2.4 Photochemical and photothermal interactions

Light can trigger chemical changes and generate heat in tissues. These photochemical and photothermal interactions form the basis for many medical treatments. Understanding how light interacts with biological molecules allows us to harness its power for therapeutic applications.

Photochemical reactions use light to drive chemical changes, like in photodynamic therapy for cancer. Photothermal effects convert light to heat, enabling targeted tissue heating. Both mechanisms offer precise, minimally invasive treatment options across various medical fields.

Photochemical Interactions

Fundamentals of Photochemical Reactions

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• Photochemical reactions occur when light energy triggers chemical changes in molecules
• Absorption of photons excites electrons to higher energy states
• Excited molecules can undergo various processes including:
  • Bond breaking
  • Isomerization
  • Electron transfer
  • Energy transfer to nearby molecules
• Quantum yield measures the efficiency of photochemical reactions
• Photochemical reactions play crucial roles in:
  • Photosynthesis (conversion of light energy to chemical energy in plants)
  • Vision (photoisomerization of retinal in rhodopsin)
  • Vitamin D synthesis (conversion of 7-dehydrocholesterol to vitamin D3 in skin)

Photosensitizers and Reactive Oxygen Species

• Photosensitizers absorb light and transfer energy to other molecules
• Common photosensitizers include:
  • Porphyrins
  • Chlorophylls
  • Synthetic dyes (methylene blue, rose bengal)
• Photosensitizers generate reactive oxygen species (ROS) through two main mechanisms:
  • Type I: Electron transfer to form superoxide anion and other radicals
  • Type II: Energy transfer to form singlet oxygen
• ROS can cause oxidative damage to cellular components:
  • Lipid peroxidation in cell membranes
  • DNA damage leading to mutations
  • Protein oxidation affecting enzyme function
• Antioxidants (vitamin C, vitamin E) protect cells from ROS damage

Applications in Photodynamic Therapy

• Photodynamic therapy (PDT) uses photosensitizers and light to treat diseases
• PDT process involves:
  • Administration of photosensitizer
  • Accumulation of photosensitizer in target tissue
  • Activation by light of specific wavelength
  • Generation of ROS to induce cell death
• PDT applications include:
  • Cancer treatment (skin, lung, esophageal cancers)
  • Acne treatment
  • Age-related macular degeneration
• Advantages of PDT:
  • Minimally invasive
  • Targeted treatment with reduced side effects
  • Can be repeated without cumulative toxicity
• Challenges in PDT:
  • Limited light penetration depth in tissues
  • Photosensitivity in patients after treatment

Photothermal Interactions

Principles of Photothermal Effects

• Photothermal effect converts absorbed light energy into heat
• Occurs when photons are absorbed by chromophores in tissue
• Common chromophores in biological tissues:
  • Melanin (skin pigmentation)
  • Hemoglobin (blood)
  • Water (infrared absorption)
• Heat generation depends on:
  • Light intensity
  • Absorption coefficient of the tissue
  • Exposure time
• Temperature rise can lead to various biological effects:
  • Protein denaturation
  • Cell membrane disruption
  • Tissue coagulation

Heat Diffusion and Thermal Relaxation

• Heat diffusion describes the spread of thermal energy in tissue
• Governed by Fourier's law of heat conduction
• Thermal relaxation time (TRT) indicates how quickly heat dissipates
• TRT calculated using the formula: $TRT = d^2 / (4α)$
  • d: target size
  • α: thermal diffusivity of the tissue
• Short pulse durations (less than TRT) confine heating to the target
• Longer exposures allow heat to spread to surrounding tissue
• Understanding TRT helps optimize laser parameters for specific applications

Applications in Photothermal Therapy

• Photothermal therapy uses heat generated by light absorption to treat diseases
• Nanoparticles (gold nanorods, carbon nanotubes) enhance photothermal effects
• Applications of photothermal therapy include:
  • Cancer treatment (photothermal ablation of tumors)
  • Hair removal (targeting melanin in hair follicles)
  • Treatment of port-wine stains (targeting hemoglobin in blood vessels)
• Advantages of photothermal therapy:
  • Non-invasive or minimally invasive
  • Can be combined with other therapies (chemotherapy, immunotherapy)
  • Potential for real-time monitoring using thermal imaging
• Challenges in photothermal therapy:
  • Achieving uniform heating in large or deep-seated tumors
  • Balancing efficacy and safety in treatment planning

Photoablation

Mechanisms and Applications of Photoablation

• Photoablation involves the removal of tissue through direct bond breaking by high-energy photons
• Occurs when photon energy exceeds chemical bond energies (typically UV wavelengths)
• Characterized by minimal thermal effects due to rapid energy deposition
• Excimer lasers commonly used for photoablation:
  • ArF (193 nm)
  • KrF (248 nm)
  • XeCl (308 nm)
• Ablation threshold defines the minimum energy density required for tissue removal
• Applications of photoablation include:
  • Refractive eye surgery (LASIK, PRK)
  • Angioplasty for clearing blocked arteries
  • Dental procedures (cavity preparation, enamel etching)
• Advantages of photoablation:
  • Precise tissue removal with minimal collateral damage
  • Reduced scarring compared to thermal ablation techniques
• Limitations of photoablation:
  • Limited penetration depth of UV light in tissue
  • Potential for mutagenic effects with prolonged UV exposure