signals are then filtered, corrected, and interpreted using statistical or physical models. Accurate analysis is necessary to distinguish true mechanical events from noise.

Common tasks include baseline correction, drift removal, spectral fitting, and event detection. The output may be a force trace, a displacement record, or a derived mechanical parameter such as stiffness or viscosity.

6 Applications

Optical tweezers have become important across many fields because they provide contact-free manipulation and quantitative force measurement. Their applications range from single-molecule biophysics to the assembly of microscopic structures. The same basic tool can be adapted to very different scientific questions.

6.1 Molecular biology

In molecular biology, optical tweezers are used to probe the mechanics of individual biomolecules. They offer a way to observe structural transitions under controlled force. This has improved understanding of folding, binding, and enzymatic motion.

6.1.1 Protein unfolding studies

Protein unfolding studies examine how force changes a protein’s conformation. A trapped bead can pull on a single protein or protein complex until it unfolds in a measurable stepwise manner. The data reveal stability, intermediate states, and kinetic barriers.

These experiments help connect molecular structure with mechanical function. They are especially informative for proteins that operate under load in living systems.

6.1.2 Motor protein assays

Motor protein assays track the movement of proteins such as kinesin, myosin, or dynein as they move along cellular tracks. Optical tweezers can measure stepping behavior, stall force, and load dependence. This provides direct insight into energy conversion at the molecular level.

Because the forces involved are very small, optical trapping is well matched to these systems. It can resolve the interaction between individual motors and their substrates with high temporal precision.

6.2 Cell biology

In cell biology, optical tweezers are used to study mechanical properties and to position living cells with minimal contact. This makes them useful for probing membranes, cytoskeletal response, and cell-cell interactions. The method can also support gentle handling during microscopic procedures.

6.2.1 Membrane mechanics

Membrane mechanics experiments investigate how cells deform under applied force. A trapped bead may be attached to the membrane and pulled to measure elasticity, tension, or membrane-cytoskeleton coupling. These measurements illuminate the physical basis of cell shape and resilience.

The technique is valuable because it can apply controlled forces without rigid tools. This reduces mechanical disturbance while still producing quantitative data.

6.2.2 Cell sorting and positioning

Optical tweezers can sort or arrange cells in defined patterns. This is useful in microenvironment studies, cell pairing, and experiments that require single-cell placement. Positioning is especially helpful when downstream observation depends on exact geometry.

Unlike bulk sorting systems, optical manipulation works at the level of individual cells. This offers exceptional precision, though typically at lower throughput.

6.3 Soft matter and colloids

Soft matter systems, including colloidal suspensions and polymer-rich fluids, are well suited to optical manipulation. Their particles are often large enough to trap easily, yet sensitive enough to reveal subtle mechanical behavior. This makes optical tweezers a powerful probe of mesoscale physics.

6.3.1 Microrheology

Microrheology uses trapped particles to measure the viscoelastic properties of fluids and gels. The way a bead fluctuates or responds to a controlled displacement reveals information about local viscosity and elasticity. This is especially valuable in complex fluids that are difficult to characterize by conventional rheometers.

Because measurements are local, microrheology can detect heterogeneity that would otherwise be averaged out. It is useful in polymer solutions, gels, and crowded biological media.

6.3.2 Self-assembly studies

Self-assembly studies examine how particles organize into larger structures. Optical tweezers can bring components together in defined arrangements and then observe whether they remain bound or reorganize. This helps test interaction models and assembly pathways.

The technique is well suited to colloidal crystals, clusters, and responsive materials. By controlling spacing and contact conditions, researchers can influence the final structure.

6.4 Nanotechnology

In nanotechnology, optical tweezers assist with positioning and assembling microscopic and nanoscopic components. Although direct trapping becomes more difficult at very small scales, specialized optical methods can still contribute to fabrication and organization. The ability to place objects without physical probes is particularly attractive.

6.4.1 Microfabrication assistance

Optical tweezers can assist microfabrication by holding parts in place during bonding, alignment, or deposition steps. This is useful when precision placement is needed but mechanical contact would be inconvenient or damaging. The method can complement lithographic and assembly workflows.

It is often used as a flexible laboratory-scale tool rather than a mass-production technique. Nonetheless, it can support prototyping and experimental device construction.

6.4.2 Particle assembly

Particle assembly uses optical forces to arrange beads, rods, or other microcomponents into ordered patterns. Holographic systems are particularly well suited to this task because they can create many traps at once. The assembled structures may serve as model materials or as precursor configurations for more permanent devices.

This controlled placement is valuable in colloidal engineering and photonic material studies. It allows researchers to test how geometry influences collective behavior.

7 Limitations and challenges

Despite their versatility, optical tweezers have practical limits. These include geometric constraints, finite force output, and sensitivity to the surrounding environment. Biological and optical side effects can also restrict what can be studied safely.

7.1 Trap strength and size constraints

Trap strength is limited by laser power, focusing optics, and particle properties. Very small particles may not be held reliably, while very large or dense objects may require more force than the system can provide. The trap volume is also confined to a small region near the focal point.

These constraints mean that optical tweezers are best suited to microscopic objects rather than macroscopic loads. The usable range depends strongly on the specific instrument design.

7.2 Sensitivity to environmental noise

Mechanical vibration, acoustic noise, and thermal drift can all disturb the trap. Air currents, imperfect isolation, and stage instability may produce measurable errors in high-sensitivity experiments. Because optical tweezers often detect tiny displacements, even small disturbances can matter.

Careful bench design, vibration isolation, and stable temperature control help reduce these effects. Nonetheless, environmental noise remains a common limitation in precision work.

7.3 Sample damage and optical heating

The same light that creates the trap can also harm the sample. Heat buildup, photochemical reactions, and prolonged exposure may alter living systems or change the measured mechanical response. This is a particular concern in sensitive biological experiments.

Researchers therefore balance force requirements against damage risk. Lower power, shorter exposure, and suitable wavelength selection can reduce the problem but not always eliminate it.

7.4 Constraints in opaque or highly absorbing media

Optical tweezers work best in transparent media where light can travel and focus effectively. Opaque or strongly absorbing materials scatter or absorb the beam before it reaches the target. This reduces trapping efficiency and may increase heating.

As a result, many applications are limited to clear liquids, thin samples, or optically accessible regions. Specialized approaches can extend the method in some cases, but the basic challenge remains.

8 History and development

The development of optical tweezers arose from earlier work on light-induced forces and progressed through advances in laser technology, microscopy, and detector design. The field matured as researchers learned to convert a theoretical idea into a practical tool for manipulating small objects. Its growth has been closely tied to the rise of modern biophysics and single-molecule measurement.

8.1 Early concepts of light-induced forces

Ideas that light could exert pressure date back to classical electromagnetic theory. Early theoretical studies recognized that momentum transfer from light should produce measurable forces. These concepts were initially of limited practical use because suitable light sources and optical systems were not yet available.

The emergence of lasers changed this situation by providing intense, focused, and coherent beams. This made precise light-matter force experiments much more feasible.

8.2 Key experimental milestones

A major milestone was the demonstration that focused laser beams could trap microscopic particles in stable equilibrium. Subsequent work refined the technique, improved calibration methods, and extended trapping to a wide range of materials. Instrumentation also became more reliable and easier to integrate with microscopes and measurement electronics.

Later developments enabled high-resolution force spectroscopy, multiplexed trapping, and manipulation of biological specimens. These advances transformed optical tweezers from a physics curiosity into a general laboratory tool.

8.3 Recognition and scientific impact

Optical tweezers have had a strong impact on multiple disciplines by enabling direct measurement at the level of single molecules and cells. They helped establish the field of single-molecule biophysics and contributed to new insights into motor proteins, DNA mechanics, and cell elasticity.

The technique is also widely recognized as an example of how optical physics can be converted into a practical method for precision manipulation. Its influence extends to education, instrumentation, and nanoscale engineering.

9 Related technologies

Several other techniques can trap, position, or probe microscopic objects. Each uses a different physical interaction and is suited to particular types of samples or environments. Optical tweezers are part of a broader family of noncontact manipulation tools.

9.1 Magnetic tweezers

Magnetic tweezers use magnetic fields to apply force to particles or molecules attached to magnetic beads. They are especially useful for pulling on biomolecules over long times with stable force control. Unlike optical tweezers, they do not rely on light and therefore avoid optical heating.

Their main limitation is that the target must be magnetic or linked to a magnetic bead. This makes them highly effective for some assays but less universal than optical trapping.

9.2 Acoustic tweezers

Acoustic tweezers use sound waves to move particles in fluid. Pressure nodes and flow patterns can trap, organize, or transport small objects. They are often attractive for larger-scale or high-throughput manipulation because sound can act over broader regions than focused light.

They are particularly useful in microfluidic settings. However, their force characteristics differ from optical tweezers, and their resolution may be less suited to single-molecule experiments.

9.3 Atomic force microscopy

Atomic force microscopy measures forces with a sharp cantilever tip that scans or touches a surface. It can provide very high spatial resolution and is widely used in nanoscale imaging and mechanics. Unlike optical tweezers, it is a contact-based technique.

AFM is often preferred for studying surfaces and stiff materials. Optical tweezers are more suitable when noncontact manipulation or working in fluid is important.

9.4 Micropipette manipulation

Micropipette manipulation uses fine glass pipettes to hold or aspirate cells and other soft objects. It is especially common in cell biology and developmental studies. The method provides direct mechanical control but involves physical contact with the sample.

Compared with optical tweezers, micropipettes are simpler in some settings and can exert larger forces. Optical trapping, however, offers greater flexibility for contact-free positioning and force measurement.