Molecular dynamics simulation of the effect of initial temperature and initial pressure on thermal behavior and cavitation dynamics in tumor tissue

Technical Summary: Molecular Dynamics Simulation of Initial Temperature and Pressure Effects on Tumor Tissue Thermal Behavior and Cavitation Dynamics

Problem Statement
Thermal-based therapeutic strategies for cancer, such as hyperthermia and ultrasound-induced mechanical stimulation, rely heavily on coupled thermal, chemical, and mechanical mechanisms. While macroscopic studies have established the role of pressure fields in enhancing heat penetration and inducing cavitation (the nucleation, growth, and collapse of micro-bubbles), the fundamental atomistic mechanisms governing thermal–mechanical coupling in tumor microenvironments remain poorly understood. Specifically, the combined effects of initial thermodynamic conditions—namely temperature and pressure—on cavitation dynamics, local thermal conductivity, and energy redistribution are insufficiently explored. Existing continuum-scale models and experimental techniques lack the resolution to capture transient atomic-scale interactions, local density fluctuations, and the precise evolution of bubble collapse mechanisms under varying initial states.

Methodology
The study employs Molecular Dynamics (MD) simulations using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) to investigate a coarse-grained tumor tissue model.

• System Construction: The computational domain (15 × 15 × 15 nm³) consists of a semi-structured polymer-like tumor matrix immersed in a TIP3P water environment. Dispersed argon atoms serve as cavitation nuclei (gas clusters) to initiate bubble formation.
• Interactions: Interatomic forces are modeled using a hybrid framework combining Lennard–Jones (LJ) potentials for van der Waals interactions and Coulombic potentials for electrostatic forces. Bonded interactions (bond stretching, angle bending, dihedral, and improper terms) are included for the tumor matrix.
• Simulation Protocol:
  1. Equilibration: The system undergoes a 10 ns equilibration stage in the canonical (NVT) ensemble using a Nosé–Hoover thermostat to reach a stable state at 300 K.
  2. Production: A 10 ns production run follows. The system is heated to 320 K over 5 ps using a Langevin thermostat. An external time-dependent sinusoidal force (simulating ultrasonic excitation) is applied to the fluid phase for the final 5 ps.
  3. Variables: The study systematically varies the initial temperature (280–310 K) and initial pressure (0–2 bar).
• Analysis: Thermal conductivity is calculated using the Green–Kubo formalism based on heat-flux autocorrelation functions. Structural evolution is quantified via the radius of gyration of the argon bubble, radial distribution functions (RDF), and spatial density/stress profiles.

Key Results
The simulations reveal distinct responses of the tumor tissue model to variations in initial temperature and pressure:

• Effect of Initial Temperature (280–310 K):

  • Saturation Temperature: The final equilibrium (saturation) temperature remained constant at approximately 320 K regardless of the initial temperature, indicating a stable thermal response independent of initial thermal history.
  • Structural Expansion: Increasing the initial temperature promoted structural expansion. The radius of gyration of the argon bubble increased monotonically from 42.79 Å (at 280 K) to 42.89 Å (at 310 K), reflecting enhanced atomic mobility and reduced effective confinement.
  • Thermal Conductivity: Thermal conductivity exhibited a non-monotonic trend, varying between 0.29 and 0.39 W/m·K. It increased from 0.30 to 0.36 W/m·K (280–290 K), dropped to 0.29 W/m·K at 300 K, and peaked at 0.39 W/m·K at 310 K. This behavior suggests a competition between enhanced atomic mobility (facilitating energy transfer) and structural constraints (limiting efficiency at intermediate temperatures).
  • Stress and Density: Higher temperatures induced significant stress localization and atomic redistribution across the entire domain, though overall structural integrity was maintained.

• Effect of Initial Pressure (0–2 bar):

  • Saturation Temperature: Similar to temperature variations, the equilibrium temperature remained stable at ~320 K across all pressure levels.
  • Structural Response: The radius of gyration showed a non-monotonic dependence on pressure. It peaked at 1 bar (42.94 Å), slightly decreasing at 0.5 bar (42.87 Å) and 2 bar (42.84 Å). This identifies 1 bar as an optimal condition for bubble expansion.
  • Thermal Conductivity: Thermal conductivity also displayed a non-monotonic trend, rising from 0.29 W/m·K (0 bar) to a maximum of 0.38 W/m·K (1 bar), then declining to 0.33 W/m·K (2 bar). The authors attribute the peak at 1 bar to an optimal balance where reduced interatomic spacing enhances interaction strength without the excessive structural compaction that restricts atomic mobility at higher pressures (2 bar).

Significance and Claims
The paper claims to bridge the gap between continuum-level observations and nanoscale mechanisms in tumor tissue modeling. By explicitly resolving interatomic interactions, the study provides atomistic-level insights into how initial thermodynamic states influence cavitation-induced thermal responses.

• Novelty: The work systematically links initial thermodynamic states (temperature and pressure) with cavitation dynamics and heat transfer in a biologically relevant environment, a combination not previously explored at the molecular scale.
• Implications: The identification of non-monotonic behaviors in thermal conductivity and structural expansion suggests that therapeutic strategies relying on pressure-assisted or ultrasound-based treatments require precise parameter optimization. Specifically, the existence of an "optimal" pressure (1 bar in this model) that maximizes both bubble expansion and thermal conductivity implies that therapeutic efficacy may be enhanced by tuning these initial conditions to balance interatomic spacing and atomic mobility.
• Limitations: The study is confined to a specific coarse-grained model and simulation timescales (nanoseconds). It does not propose new experimental protocols or clinical applications but rather offers fundamental insights to inform the optimization of existing thermal-based cancer therapies.