© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

9.3.5.1 Morcellation and Mincing

Morcellation is the fragmentation of biological materials as they are excised from the body; mincing is the progressive reduction in size of tissue segments by mechanical means. Chunks of biological material may be excised from the body using a tool such as the core sampler described in Section 9.3.2 (3). Cutting is required because many connective tissues have strengths on the order of 10-100 atm but the body is only pressurized to ~1 atm, so vacuum suction alone is insufficient to pull coherent tissues apart.

In the simplest design, morcellated material is passed through successive banks of fixed diamondoid cutting blades or rotating diamondoid cutting disks positioned at various angles with progressively more closely-spaced blade edges. The target material is minced into nanometer-sized bits. Each blade is geared for maximum force application at relatively slow speed and turns in a tightly fitted sleeve, providing an automatic self-cleaning action. More complex designs might include paddlewheeling-bucket or orthogonal-blade arrays whose interiors are cleared by pistons after completion of the mincing action on material compacted into the cutting chamber by a larger piston. Table 9.3 shows that diamond and other potential blade materials are far stronger than all forms of biological matter, including dental enamel which is the hardest natural substance found in the human body.

To quantify the mincing action, consider a blade of length L_blade, cutting edge width W_edge, and compressive strength T_blade (N/m²) which applies a force F_blade to a biological sample of length L_sample and thickness x_sample and tearing strength T_sample. Assuming L_blade = L_sample, then the minimum force required to cut the sample is F_min = T_sample L_blade W_edge < F_blade, the chopping energy per full blade stroke is E_stroke = F_blade x_sample, and the continuous power requirement of the blade (neglecting drag during the return stroke) is P_blade = E_stroke n_blade where n_blade is chopping frequency in Hz. Taking L_blade = 1 micron, W_edge = 1 nm, x_sample = 1 micron, T_sample = 10⁸ N/m², and n_blade = 1 KHz, then F_min = 100 nN, E_stroke = 0.1 pJ, and P_blade = 100 pW.

Consider next an ideal morcellation system which accepts a cubic biological sample of volume V_sample = L_sample³ and minces it by progressive halving into final pieces of volume V_minced = L_minced³ in a minimum number of N_cut = log₂ (V_sample / V_minced) cuts (analogous to "oct-trees" in computer science³¹⁵⁵). The energy required to make the i^th cut is T_sample W_edge [V_sample / 2^(i-1)]^2/3 and after the i^th cut there are 2ⁱ intermediate pieces, hence the total energy E_minced required to make all cuts down to the final pieces is:

{Eqn. 9.54}

The minimum size of the final pieces depends on the character of the sample material, the edge sharpness and speed of the blades, and numerous other factors, but in most cases cannot be smaller than 1-10 nm -- natural biomolecules like hemoglobin, insulin, or albumin are 5-10 nm in diameter (Fig. 3.15). A crude limit is E_stroke >~ E_bond, where E_bond = 180-1800 zJ for individual covalent bonds (Section 3.5.1), which implies a minimum final piece size of L_min ~ (E_bond/T_sample W_edge)^1/2 assuming L_blade = x_sample = L_min. Taking E_bond ~ 1000 zJ, T_sample = 10⁸ N/m² and W_edge = 1 nm, then L_min ~ 3.2 nm.

Eqn. 9.54 may now be applied to a specific morcellator configuration. Taking V_sample = 1 micron³, V_minced = 1000 nm³ (L_minced = 10 nm), W_edge = 1 nm, T_sample = 10⁸ N/m², and n_blade = 1 KHz, then N_cut = 20 cuts, blade velocity v_blade = n_blade V_sample^1/3 = 1 mm/sec, total mincing time t_minced = N_cut / n_blade = 20 millisec, volume mincing rate 'V_minced = V_sample / t_minced = 50 micron³/sec, total energy to mince a single sample of volume V_sample is E_minced = 40 pJ, and continuous power dissipation while the morcellator is in operation is P_minced = E_minced / t_minced = 2000 pW. Care must be taken to minimize sample adhesion in blade surface design (Section 9.2.3).*

* M. Krummenacker notes that mechanical bond cutting might leave behind reactive fragments, possibly including radicals, that could dehydrogenate the diamond blade surface via radical abstraction, leading to chemomechanical blade wear. A fluorine-passivated diamond blade might be more resistant to such wear, and hydrogen bonding with fluorine increases protein-diamond friction, possibly giving better cutting.

Once reduced to multi-nanometer-scale chunks, biological refuse may be disposed of directly (Chapter 16) or may be further digested using inorganic acids, lysosomal enzymes (Sections 8.5.3.8 and 10.4.2), collagenases,³⁵⁹ artificial enzymes, or even direct molecular mechanodecomposition (Section 9.3.5.3.5) which is the inverse of mechanosynthesis. A discussion of the biocompatibility of minced tissue and methods of final disposal is deferred to Chapters 15 and 16.

Last updated on 21 February 2003